I have a 99 chevy sub with the 5.7 and I had the seal replaced under the intake manifold and now it miss fires when I climb a hill and get to peak it will missfire badly. It will never do it when your climbing the hill or on flat ground but only when the engine has been enganged on the hill and then comes off of a little. It only happens at speeds over 50 MPH. I have replaced the plugs, wires, cap & rotor, and the fuel filter. The dealer wants to replace all these parts again and charge me 500 for it. I need some Ideas. I took it to another mechanic and he sees it misfire with the scope while driving but can't pinpoint it.

Thanks

Tony

Wow! you described your problem as if you have been driving my '99. I plan on cleaning the EGR valve via some shade tree advice this weekend. I will let you know what happens.

what kind of wires did you buy? if you replaced them with the ones from advance or autozone take them back also check all the plugs to see if the porcelin is loose at the point where it meets the nut.i own a automotive shop i have had some bad experiance with advane and autozone wires.always use standard ignition or borg warner wires. the plug situation is somthing a ran across an a 1994 chevy truck a couple of weeks ago. i did a quick tune up on it and it ran worse i found the #3 plug missfiring around the porcelin.

guess where i got my wires from? its been missing since i bought it and the engines been rebuilt still the same but worse ive put plugs(ac delco),cap , rotor, wires, fuel filter on 3 times! oh yeh 1 o2 sensor still the same problem the longer i drive the more it stumbles and it has strong exhaust and ive checked egr valve seems to be working .

I have just experienced the same problem with my '96, 5.7 Sub.
I too just had the intake manifold seal replaced about 1,500 miles ago--seems like a pattern is forming. My Sub now skips, jerks and seems like it will stall (but never has) when going up even the smallest incline, and at acceleration when over 50 mph.
Has anyone in this thread, or otherwise, nailed down the cause?
Thanks in advance.
RJN

do any of you have a check engine light?

i do not have a check engine light on nor do i have any codes shown.runs fine while driving just stumbles while idling or at a light.replaced my iac a few weeks ago it stopped working.seems to drag taking off ,maybe slow shifting trans originally suspected trans problem but some said engine was bogging down after having it rebuilt had the same miss as before and still shifts slow, goes into 2nd at 15mph or more depending on how fast i take off, 30 mph on 3rd.oh ive heard a rumor about cleaning up under the throttle body could this be true?have no vacuum leaks that i could find

Re: Check Engine Lights
Concurrent with the my sputtering/acceleration problem, my "Service Engine Soon" light came on and has stayed on since. I assume that the stealership will be able to track down an engine fault code; however, it is my hope that someone here will have some good insight for all of us.

I too have been told that it is likely the EGR valve. If so (and forgive my novice), but where is it located? I understand that giving the EGR a gentle knock may dislodge the carbon that is potentially clogging it.
Any thoughts?
Thanks in advance
RJN

it should be located to the left rear of the throttle body sort of circular in shape with a vac line going to it. ive already removed my and cleaned and reinstalled with new gasket still no change in performance. next project will be to remove throttle body and injectors and clean them but guess what ? the gasket set is kinda expensive to be so small! i wonder if it could be the fuel regulator anybody know how to check it? btw already replaced fuel filter twice

Well, this sounds too familiar too me as well and I have been told "sticky fuel injectors" I had them cleaned and it seemed to help for a while, but true to its nature the problem came back. $474 dollars down the tubes. It missfires as I crest a hill and will eventually go away, check engine light comes on and reads "random misfire" code. I would love to hear of any solutions as well, I am a thrifty Do it yourself mechanic, but this one is kicking my but. this is the first time in 15 years I have gone to a mechanic only to get what I thought, money taken problem still there.

i know what u mean,i very seldom pay someone else to fix my vehicles! damn near never only when i cant figure it out

I have been having the same problem with my 1996 5.7 and changed all of the o2 sensors and it was still giving a code but no as often. I brought it to the dealer and they told me that it was a clogged cat. This is a normal problem because there is a federally mandated warranty for 8 years or 80,ooo miles and mine had 82,000 on mine. Guess what no warranty so I bouht mine at napa for 290.00 instead of 651.00 from the dealer.

I have been having the same problem with my 1996 5.7 and changed all of the o2 sensors and it was still giving a code but no as often. I brought it to the dealer and they told me that it was a clogged cat. This is a normal problem because there is a federally mandated warranty for 8 years or 80,ooo miles and mine had 82,000 on mine. Guess what no warranty so I bouht mine at napa for 290.00 instead of 651.00 from the dealer.
what did they do to it to determine it was clogged?

Have ya'all tried a $5 can of Seafoam? Seems its a miracle tonic for all that ails an engine.

whats that ive never heard of it before?

I guess I'm kinda half kidding - its worth a $5 try. Its a fuel additive you can let the engine suck up via the brake vacuum hose and also put in the gasoline. It cleans out the carbon buildup. Do a search for seafoam on most any automotive forum.

Make sure that your Distributor cap is tight. The base that the cap screws into is plastic and will crack and become loose over time. if this happens the truck will misfire and jerk like crazy.

I had to replace the entire distributor because the holes were the cap screws into was cracked and wouldnt allow me to tighten the cap down. i rigged it for a while and that fixed my misfiring, but i just bit the bullet and bought a distributor for it.

what kind of vehicle do u have

i would make sure your dist cap is tight, The dist. cap has plastic holes that the cap screw into and they will crack over time... mine did, and i had to rig the cap so it woldnt move as the rotor was spinning around. once i secured the cap.. it stopped misfiring.. i ended up getting a new distributor just because, but rigging the distributor holes with JB weld so the cap could have someting to screw into worked.

With this fuel system fuel pressure is very important to over come the poppet nozzles in the fuel injectors. If the fuel pressure drops below the poppet nozzle threshold the injector will not flow any fuel, (if fuel pressure is even as high as 45-50 PSI it will not run, usually takes 50-52 PSI to open the poppet nozzles) that is why fuel pressure and fuel volume are so critical. Under heavy load (driving at highway speeds up a hill or under heavy acceleration) the fuel system is being asked to work it's hardest, if there is a weak component that is when it will show up. Check fuel pump volume, fuel pressure regulator, and fuel system's ability to maintain fuel pressure when shut off. If your system does not maintain fuel pressure when shut down or the pump volume is low it should miss under heavy accel. or going up hills at highway speed. After verifying the fuel supply system make sure the injectors are clean. (In the range of $100.00 around here.) $474 wow is that all that was done is clean the injectors? I really like this fuel system, it is just not understood by most because it does not react like all the other electronic fuel injection systems. Last but not least make sure the spark plugs and wires are fine, and yes I prefer OEM. Hope it helps.

on these vortec motors u can not adjust the timing its computerized
however if the distributor has been tweeked a little the computer does not know that and it will cause CROSS firing... take it to a dealer and have it reset to specs..... if you dont beleave me read a chiltons book..]

