Intermittent miss at light load.

My Blazer 95 W code has had an intermittent mis-fire only at part throttle for a long time. It is getting worse now, any ideas on how to check it out?

I would suggest checking your plugs and wires, the 95s wires run really close to the exhaust manifolds and they tend to dry up and crack easily. If they have never been replaced then they will probably come out in pieces...

Plugs, wires, cap and rotor are all new about a few months ago. The problem is still there though. It is so intermittent it is hard to make it act up when I want it to.

All I have observed indicates secondary ignition components. I am pretty sure that it is worse when it is raining with the lights on.

Some more info.

It is not consistant, usually it runs great.
It has been there about 2 years I estimate.
No codes , no light
Never happens at WOT
Never happens at 1/2 throttle or more
Never happens above 80 mph
Never happens at idle

CPI injector and nutkit are probably leaking.

Hmm, I was thinking spark, never thought of fuel problem, I did raplace a small plastic line under the intake a few years ago, it was leaking gas into the intake. I think I know how to check it.

Pressure up fuel system with key on and then turn key off and wait for pressure to drop or hold? Maybe could be a leak AFTER this pressure test, but why would a leak or rich mix on a cyclinder or two cause an intermittent misfire?

How else can I verify this without a lot of disassembly?

Remove the top vortec cover and then remove the round IMTV valve and peer down into the plenum and looked for washed spots.

Also smell for gas and look for pools of gas in the plenum.

I think it is not fuel problem, it is a misfire. I had it in my head that it was worse when wet and/or night out with the lights all on.

Then I thiought it was worse when the engine was cold. Today It was rainy, very humid and so I started it up, turned on the lights and ac fans and took off imediately, It didn't act up until about 10 minutes into the drive.

So much for bith of those theories, lol. bummer very inconsistant problem.

Worst case scenario is at constant to very slight acceleration between 1000-3000 rpm and between 30-55 mph. It must be used to those parameters, that's what I usually run .

I hope it dies soon so I can figure it out.

I think it is not fuel problem, it is a misfire. I had it in my head that it was worse when wet and/or night out with the lights all on.

Then I thiought it was worse when the engine was cold. Today It was rainy, very humid and so I started it up, turned on the lights and ac fans and took off imediately, It didn't act up until about 10 minutes into the drive.

So much for bith of those theories, lol. bummer very inconsistant problem.

Don't ever assume it is not one thing over the other.

The CPI injector leaking can and will cause a misfire seeing it cannot hold the fuel pressure needed to open the poppets and limit the fuel going into the cylinder.

It could also be a coil that is gone.

Yes, thanks. The coil is a real good bet, I may become a "parts changer" and replace it if it isn't too expensive, I do not know how to check it other than to inspect it very well or replace. I can't dump a lot of money into this right now.

It finally got so bad and after watching it in the dark I decided to buy a coil and put it on. I seen a nice little light show on both edges of coil by the metal bracket so before I got a new one I insulated the coil from any metal mountings and cleaned the sides up real good.

A few days later I concluded it was "fixed" almost so I got a coil and replaced it. It was "fixed" for a few days (with 2 different drivers that are aware of problem)

A few days later I conclude that it is 50-90% better, but is not fixed at all. Ran new coil enough to conclude that it is still misfiring very intermittently, but not as bad.

A few days later I put old coil back on and ran for a week or so concluding it is worse than the new one. I also have drowned it in enough water to make an off-roader jealous and when it dries off it runs exactly the same.

After putting old coil back on it runs bad when warming up, but when warm seems to run ok and not miss. The other nite I saw another little light show at the sides of the coil and also found some intermittnt sparks jumping to my dipstick tube and also the coil wire was jumping to the metal braket.

Ho, humm, insulated the tube and some wires that were close to metal and moved coil wire away from metal braket, time to test drive.

Get the new coil on there, the old one is pretty bad if it is arching like that.

I had replaced my cap/rotor with Wells and in 1 year it was arcing inside the palstic casting between wires (not visible even looking at the inside) so just a point to consider - many aftermarket parts are not equal as I chased and traced this for a few days - a simple cap/rotor change cured and only way to tell for sure. If all is OK you replaced already then look at the crank sensor ($60) and ign module (in dist on yours $75).