I will said that it has to be something related with the intake gasket area because I just have mine change and the same day after pickup the truck from the repair shop on my way home was when I found the problem, before this job (intake gasket) I did never have the problem......Sub 99 K1500 77K miles.

I have a 99 chevy sub with the 5.7 and I had the seal replaced under the intake manifold and now it miss fires when I climb a hill and get to peak it will missfire badly. It will never do it when your climbing the hill or on flat ground but only when the engine has been enganged on the hill and then comes off of a little. It only happens at speeds over 50 MPH. I have replaced the plugs, wires, cap & rotor, and the fuel filter. The dealer wants to replace all these parts again and charge me 500 for it. I need some Ideas. I took it to another mechanic and he sees it misfire with the scope while driving but can't pinpoint it.

Thanks

Tony
I have this exact condition with my 97 yukon. The dealer replaced the intake manifold gasket. Now on high throttle settings like pulling my boat up a long hill all is well until I crest the hill and back off a bit. Then a really heavy misfire situation lasts for about 5 - 10 seconds. I think vacuum leak causing preignition might be the problem??? This week I got a flashing check engine too under the same load conditions. I really don't want to go to the dealer without a better idea. I have replaced plugs HT leads pcv valve and fuel filter. Any ideas??

With this fuel system fuel pressure is very important to over come the poppet nozzles in the fuel injectors. If the fuel pressure drops below the poppet nozzle threshold the injector will not flow any fuel, (if fuel pressure is even as high as 45-50 PSI it will not run, usually takes 50-52 PSI to open the poppet nozzles) that is why fuel pressure and fuel volume are so critical. Under heavy load (driving at highway speeds up a hill or under heavy acceleration) the fuel system is being asked to work it's hardest, if there is a weak component that is when it will show up. Check fuel pump volume, fuel pressure regulator, and fuel system's ability to maintain fuel pressure when shut off. If your system does not maintain fuel pressure when shut down or the pump volume is low it should miss under heavy accel. or going up hills at highway speed. After verifying the fuel supply system make sure the injectors are clean. (In the range of $100.00 around here.) $474 wow is that all that was done is clean the injectors? I really like this fuel system, it is just not understood by most because it does not react like all the other electronic fuel injection systems. Last but not least make sure the spark plugs and wires are fine, and yes I prefer OEM. Hope it helps.

My 97 Yukon has this set of problems. Same engine. The intake manifold gasket started to leak coolant and the dealer pulled off the fuel system to replace it. When I got the truck back( with a new oil pressure sender, water pump and spark leads $1500 total) I had a little induction growl on throttle opening but unless the radio was off I couldn't hear it(can't hear much anyway says my wife) On a fishing trip many weeks later, I pulled my boat up a long hill. The initial throttle up works very well. strong pull and no miss. Its when I back off at the crest of the hill that the miss occurs. The first time it happened I was on cruise but I can reproduce the effect every time I go to a high throttle situation and back off. When I come back on the throttle I briefly have a heavy miss.

My latest fishing trip has escalated the symtom with a flashing check engine light on the initial throttle up but still no miss until the throttle down throttle up cycle as the load is reduced.

Like everyone, I need reliability and I'm worried about getting stalled somewhere but my fear of the dealer's costs and my doubts about their abilities keep me looking for a diagnosis before I try to fix this.

I do all routine maintainance and have replaced the fuel filter and pcv valve and installed new plugs. I also have an OBDII code reader and there are no codes present.

I'm wondering if the heavy load heats up some component that causes preignition in the multiport Fuel injection throttle body. Combined with a vacuum leak could that be the problem?

I'm at the point of abandoning GMC forever. I have a seven year old truck with 54 000 miles on it garage stored and carefully maintained and I am losing trust in it's reliablity

I really appreciate your ideas cause I really need a fix.

After reading all the posts on this problem and seeing the same pattern it would seem to me that the problem is in the EGR system.
I am thinking maybe dirt or something has got into the EGR passages ??
Fireplug


My 97 Yukon has this set of problems. Same engine. The intake manifold gasket started to leak coolant and the dealer pulled off the fuel system to replace it. When I got the truck back( with a new oil pressure sender, water pump and spark leads $1500 total) I had a little induction growl on throttle opening but unless the radio was off I couldn't hear it(can't hear much anyway says my wife) On a fishing trip many weeks later, I pulled my boat up a long hill. The initial throttle up works very well. strong pull and no miss. Its when I back off at the crest of the hill that the miss occurs. The first time it happened I was on cruise but I can reproduce the effect every time I go to a high throttle situation and back off. When I come back on the throttle I briefly have a heavy miss.

My latest fishing trip has escalated the symtom with a flashing check engine light on the initial throttle up but still no miss until the throttle down throttle up cycle as the load is reduced.

Like everyone, I need reliability and I'm worried about getting stalled somewhere but my fear of the dealer's costs and my doubts about their abilities keep me looking for a diagnosis before I try to fix this.

I do all routine maintainance and have replaced the fuel filter and pcv valve and installed new plugs. I also have an OBDII code reader and there are no codes present.

I'm wondering if the heavy load heats up some component that causes preignition in the multiport Fuel injection throttle body. Combined with a vacuum leak could that be the problem?

I'm at the point of abandoning GMC forever. I have a seven year old truck with 54 000 miles on it garage stored and carefully maintained and I am losing trust in it's reliablity

I really appreciate your ideas cause I really need a fix.

My 98 Silverado has had this symptom for a year now. Dealer checked it for free as they did the CSPI to MFI injector swap on a GM policy program. The result is the 5.7's have a known problem with the valves developing an actual mechanical sticking. Mechanic said live with it or rework the heads. Seafoam through vacuum port and marvel mystery at oil change made it a bit better but in the right conditions it shakes like hell!
:crying: :crying: :crying:

OK, after taking back the truck to the shop we discover a engine code showing problems with the Camshaft Position Sensor we unplug the sensor and start the engine and the problems was there like if the sensor was dead, so we repace the sensor and the problem disapear, so I'm very happy!!!!
The check engine light never came up so we didn't know about any codes, we just plug in the scanner and magic the code came and the poblem faound. Just want averybody to know the end of my problem to try to help some.Good Luck!!!!!!

Luis
10 years working on computers
0 years working on cars

With this fuel system fuel pressure is very important to over come the poppet nozzles in the fuel injectors. If the fuel pressure drops below the poppet nozzle threshold the injector will not flow any fuel, (if fuel pressure is even as high as 45-50 PSI it will not run, usually takes 50-52 PSI to open the poppet nozzles) that is why fuel pressure and fuel volume are so critical. Under heavy load (driving at highway speeds up a hill or under heavy acceleration) the fuel system is being asked to work it's hardest, if there is a weak component that is when it will show up. Check fuel pump volume, fuel pressure regulator, and fuel system's ability to maintain fuel pressure when shut off. If your system does not maintain fuel pressure when shut down or the pump volume is low it should miss under heavy accel. or going up hills at highway speed. After verifying the fuel supply system make sure the injectors are clean. (In the range of $100.00 around here.) $474 wow is that all that was done is clean the injectors? I really like this fuel system, it is just not understood by most because it does not react like all the other electronic fuel injection systems. Last but not least make sure the spark plugs and wires are fine, and yes I prefer OEM. Hope it helps.

The thing is, that when the high throttle setting is first applied, even on a long climb, there is no problem. Its only when the throttle is closed and then reapplied that the miss occurs. Is there a fuel pressure regulator that could be sticking in a high flow situation that closes too slowly causing a temporary over pressure???? Going after the fuel pump is beyond my comfort zone since I believe it is in the fuel tank and I wouldn't want to drop the tank in my garage. If there was a regulator on the induction system though I would try that.

I have experienced this mystery problem on a trip this weekend in my 99 burb. The code that was stored designated a misfire in the #7 cylinder. Felt like the trans was going to exit the bottom of the darn truck.

The burb is in the mechanics this AM. I will post whatever he finds.

I have experienced this mystery problem on a trip this weekend in my 99 burb. The code that was stored designated a misfire in the #7 cylinder. Felt like the trans was going to exit the bottom of the darn truck.

The burb is in the mechanics this AM. I will post whatever he finds.

Hope he doesn't come up with sticking valves. I'm going to have to be very convinced before I let them start pulling the heads.

I have been tring to solve the misfire issue for almost two years. This past week $300 more, with the same result. Don't spend another dollar sell it trade it but don't put another dime into the thing. Too many people have the same problem and after reading all these posts nobody has found the smoking gun. I am told there is a problem with the design of the heads and they must be REPLACED with NEW ones. But no guarantee. My advice sell it!!

It cant be sticking valves.
Everyone in here has said the problem started after having a intake manifold gasket done right??
So why would it not do this problem before the gasket was done??? only after.
I have 104564 miles on mine (5.7) and it has never had a gasket done and has no miss at all.
I just dont buy the sticking valve Job
My money in on a sticking egr valve. If you take a running engine at idle and open the egr valve what happens the engine misses and or stalls now add a heavy load and bingo you have a very bad miss that was not there before the gasket was done.
Fireplug

MIne did it before I had to replace the gasket, about 20K miles before. I replaced my cam sensor and it did not help much. I have not found a scanner to check the cam retard but I have moved the dist around a bit. I too think it sucks. A truck having this kind of common problem. I am leaning toward a Tundra if I ever buy something new. GM, Ford, and Dodge don't seem to care after they have your money. Lots of Honda and Toyota cars for sale in my local paper with near 200K on them and there is still a buyer for them.

My 94 has done this often and usually I find a bad wire or plug. Get out at night and see if you have any arching going on. I bet you find a light show.

I too saw a light show under the hood and replaced all plugs (AC/Delco) and wires (Bosch). The light show has tamed considerably, but still see a constant flashing around one particular spark plug. I don't know the numbering system, but it is on the left (driver side) second back from the radiator. I swapped plugs and the problem stays at the same cylinder. Don't know why. Anyone?

That would be cylinder number 3
1,8,4,3,6,5,7,2, is the fireing order.
Driver side has cylinder 1,3,5,7,Pass side 2,4,6,8,
The wire that is still sparking please check that the rubber boot is all the way down on the plug.
There should be NO sparking at all . If there is that means there is a voltage leak in the wire.
I have seen new wires leak so try switching wires and see if the sparking goes with the same wire.
You can use a spray bottle with water in it and spray a mist on it to see where the wire is bad.
Fireplug

Yes, #3 is the one.
I will try swapping wires. The boot is all the way on the plug. Thanks for the tips!

I had the severe miss-firing probem on my 99 Sub. The mechanic was unable to get a fix on the problem but suggested I replace the plugs, cap and rotor. This was done using all delco parts (platinum plugs) and I have not had the problem since. I think if it does return then I will replace the EGR valve as it is not expensive and easy to get to. I did not replace the plug wires yet but will if the problem returns.

The mechanic checked all the related sensors and found all to be in working condition. I think the problem is a result of one thing going wrong that is causing a cascade effect.

Another forum, Ten Mag Chevy truck had a thread running, person with the problem like described here reports a cracked valve seat..... Some Beach!!!!!

Told by dealer its the injectors, which they say they now have a service bulletin that reccomends replacing them for about $850 verus cleaning them for about $250.

Has anyone replaced the injectors? Did it fix it? I may just clean them and trade it in - my 2002 is paid off now anyway!

I replaced injectors in my 96 K1500 but not because of misfire, I had a couple leakers and needed a new fuel pressure regulator. Cost me $270 for parts delivered to the door, took two hours to replace.

After I paid to have my intake manifold gaskets replaced I had a misfire at light load cruise- 60 mph with foot barely on throttle- and I'd just feel it misfire a little. Hooked my auto xray scanner up to it and checked the cam retard- it was -11 degrees and its supposed to be 0 degrees +/- 2 degrees. Once I got it to 0 degrees the misfiring was gone. I wonder if you would get misfiring under load(or heavy acceleration) with the cam retard too positive

That would be cylinder number 3
1,8,4,3,6,5,7,2, is the fireing order.
Driver side has cylinder 1,3,5,7,Pass side 2,4,6,8,
The wire that is still sparking please check that the rubber boot is all the way down on the plug.
There should be NO sparking at all . If there is that means there is a voltage leak in the wire.
I have seen new wires leak so try switching wires and see if the sparking goes with the same wire.
You can use a spray bottle with water in it and spray a mist on it to see where the wire is bad.
Fireplug
DESCRIBE EXACTLY HOW YOU USE THE SPRAYMIST TO FIND BAD WIRES! I HAVE THAT SAME PROBLEM WITH THE LIGHT SHOW UNDER THE BOOTS!

With the engine running spray a mist of water on all ign parts , wires , plugs , coils and look for any sparks. If there are any sparks the there is a problem. Lets say the ign wire boot has sparks comming from it . The wire is bad or the spark plug is cracked. To pin point the correct 1 switch the sparking wire with a nonsparking wire and see if the light show goes with the wire or stays at the cylinder if it stays at the cylinder then you have a cracked spark plug. also mad sure that the ign wires are all the way down on the plug and snap or click tight.