If you have arching, you have to get that fixed first. Insulating it from other things is not fixing it, it should have no arching.

Ya, my new coil is on the way. Insulating it was merely an attempt to see if the arching was the intermittent miss, it was not.

I had replaced my cap/rotor with Wells and in 1 year it was arcing inside the palstic casting between wires (not visible even looking at the inside) so just a point to consider - many aftermarket parts are not equal as I chased and traced this for a few days - a simple cap/rotor change cured and only way to tell for sure. If all is OK you replaced already then look at the crank sensor ($60) and ign module (in dist on yours $75).

Thanks man, I did price a Wells coil. It seemed to be a cheap one. I was advised against the "cheap" coils. Lots of aftermarket stuff in junko compared to OEM.

As I'm learning about scanners and these computer stuffs I think I may have narrowed it down a bit more. Someone chech me out and see if this makes sense.

As I take off with a cold engine it runs without missing a beat, I hold a constant speed (42-65 mph) and the very second it goes to "closed loop" is when it falls apart and starts mis-firing. I notice it is exactly as the converter locks up.

How does one tell when it is in closed loop without a scanner? This is new term for me. Can I force it to stay in open loop so I can drive it and verify that it is not missing in open loop?

The check engine light must be one when it does this.

I would still scan for codes, not all of them trip the light to come on.

No, the MIL is never on. Hasn't been on for a year or so. I may get it scanned for free or gwet the scanner myself. I am finding out it can have codes or pending codes with no light.

One shop wants it with the old coil on it, As son as my coil gets here I will install it and drive crap out of it, lol.

I kick my own butt, I put the new coil on and no difference, same missing, same lightshow around the coil. Most of my stray sparks are around the coil wire itself so I made another new one and will try it out.

go to somewhere that sells standard ignition they give a lifetime warranty with everything caps rotrs wires coils everything even o2 sensors...

I did have lifetime belden wires on there, but changed them all with the cap and rotor and plugs cuz it all had a lot of miles. The wires were free, but now that I think back the new ones looked kinda old, like they had been seting around a lot. Maybe they were very old stock.

I have scanner and some software on the way to help me, I will also check under dist cap to see spark relationship to post.

Coil wire , no different. No trouble codes present according to AZ, but I finally figured out something.

It only misses (at light throttle) between 5 minutes and 20 minutes of running. I think this is why I had a hard time making it act up and thought that a new coil "fixed" it or whatever I seemed to try magically fixed the problem. 95% of my drives are exactly 15-20 minutes long.

I assume that it is going to "closed loop" at around 5 minutes, but what happens at around 15-25 minutes?

I'm going to time it to see if it will repeat. Once started for the day, it will run perfect all day long as long as it is not allowed to cool off all the way. It seems to be time related (or temp) can't think odf anythinjg else can anyone else think of what happens at 20 minutes or so?

Cpi Injector Leaking!!!

I'll be checking into that soon. A lean cylinder will cause a spark to "not jump the gap" or to not fire at all. One thing is for sure , it is getting worse and I am geting real good at making it act up (not like before)

One test I did do pointed to the back 3 cylinders and after seeing some nice pics here on the forum, they are all a common plenum, hmm.

The problem reminds me of increased timing advance pulling the spark away from the correct distributor post. I have my old cap and may make a "cut-away" cap to check it and see. It has also been suggested that the shaft/gears can be worn causing the rotor to pull away from the correct post (like maybe wobbly or run-out, this a new one to me, but was a possibility from a good shop)

had the same problem on my 97 LS 4.3L 4WD. new plugs and wires didn't do the trick so i cleaned off the contacts in the cap/rotor and added dielectric grease to them. also cleaned out the EGR. problem solved.

I'll be checking into that soon. A lean cylinder will cause a spark to "not jump the gap" or to not fire at all. One thing is for sure , it is getting worse and I am geting real good at making it act up (not like before)

No, you are wrong, fuel mixtures don't control any part of how the spark jumps across the electrodes.