I have a 99 chevy sub with the 5.7 and I had the seal replaced under the intake manifold and now it miss fires when I climb a hill and get to peak it will missfire badly. It will never do it when your climbing the hill or on flat ground but only when the engine has been enganged on the hill and then comes off of a little. It only happens at speeds over 50 MPH. I have replaced the plugs, wires, cap & rotor, and the fuel filter. The dealer wants to replace all these parts again and charge me 500 for it. I need some Ideas. I took it to another mechanic and he sees it misfire with the scope while driving but can't pinpoint it.

Thanks

Tony

Hi,
check your timing...maybe distributor was loose enough to be turned by mechanic torquing down the manifold.

It cant be sticking valves.
Everyone in here has said the problem started after having a intake manifold gasket done right??
So why would it not do this problem before the gasket was done??? only after.
I have 104564 miles on mine (5.7) and it has never had a gasket done and has no miss at all.
I just dont buy the sticking valve Job
My money in on a sticking egr valve. If you take a running engine at idle and open the egr valve what happens the engine misses and or stalls now add a heavy load and bingo you have a very bad miss that was not there before the gasket was done.
Fireplug

Um, mine does it and I have not had the gasket job done. My suburban has 90K miles on it. Haven't put any money into it yet. Love my truck wouldn't sell it, just want to fix it. think there might be something to the fuel system cleaning idea. Had a 77 chevy, put bad gas into it one time, it ran like crap for months. I went through 30 plus fuel filters, and it always misfired until it eventually stalled, at which time (rain, hail, snow or never sun) I had to replace that darn fuel filter on the side of the road.
Jodi

Um, mine does it and I have not had the gasket job done. My suburban has 90K miles on it. Haven't put any money into it yet. Love my truck wouldn't sell it, just want to fix it. think there might be something to the fuel system cleaning idea. Had a 77 chevy, put bad gas into it one time, it ran like crap for months. I went through 30 plus fuel filters, and it always misfired until it eventually stalled, at which time (rain, hail, snow or never sun) I had to replace that darn fuel filter on the side of the road.
Jodi

Well - I don't usually go for better living through chemistry - but I blew a can of intake cleaner - cat converter safe - through the open throttle body after removing the zip tube/ resonator box (the box on top of the engine, comes off with the thumb screw) and my problem has gone into hiding. There was a film of carbon in the plenum and I believe that indicates some backfire condition and the cleaner dissolved it. There is a post in this thread where a guy used "Seafoam" whatever that is and got some positive results. Nobody else has. I don't like it because I have no idea what I changed but yeasterday I climbed "the hill" at full throttle full load and I did not get the miss.

What type of cleaner did you use? After reading this thread, I to may be having this same type of problem.....has any one noticed if the problem gets worse if you take a hard left under heavy exceleration?

What type of cleaner did you use? After reading this thread, I to may be having this same type of problem.....has any one noticed if the problem gets worse if you take a hard left under heavy exceleration?

I used a Solder Seal/Gunk spray called INTAKE MEDIC

My father is an old time mechanic and used to work on cars, before they became computerized. He is pretty knowledgeable and I get 99% of my information from him. From what he has always taught me, I don't think it is a good idea to stick anything in your engine that is a sealant. I could be wrong, but I would be leary since you don't really know what it will choose to seal and how dearly you will have to pay in the long run.

Solder Seal/Gunk is a brand name, and yes, they do make a sealer for your radiator! Intake Medic is an entirely different product however! :)

We have had misfire problems with our 1999 Suburban for quite some time. It usually showed up when going up a hill. The 'service engine light' would come on but when the codes were checked by a dealer or our mechanic there was only a 'random misfire' listed. I came to this site to because I was interested to see what other people had to say about what seems to be a chronic problem.

We purchased our 1999 in 2000. It was previously owned by a GMC sales manager. The first time we had problems was in 2002. At that time we replaced the plug wire set and the platinum plugs.

In 2003 we replaced the fuel pump.

In 2004 we replaced the distributor cap and the rotor. We replaced the platinum plugs again.

Later in 2004 we had the fuel injectors cleaned and the gas filter replaced and had the codes set by a dealer.

We had put on an after-market K&N airfilter. The beginning of 2005,we replaced the air filter and it ran fairly well for a few months.

But this month the 'service engine' light came on again. When we took it in the 'recommended fix' was to replace the CSFI fuel injection system with MFI injectors which we did. The water pump was leaking and had to be replaced also. When the suburban was taken for a test drive before we took it home, they checked for misfires and found that the #3 & #4 cylinder had a few misfires but cylinder #5 had 500 misfires on the test drive. So obviously changing to the MFI fuel injection was not the fix for us. We trust our mechanic and he has tried to fix the problem but it is ellusive!

The next suggestion was to replace the air sensor or replace the manifold intake gasket. We're not sure if we should or want to try this next.

My husband is just about ready to take JeffV's out and sell it, but I like the suburban and would like to fix the problem.

Any new suggestions?? Has anyone else who has had this problem been able to find a fix that has lasted??

The posts here have given us some additional things to consider and I appreciate the information.

Did anyone find a permanent solution to the misfire problem?

Did anyone find a permanent solution to the misfire problem?

Yes. It took some time - a lot of time, and on a whim, we changed the cam sensor. The misfire has gone away and I can now pull a nice steep hill going 70 or so MPH in all of the 4 miles per gallon glory that a Burb has when pushing it like that!

Up until that point, here's what was done, all of which did NOT resolve the problem:

New plugs
New wires
New cap and rotor
New fuel injector (#4 was the initial cylinder)
New intake manifold gasket
Crank sensor
Checked fuel pressure and cylinder compression (again #4)

Thanks Suburban Lee,
I'll pass on the information to our mechanic about the cam sensor.

Our suburban is scheduled to go in on Tues for intake manifold gasket replacement and some kind of air sensor.

Has anyone contacted GMC about the numerous problems that some of us have had with the misfire and rough running problems of this model?

Thanks Suburban Lee,
I'll pass on the information to our mechanic about the cam sensor.

Our suburban is scheduled to go in on Tues for intake manifold gasket replacement and some kind of air sensor.

Has anyone contacted GMC about the numerous problems that some of us have had with the misfire and rough running problems of this model?

I have read a lot of threads about this. I also have a 1999 C1500 Suburban. Same problem as everyone. Except I had to have the lifters replaced at about 85,000 miles. But when the lifters were replaced, of course the intake gasket was also replaced. The misfiring and shaking stopped or about 15,000 miles. But, IT's BACK!!!! One thing I have noticed is that when I run Super Unleaded gas, the misfiring is almost not there! I also have changed:

Distributor
Plugs (3 times; always using Platinum)
Plug wires
Fuel Filter
Coil
Distributor cap
Rotor
And, of course lifters!!!

Any ideas on why running hi-octane gas helps the misfiring not is so apparent?

I have read a lot of threads about this. I also have a 1999 C1500 Suburban. Same problem as everyone. Except I had to have the lifters replaced at about 85,000 miles. But when the lifters were replaced, of course the intake gasket was also replaced. The misfiring and shaking stopped or about 15,000 miles. But, IT's BACK!!!! One thing I have noticed is that when I run Super Unleaded gas, the misfiring is almost not there! I also have changed:

Distributor
Plugs (3 times; always using Platinum)
Plug wires
Fuel Filter
Coil
Distributor cap
Rotor
And, of course lifters!!!

Any ideas on why running hi-octane gas helps the misfiring not is so apparent?

Also, there has been some posting on other forums regarding a HSTSA Bulletin # 3081. It talks about replacing the EVAP Canister Vent Hose Assembly. Anyone try that?

I have read a lot of threads about this. I also have a 1999 C1500 Suburban. Same problem as everyone. Except I had to have the lifters replaced at about 85,000 miles. But when the lifters were replaced, of course the intake gasket was also replaced. The misfiring and shaking stopped or about 15,000 miles. But, IT's BACK!!!! One thing I have noticed is that when I run Super Unleaded gas, the misfiring is almost not there! I also have changed:

Distributor
Plugs (3 times; always using Platinum)
Plug wires
Fuel Filter
Coil
Distributor cap
Rotor
And, of course lifters!!!

Any ideas on why running hi-octane gas helps the misfiring not is so apparent?

This is my second burb. In total, I've put about 150K miles on these vehicles and I've never run high octane gasoline. Just me, but I think almost not there is not a qualifier. If there's a misfire, there's something wrong. If there's nothing wrong, then I don't care what kind of fuel you run through these vehicles.

I'm curious to hear from GrandmaK as to whether the CAM sensor proved to be a solution to the problem.

OK, after taking back the truck to the shop we discover a engine code showing problems with the Camshaft Position Sensor we unplug the sensor and start the engine and the problems was there like if the sensor was dead, so we repace the sensor and the problem disapear, so I'm very happy!!!!
The check engine light never came up so we didn't know about any codes, we just plug in the scanner and magic the code came and the poblem faound. Just want averybody to know the end of my problem to try to help some.Good Luck!!!!!!

Luis
10 years working on computers
0 years working on cars

I wonder if the intake could be icing??????

Suburban Lee: Did you replace the Cam Sensor Position Module? Thanks.

Suburban Lee: Did you replace the Cam Sensor Position Module? Thanks.

Cam Sensor Position Module?? Hmmm, sounds fancy. I referred to it as the Cam sensor. I assume we're talking about the same thing. Small part, about 2 by 1 inches, around $25 or so. If this doesn't answer your question, send me an e-mail.

Cam Sensor Position Module?? Hmmm, sounds fancy. I referred to it as the Cam sensor. I assume we're talking about the same thing. Small part, about 2 by 1 inches, around $25 or so. If this doesn't answer your question, send me an e-mail.

Having problems sending u an email. But, the part sits on top of the distributor and has a three (3) pronged plug (female I believe). A picture of the part can be found at the following link: http://www.rockauto.com/ref/ACDelco/Detail.html?213-920.jpg.

Is this what you had changed?

Thanks.

Yup, that's the part. After all it put me through, I thought about hanging it from the rear view mirror!

Yup, that's the part. After all it put me through, I thought about hanging it from the rear view mirror!

Well, I had the "knock sensor" changed and it seems to have helped. I can pull my 27' StarCraft Travle Trailer (4,050lbs) up a small hill, and no misfiring, shaking or pinging. BTW, I also had a terrible time with pinging; sounded like a bunch of monkeys with ball peen(spelling??) hammers inside banging away.

Anyways, my mechanic has the camshaft sensor, but I am going to wait for a couple more days and see.

Well after reading all of the threads I changed the Cam Sensor Position Module this weekend and so far no shake rattle and roll when underload and shifting into overdrive. I will add this good luck lineing up the notch on the dist. plate.

I just discovered this forum, and this thread, so I guess I'll throw my hat into the ring as well.

We too have a '99 suburban with the 5.7. Bought the truck from a used car lot with 50K on the clock, so I don't know what was done before that, but it was pretty much a cream puff. We've had the truck about three years, and noticed the mis-firing problem pulling a camper on a trip out West the first summer we had it. First sign of trouble was a check-engine light that came on during a long easy hill at speed. Next day the light had gone off again, but it did this several more times. Finally on a really strenuous grade on I-70 in the Rockies, I got the blinking check-engine light and the shuddering. Slowing down cured the problem.

After arriving home, we continued to have problems even when not pulling a trailer. Exactly as the OP described, it would start shaking badly when cresting a hill. At first I thought it might have to do with what gear it was in, i.e. an RPM related problem. The local dealer checked the codes and detected a misfire (don't remember what cylinder), and a slipping transmission. They serviced and adjusted the tranny, and recommended I change out the cap, rotor, and plugs for the misfire. I did that, but didn't use GM parts. The problem continued. I had another dealership check it out: misfire diagnosis again. I replaced the cap, rotor, and plugs again using all GM parts this time, to eliminate that as a possibility. Since then we moved, and I haven't had it back to a dealer. The problem seemed to have lessened, but then where we live now doesn't require as much hiway driving. Recently we noticed the misfire problem is still there when nearing the top of a long hill.

Separately, we took the Burb in to an independent mechanic for what I thought was going to be a water pump, but turned out to be the intake manifold gasket. They also diagnosed a misfire, but I didn't ask them to check into it. The misfire is still there after the new gasket, same as it was before, so in this case they don't appear to be related. OTOH, perhaps whoever owned it before us already had the gasket replaced . . .

So after reading through this thread, the possibilities seem to be the cam sensor module, a sticky EGR, injectors, or carbon buildup in the intake. Is there any follow-up information that previous posters would care to add?

Oh great! Something else to look forward to with my '99 Suburban! I just had the intake manifold gasket replaced 2 weeks ago and I have noticed that it seems to be skipping at idle. We are suppose to drive it to the mountains at Thanksgiving so I guess I'll get to see if I am having the same problem everyone is describing with it missing at load going uphill. Let's see, it has 65,000 miles on it and so far I have had to replace the transmission at 60,000 miles to the tune of 2600.00. The old transmission was serviced per GM instruction. The intake manifold went out at 65,000 miles to the tune of 504.00. That is a bunch of coin!!

This is the nicest car I have ever owned from the heated leather seats to the self-dimming rear view mirror. It rides and drives great and the MPG is not bad for the size vehicle--actually got 19 on last trip. But if this thing keeps busting my chops I am going to have to unload it! I have an 88 Silverado with the old TBI 350 and it has 205,000 hard miles on it and none of the problems the Suburban has presented. It still has the original engine and transmission.