The problem reminds me of increased timing advance pulling the spark away from the correct distributor post. I have my old cap and may make a "cut-away" cap to check it and see. It has also been suggested that the shaft/gears can be worn causing the rotor to pull away from the correct post (like maybe wobbly or run-out, this a new one to me, but was a possibility from a good shop)

What, advanced timing do what?! Seriously, where are you getting these ideas from?

You have a fuel problem, fix it and stop focusing so much on the cap and rotor and coil if they were replaced.

A lean mix, advanced timing, higher cylinder pressure can all cause a misfire. A rich mix is easy to fire, a lean mix is much harder to fire.

Setting the cap/rotor relationship at a specific advance point and then advancing or retarding spark starts to pull the spark away from the particular post. Advancing it will cause the spark to lead the post, retarding it will cause it lag behind and jump backwards towards the post.

In extreme cases such as low compression blower apps where the range of advance is very great from low vac high rpm and a lot of advance to high boost and low rpm and not much advance causes one to set the cap/rotor relationship to the hardest to fire spot (high cylinder pressure)

The problem shows up mostly on adjacent cylinders causing a cross-fire and back fire with small diameter distibutor caps. My problem never backfires, and I assume it is because there is never an adjacent cyclinder with mixture to fire, it just jumps out to a ground somewhere on the engine, another wire, or within the cap itself and simply shows up as one missed spark.

Cpi Injector Leaking!!!

Can you explain how a fuel leak will cause a misfire at very light throttle and from 5-30 minutes of driving, yet be ok at any other throttle position and time?

Also I would like to know what arrangment the injectors are in and how they deliver fuel.

like is there 6 of them and are they at each intake port? Do they continuosly spray fuel? just varyingthe amount? or do they turn on and off and if so are they timed? If so when do they spray, and for how long?

i also had a 95 blazer that did the same thing it seems like a lot of them get a slight misfire i had to change the whole fuel injection assembly it was like 3oo bucks

I repaired a leak under plenum a number of years ago on mine, as I recollect it had a plastic line with a tiny pin hole in it and would only show up if it sat for 30 minutes-3 hrs.

My symptom is misfires between 5 - 30 minutes at very light load. When it is acting up you can see spark jumping on many of the wires. I'm tempted to put on some better insulation wires, but I'm convinced it is only a symptom of my real problem.

A lean mix, advanced timing, higher cylinder pressure can all cause a misfire. A rich mix is easy to fire, a lean mix is much harder to fire.

Wrong, a rich mixture is harder to fire seeing there isn't enough oxygen in the intake charge to fully combust the large amount of fuel. When an engine is running lean at idle you won't really see a whole lot of a difference but when it is rich it will be sitting there puking out black smoke trying to stay alive while running very poorly.

Running lean makes the mixture OVERLY combustable seeing it will cause PREIGNITION, pinging, and detonation seeing it will ignite too early because of the increased combustion temperatures.

Setting the cap/rotor relationship at a specific advance point and then advancing or retarding spark starts to pull the spark away from the particular post. Advancing it will cause the spark to lead the post, retarding it will cause it lag behind and jump backwards towards the post.

You REALLY need to go back an re-read what you thought you have read about the ignition system. No one advances the distributor cap and then advances the distributor so who ever told you that or taught you that is full of it.

You advance the distributor ONLY, not the cap and the distributor separately.

The jumped to and from the cap posts is a little ridiculous bud, and you really need to rethink and relearn what you know about the internal combustion engine.

In extreme cases such as low compression blower apps where the range of advance is very great from low vac high rpm and a lot of advance to high boost and low rpm and not much advance causes one to set the cap/rotor relationship to the hardest to fire spot (high cylinder pressure)

Again, completely wrong. The timing is retarded under blower conditions, not the cap and rotor relationship. In a computer controlled application, the timing is retarded on the fly by an external control where as if you have a carberated system, the timing is retarded to lower combustion temperatures, richen up the combustable mixture and lessen and preignition.

Again, you need to relearn about timing and what it means.