:lol: :rofl: :lol: :rofl: :iceslolan
I have to laugh at this thread...I have posted the fix for this concern many times here in the forum and it appears that no one has bothered doing a search to find the answer.

Misfires on these engines that occur under load (climbing hills or cruising at highway speeds) are usually caused by the valves sticking in the guides (after proper diagnosis of the fuel and ignition system rules out fuel or spark).
The cause is carbon buildup on the valve stems that cause the valves to stick in the guides under higher RPM (acts like valve float)
There are a couple paths to follow here.
You can use a product such as Seafoam in the oil and gas to help with lubercation of the valve stems.
Decarbonizing of your engine with GM Top engine cleaner will help.
Or if one of those 2 don't work the next thing is to remove the heads and have them professionally cleaned and the guides replaced.

Laugh all you want, assuming that each and every problem with a similar sympton has the same cause is narrow minded. There are many causes that could contribute to a misfire condition, from as simple as bad ignition wires to your carbon-sticky valve issue. Truth is, it may not be a valve problem (something that we explored while diagnosing my same issue).

In your case, it was sticky valves, in my case it happened to be the CAM position sensor. How about we just pass the information and let those searching for it make the decision as to what to fix?

I know I'll be starting with the cheapest solution, then working my way up :)

Laugh all you want, assuming that each and every problem with a similar sympton has the same cause is narrow minded. There are many causes that could contribute to a misfire condition, from as simple as bad ignition wires to your carbon-sticky valve issue. Truth is, it may not be a valve problem (something that we explored while diagnosing my same issue).

In your case, it was sticky valves, in my case it happened to be the CAM position sensor. How about we just pass the information and let those searching for it make the decision as to what to fix?
Once again someone reads half of what is written and jumps to a conclusion.
Go back and read ALL of my post..including this part
“Misfires on these engines that occur under load (climbing hills or cruising at highway speeds) are usually caused by the valves sticking in the guides (after proper diagnosis of the fuel and ignition system rules out fuel or spark).”
How can this be an “assumption that each and every problem with a similar sympton has the same cause”.
After ruling out the obvious (fuel and spark), an often overlooked area is the valvetrain, especially on these engines.
I personally have never seen a CMP sensor on these vehicle cause a misfire at highway speeds.
Usually any concerns related to the CMP sensor that I have seen revolve around worn Dist housing bushings or worn Dist driven gears.

The VCM uses the CMP signal to determine the position of the #1 piston during its power stroke. This signal is used by the VCM to calculate fuel injection mode of operation.
If the cam signal is lost while the engine is running, the fuel injection system will shift to a calculated fuel injection mode based on the last fuel injection pulse, and the engine will continue to run. The engine can be restarted and will run in the calculated mode as long as the fault is present .

So where your misfire and change in engine performance comes from is the change from a individual injector firing strategy to an all fire injector strategy.

Guys,

I don't have the technicals knowledge on car engines but I was a mecanic on jets for 6 years.
What I learned was mostly that carbone deposit can be removed with the good chimical.
I've been struggling with my GMC Yukon for 6 months and spent $700 trying to fix all of the above.
Tired of the bad advices I decided to buy a quarter of SEA FOAM (2 bottles of 433 cl) at Autozone for $13.
Engine running with a gas tank almost dryed ( 2 gallons remaining) I poored the 2 bottles in my tank.
After 15 minutes at iddle speed my engine caughed and stoped.
Then I poored 5 gallons of fresh gas and crancked the baby.
After 10 second of caughing black smoke, my V8 sounded like a brand new one.
Some will tell me that there is a risk of destroying my fuel pump with that much SEA FOAM concentrated in only 2 gallons of fuel.
Maybe, but it worked better than anything I did before and cost only 13 bucks without destrying anything.
I dont say it will work for you but it worked for me

I had a similar problem, this might be another solution. But it is an expensive one!
The Problem:
99 Suburban Big Block- purchased with 59,000 miles and driven 500 miles no problems. First towing trip, towing a car trailer, first long steep hill, it feels like the transmission is slipping or someone unplugged 5 spark plug wires. Check engine light is on. After turning off the motor and turning it back on, it seems fine. Next hill same problem. Good news- it is under a used vehicle warranty. Bad news- when I took it to the dealer, since the check engine light was not on, they could not diagnose.
The Solution:
Long story short, I couldn't duplicate it except to hook up the trailer and tow up a long steep hill. I then drove to the dealer, with very little power, and did not turn it off. They connected their scanner- here's where my memory goes fuzzy- and they adjust some things. No change. He disconnects the connector (electrical) on the Mass Airflow Sensor and likes what he sees.
What Filter do you use?
He asks a question that surprised me.. Did I use a K&N Filter? The oil from those causes problems with the MAS. In my case no. They replaced the MAS and I have had no problems since.
Now I know why the original owner traded it in!!
The symptoms I had don't sound exactly like some who have posted here, but they do a little. Sorry I wasn't real detailed on the dealer troubleshooting. That was two years ago.

I have had the same misfiring problems on my 99 Chevy Suburban (143,000 Km). Everytime I went up a hill and the rpm's were sustained between 2500 and 3000, the engine would noticably misfire and the codes consistently reported it was cylinder #6. I have replaced sparkplugs/wires, distributor, #6 fuel injector, flushed the engine to unstick valves, and I still experienced misfiring and codes. Finally, I pulled the pre-cat O2 sensor and took my burb for several test drives up and down some steep hills. No misfiring and no codes! I had a bad cat converter. Temporarily pulling the O2 sensor and taking a test drive might be a useful step in the process of helping to diagnose other engine misfiring problems. I wish I had done this before replacing the injector.

After reading this thread and one titled "99 suburban wont start" I realized that the problems are fairly similar and require aproximatley the same repair. My 99 suburban wouldnt start one day and the next it did. I found this thread very informative and wanted to let anyone with these same issues know that after only one year of just having a complete tune-up I took my suburban (that wouldnt start) to the mechanic only for them to tell me that I needed yet another $424 tune-up! Theres no way! Turnes out the spark plugs did not need to be replaced, nor did the distrubutor cap, nor the wires. BUT the rotor button was a little burnt up. After buying a new one, a Haynes Manual, replacing the cam sensor, and feeding it some sea foam, my suburban starts up without any hesiatation, purrs like a kitten, and seems to have a little more pep. This was my fix....so far, so good.

Well after reading all of the threads I changed the Cam Sensor Position Module this weekend and so far no shake rattle and roll when underload and shifting into overdrive. I will add this good luck lineing up the notch on the dist. plate.I am having similar probs in my 97 suburban 4x4 with 33's and the hyperdyne programmer installed-with k&n filter. All the way up to like a month ago I could pull my 30' boat up ridiculous hills with that little small block and have no probs reving up to 5k etc. Now Im fighting with the dreaded po300 code but only under a load . the truck will kick down at speed to 5k then shift to o/d after that "stumble" and codes if I stay in the throttle. I have to let off for about 15 sec for it to run right again. Replaced plugs, wires, cap, rotor, fuel filter, cam sensor, fuel pres test 55@idle 65@load. Ran injector test thru scan tool . The dealer said I had a clogged cat, but I took it to a exhaust shop and that wasn't the case--pissing me off considering I work on blower motors and outdrives all day but can't fix my little sm. block!

:lol: :rofl: :lol: :rofl: :iceslolan
I have to laugh at this thread...I have posted the fix for this concern many times here in the forum and it appears that no one has bothered doing a search to find the answer.

Misfires on these engines that occur under load (climbing hills or cruising at highway speeds) are usually caused by the valves sticking in the guides (after proper diagnosis of the fuel and ignition system rules out fuel or spark).
The cause is carbon buildup on the valve stems that cause the valves to stick in the guides under higher RPM (acts like valve float)
There are a couple paths to follow here.
You can use a product such as Seafoam in the oil and gas to help with lubercation of the valve stems.
Decarbonizing of your engine with GM Top engine cleaner will help.
Or if one of those 2 don't work the next thing is to remove the heads and have them professionally cleaned and the guides replaced.
My 98 Suburban had all the symptoms, going up the hill miss when pulling a trailer. New original cap, wires, cam sensor & two mechanics both reported # 5 Miss. but no solution. Guess what. The seafoam guy had my answer. One can in the tank, one can in the oil, and one down the brake vacum line (actually did this twice) and up the hill we went with no knock and no miss. Thanks.

i have 97 tahoe 5.7,havin a problem goin up hill, feels like im losin power,petal to the floor ,but still,no power,does anyone know of problem

Hello all I have a 1998 GMC 1500 truck with the 5.7L engine. It has about 85,000 miles on it. I first noticed the misfire problem after we bought a travel trailer and pulled it up a pass to go camping. The light came on and it was a random misfire under load. I started with the tune up, plugs, wires, cap, roter. And I had an injection cleaning and new fuel filter. Well the problem was still there and it started doing it without the trailer at the top of a long hill we live on. Just as I would get to the top and start to let off the gas it would start shaking really bad but it would go away after taping the gas a few times.(not enought to make the light come on) Right before a camping trip last fall it did it again with the trailer on a slight incline on the freeway. We turned around and headed back to the shop. They thought maybe they would try the EGR valve and that did the trick! We pulled over a 5000 foot pass both ways and not a problem. Well a month ago (4 mo. after the EGR) it started missing again!!! So then I tried the seafoam in the pcv line but I'm not sure it got to all the cylinders? The pcv enters on the side of the intake so I'm not sure if its one big cavity under there or not. I also put the seafoam in the gas and I will put some in the oil today. The seafoam in the intake didn't do the trick so I might try the egr again, but I'm not sure why the egr would work for 4 months and then start missing again ? Poor quality EGR ?? I can't afford a new truck but this misfire thing is really pissing me off!! It takes the fun out of camping because I worry the whole time if we'll make it there. I did put a K&N filter in last year and I thought I read someone posted about it could foul up the Mass airflow sensor?? I just don't know what to do.

I have a 99 chevy k1500 truck it was doing the same thing on misfiring going uphill or under a load. We changed everything Injectors, wire, plugs.sensor..etc. over 2000.00 worth of parts STILL NOT FIXED.
I changed the fuel cap with a factory one perfect now!!!! 10.00 FUEL CAP Make sure factory though.
Thought I would share my fix.
Lisa

I'm having similar problems but for some reason the plug in cylinder four keeps fouling out. About every two weeks it will start to idle rough and the check engine light comes on after each cleaning. Any ideas as to why this keeps happening?

Same problem, misfiring, 5.7 vortec, 140k miles. Just installed NEW heads (old ones cracked), polished rough edges in heads, cleaned piston tops like new, NEW fuel injection with electronic injectors, plugs, wires, distributor cap, rotor (all Delco or better, no crap), new thrust bushing in distributor, new crank sensor. no codes, cam retard ok, ran with GM Tech 2 scanner, everything normal. Popping is now WORSE with everything in great shape. disconnected EGR, no help. Can't drive over 60 without serious popping/misfire. Ran on ignition scanner..trace on primary side was perfect. Fuel pressure 55+ lbs. Replaced new injector system with old poppet valve injection unit..no difference. Really frustrated. Cam sensor in distributor just tells brain which cycle crank sensor is reading..at 2500 rpm shouldn't make much difference in injector pattern. TPS? Looked ok on Tech2. O2's were banging away. Short/long term fuel trim were ok. Cleaned Mass airflow sensor, no vacuum leaks, no slop in timing chain, distributor gear like new.

Frustrated as hell. It really feels like ignition problem, but nothing apparent. It's a small block Chebby..ain't new stuff. Someone out there MUST have figgered this out..talk to me!

Just read the fuel cap post again..I have disconnected the evap cannister and run it,,no help. There IS a breather on the top of the cannister..pulled up the plactic cover and cleaned the leaves out. No difference. The evap pipe goes across the motor and is connected to the purge valve on the passenger side of the upper manifold. It then breathes into a pipe that sticks up under the throttle body. BTW, cleaned the throttle body and IAC passage. No help.

I don't have time to read all the replys to see if this was solved. I had same problem with 99 5.7 vortec in Van. After a year of looking for a solution to no avail, I went to dealer and they diagnosed as worn distributor shaft bushing or distributor shaft gear can't remember which. That fixed it. It was not the tranny, fuel injectors, tune up, wires, torque converter,intake manifold gasket etc. that was suspected by other mechanics. Hope this helps someone because it drove me crazy. Dale

Hello all ,I had the same problem you all do , first I replaced the cap ,rotor ,pluges ,platinum # 2 plugs ,coil and ignition module , still no good. Then I ran into a old mechanic and he told me to put Sea Foam in my gas tank and in my oil to. Advance Auto sell's it $6.97 a can . Well I did it and 2 tank fulls later no more miss fires , it;s the motor that is gumed up from the gas . BigNed

I have come to believe that this is a sensor induced problem. It could be ice forming in the intake, seeded by carbon (soot) bits. Spraying or adding cleaner to the fuel/induction system seems to fix it.

I don't think its sticking valves or some mechanical gremlin.

It can be a bad sensor by itself but trying cleaner is cheap and a quick fix if it works.