The problem shows up mostly on adjacent cylinders causing a cross-fire and back fire with small diameter distibutor caps. My problem never backfires, and I assume it is because there is never an adjacent cyclinder with mixture to fire, it just jumps out to a ground somewhere on the engine, another wire, or within the cap itself and simply shows up as one missed spark.

NO!!!!!! Get off your ass and get the upper plenum cover off and you will notice that the poppets fro the CPI injector are plugged or the fuel pressure regulator is leaking causing a lack of fuel to go to some cylinders.

If you think it is the cap, then replace the cap and rotor, but other than that on a 1995, you can't touch the timing so don't even start to mess with it.

I own this year and engine and I know it from stem to stern and the problem is always REALLY simple.

You need to realise that you are overcomplicating this situation and ignoring the obvious and for what reason I have no clue.

If the ignition system is new then MOVE ON TO SOMETHING ELSE.

It isn't some ridiculous hypothesis about spark jumping in and out of posts, it is about one of three things.

Air, fuel, or fire.

Now that you have made sure the fire is fine, MOVE ON TO FUEL! and stop sitting there babbling on about the ignition system.

after fixing the CPI i was still having a missfire on my 92 4.3. turned out to be a bad distributor bearing. i suggest tearing down the CPI and inspecting the spider assm. and checking the distributor at the same time since it will be easier to get to with the vortec plenum removed. here's a good post with lots of pics: http://www.automotiveforums.com/vbulletin/showthread.php?t=518850&highlight=spider

Thanks, I may just do that. Was it perhaps the top bushing/bearing in the distributor housing? I broke my housing the last time from overtightening the screws and may even replace it. That is also probably what the shop was referring to when he said "worn out dist gear"

Thanks, I may just do that. Was it perhaps the top bushing/bearing in the distributor housing? I broke my housing the last time from overtightening the screws and may even replace it. That is also probably what the shop was referring to when he said "worn out dist gear"

No, the gear is on the bottom.

You don't have aproblem with your distributor, you have a problem with either the cap not being installed properly or the fuel system.

A worn gear can cause end thrust in the shaft and some backlash throwing that rotor/post relationship off. I don't think the cap can be on wrong (other than me overtightening screws)

And what if you breaking the screw points is not allowing the cap to seat properly and therefore is allowing moisture into the cap and causing the misfiring.

This is why after 5-10 minutes it stops as the heat from the engine drives off the excess moisture.

A worn gear would cause a misfire all the time. Not just 5-20 minutes after starting the truck.

My cap WAS bolted down as good as a new, but I will check it to make sure is still tight. A good idea about cap and moisture, that is helpful. I've ran enough water over all secondary ignition components to make an off-roader jealous and just a short drive clears up any moisture missing. I attempted to duplicate problem like that, but could not.

My problem is completely undetectable until the tork convertor locks up. That is how I assume it is going to closed loop and the very second it does is when it starts missing (as long as the vac is between maybe 10-20")

It gradually gets better until about 30 minute or so and does not nessecarily have to be driven the whole time, but after the time expires the problem cannot be detected.

I do have a theory about the leak in the intake, but will get set-up with a pc to verify all values to be sure. I kinda want to watch stuff oncreen while it is acting up.

My cap WAS bolted down as good as a new, but I will check it to make sure is still tight. A good idea about cap and moisture, that is helpful. I've ran enough water over all secondary ignition components to make an off-roader jealous and just a short drive clears up any moisture missing. I attempted to duplicate problem like that, but could not.

My problem is completely undetectable until the tork convertor locks up. That is how I assume it is going to closed loop and the very second it does is when it starts missing (as long as the vac is between maybe 10-20")

It gradually gets better until about 30 minute or so and does not nessecarily have to be driven the whole time, but after the time expires the problem cannot be detected.

I do have a theory about the leak in the intake, but will get set-up with a pc to verify all values to be sure. I kinda want to watch stuff oncreen while it is acting up.

I would take this to the dealer and get them to attach a Tech 2 to it and take it for a drive, they will be able to pin-point the exact problem.

Don't just throw random parts and hypothesis at it, get some actual testing done.

Still, have you chanecked your CPI injector yet?