We too have a '99 suburban with the 5.7. Bought the truck from a used car lot with 50K on the clock, so I don't know what was done before that, but it was pretty much a cream puff. We've had the truck about three years, and noticed the mis-firing problem pulling a camper on a trip out West the first summer we had it. First sign of trouble was a check-engine light that came on during a long easy hill at speed. Next day the light had gone off again, but it did this several more times. Finally on a really strenuous grade on I-70 in the Rockies, I got the blinking check-engine light and the shuddering. Slowing down cured the problem.

After arriving home, we continued to have problems even when not pulling a trailer. Exactly as the OP described, it would start shaking badly when cresting a hill. At first I thought it might have to do with what gear it was in, i.e. an RPM related problem. The local dealer checked the codes and detected a misfire (don't remember what cylinder), and a slipping transmission. They serviced and adjusted the tranny, and recommended I change out the cap, rotor, and plugs for the misfire. I did that, but didn't use GM parts. The problem continued. I had another dealership check it out: misfire diagnosis again. I replaced the cap, rotor, and plugs again using all GM parts this time, to eliminate that as a possibility. Since then we moved, and I haven't had it back to a dealer. The problem seemed to have lessened, but then where we live now doesn't require as much hiway driving. Recently we noticed the misfire problem is still there when nearing the top of a long hill.

Separately, we took the Burb in to an independent mechanic for what I thought was going to be a water pump, but turned out to be the intake manifold gasket. They also diagnosed a misfire, but I didn't ask them to check into it. The misfire is still there after the new gasket, same as it was before, so in this case they don't appear to be related. OTOH, perhaps whoever owned it before us already had the gasket replaced . . .


Update to my own post: The Sub now has 160K on it, and still runs like a dream except for the misfire problem. Amazingly, I still have not tried SeaFoam, though I intend to do so finally this weekend. Since the last post, I replaced the fuel filter, the CMP sensor in the distributor, the fuel pump (it failed), and both cats (one was dead/clogged). 02 sensors tested fine. I also had the transmission flushed and serviced, since it was due, and I wanted to check for any possibly related problems. There were none.

Just returned from a trip through the Ozarks and Appalachians, misfire problem is as bad as ever, and I wasn't towing anything. Seems as long as I keep it in 3rd and under 65mph, the problem goes away. I have high hopes for the SeaFoam, because if that doesn't do the trick the next step is tearing into the intake and heads......

I still want to know why this problem seems to affect the '99s. What changed?

I replaced my Crank sensor, problem solved.

99 suburban here and same problem! 128,000 miles

Started doing this a couple of years ago when pulling the boat but got so it would missfire without pulling the boat but only when going up a hill. Premium fuel helped reduce the problem.

My mechanic cleaned the popet valves last week and my family took it on a road trip this weekend running regular unleaded. No missfires, no shakes but I am getting a knocking (rattling) sound when accelerating. I don't know whats going on but its back to the mechanic. $$$$.$$ :banghead:

Anyone ever come up with a solution to this? My mechanic and I suspect the problem is carbon buildup.

P0305 code on a 99 suburban.

I have the same problem with my Suburban. SVS Engine soon goes on when going on a uphill. Changed all plugs and wires. I was told to change the coil by another mechanic. Can you tll me with your experiences what did you do to fix the problem. Truck is well maintained but running out of ideas. I reside in Escondido. Please email me if you have an idea or remedy to the problem I am having. It would be greatly appreciate it.

Thanks...

Welcome to AF, this also can be caused by carbon deposits on the valve stems causing the valves to hang up in the guides, GM has a TSB on this issue covering your year I believe, the problem is pinpointing this as the issue, a top engine decarbonizing treatment is performed and if the condition clears up this indicates that is the problem, but this is a temp. repair, the heads would have to come off.

I thank you very much for your advise. I try it out.

thanks,

I have a 99 chevy sub with the 5.7 and I had the seal replaced under the intake manifold and now it miss fires when I climb a hill and get to peak it will missfire badly. It will never do it when your climbing the hill or on flat ground but only when the engine has been enganged on the hill and then comes off of a little. It only happens at speeds over 50 MPH. I have replaced the plugs, wires, cap & rotor, and the fuel filter. The dealer wants to replace all these parts again and charge me 500 for it. I need some Ideas. I took it to another mechanic and he sees it misfire with the scope while driving but can't pinpoint it.

Thanks

Tony
I have a 99 chevy had the same problem one of the valves in the transmission was blocked have a transmission anaylisis done

Can someone explain how cam position sensor helps this condition? It's about the only part I haven't changed on my '99 Tahoe 5.7 to try to cure this poblem. I have done plugs, wires, EGR, Dist cap, rotor, Seafoam, cleaned mass airflow, still no cure.:banghead:

guys same problem misfire codes and problem here. Car misfires heavily at idle lots of black smoke poor MPG. We replaced rotor, cap, wires, a lot of plugs, coil, coil module, inatake valve seals, crank position sensor, and still the same problem. Intake gasket has been replaced 5 times. We tried the GM OEM twice one the autozone one the advance auto parts one, even a home made one out of gasket material. The car will run fine upon the change of the intake gasket (it even clears codes automatically) but within 24 to 72 hrs it will end up in the same condition.

guys same problem misfire codes and problem here. Car misfires heavily at idle lots of black smoke poor MPG. We replaced rotor, cap, wires, a lot of plugs, coil, coil module, inatake valve seals, crank position sensor, and still the same problem. Intake gasket has been replaced 5 times. We tried the GM OEM twice one the autozone one the advance auto parts one, even a home made one out of gasket material. The car will run fine upon the change of the intake gasket (it even clears codes automatically) but within 24 to 72 hrs it will end up in the same condition.

Dead thread. But, have you checked the O2 sensor(s) and the fuel pressure regulator?

Yes fuel pressure is good and 02 are good too. The problem obviously is the intake gasket when we replace it the problems goes away then it returns as the days go past. Why this happens beats me. Why do you say the thread is dead?



Dead thread. But, have you checked the O2 sensor(s) and the fuel pressure regulator?

I have fixed the problem. Thanks for the SeaFoam advise. I have poured Seafoam in my crankcase and fuel tank and fixed the problem overnight. I even passed my CA smog with flying colors. I highly recommend Seafoam to everyone who has been having the same problems with their Suburban.

Thanks again.

Yes fuel pressure is good and 02 are good too. The problem obviously is the intake gasket when we replace it the problems goes away then it returns as the days go past. Why this happens beats me. Why do you say the thread is dead?

The thread was dead because it was over a month old. When a thread is dead, start a new one.

The problem will come back. Sticking valve guides in the heads is most likely your problem. Seafoam will free it up for awhile but be ready for the return of the miss or as I call it the 1500-2000 RPM Shake:grinyes:

We discourage continuing a thread for more than 3 months, all the topics/possibilities have been thrashed over sufficiently at this point.