My cap WAS bolted down as good as a new, but I will check it to make sure is still tight. A good idea about cap and moisture, that is helpful. I've ran enough water over all secondary ignition components to make an off-roader jealous and just a short drive clears up any moisture missing. I attempted to duplicate problem like that, but could not.

My problem is completely undetectable until the tork convertor locks up. That is how I assume it is going to closed loop and the very second it does is when it starts missing (as long as the vac is between maybe 10-20")

It gradually gets better until about 30 minute or so and does not nessecarily have to be driven the whole time, but after the time expires the problem cannot be detected.

I do have a theory about the leak in the intake, but will get set-up with a pc to verify all values to be sure. I kinda want to watch stuff oncreen while it is acting up.

I would take this to the dealer and get them to attach a Tech 2 to it and take it for a drive, they will be able to pin-point the exact problem.

Don't just throw random parts and hypothesis at it, get some actual testing done.

Still, have you checked your CPI injector yet?

A worn gear would cause a misfire all the time. Not just 5-20 minutes after starting the truck.

Actualy my distributor had a bad upper shaft bearing that caused a cold start misfire that would go away after a few mins until it got worse. The engine was warm and the mechanic miss diagnosid and replaced the CPI spider and fuel lines and test drove it. Almost $900 and it did the exact same thing the next day when it got cold again.

Hope you didn't pay for it.

You don't have to pay for them doing a trial and error shot on your truck.

If you paid, you got screwed.

it needed to be done anyways. they tried to tell me they couldn't put the old parts back but they came way down when i said pull out all your parts and tow it to my house and i'd do it myself. they knocked off all labor and marked the parts down to cost to get my screaming ass out of the waiting room.

My 2 large local dealers will r&r parts until customer runs out of money or they happen to hit it by guessing. They seem to be parts changers and while they talk pretty impressively thier understanding of thier own systems seems quite limited to what a book, flowchart,computer,scanner tells them.

It is quite easy to pin them down with some questions about a refund for a misdiagnosis and an unnessecary r&r regardless of the cost. Here are some typical responses I get.

We can tell you anything about your car, when you floored it, how warm it ran how fast you drove, when you drove, ect.

Our mechanics do not make mistake when diagnosing, they are good.

If it doesn't fix it , we will keep trying until we get it right.

We will warrant the unneeded repair for 90 days.

I would take this to the dealer and get them to attach a Tech 2 to it and take it for a drive, they will be able to pin-point the exact problem.

Don't just throw random parts and hypothesis at it, get some actual testing done.

Still, have you chanecked your CPI injector yet?

I'd like to see a tech 2. Whatever that is.and couldn't agree more about throwing random parts on it. That drives me nuts to replace something that is not faulty (like my coil) :screwy: By the same token I will not waste time doing random labor without testing and being pretty sure I am working towards the correct direction. I'm not yet convinced I have a fuel problem under plenum and it would drive me nuts also to take it apart just to verify visually that all looks well.

Proccess of elimination is very powerful, but can be very time consuming and extremely costly (like at the dealers above) I'd rather pay someone hrs of diagnostics to figure it out than to pay someone to use proccess of elimination to fix it. If one would install parts "just to see if it fixes it" and not charge for all the mistakes I wouldn't much care.

Well , I got my scanner and software and have prety much confirmed what I thought was going on.

When first starting and in open loop, it runs perfect.
The very second it goes to closed loop AND the timing shows (-35deg)
It misfires real bad, anything below 30 degrees and it runs perfect again.


Not real sure why it shows negative and I don't even know if the value is accurate, but it really irrelevent as it warms up the magic number is about 32 degrees. Any more than that makes it miss, any less and it runs perfect.

The timing map for this vehicle is very strange besides. Is the actual reading I'm geting the actual timing and why is it negative?

I've been reading this thread with great interest. My 96 Blazer has been doing the same thing for some time now. It's had a cap and rotor, two sets of plugs and wires, new O2 sensors, new coil and MAP sensor. All to no avail. My brother is a twenty+ year mechanic and says it is probably the spider injector assembly. I replaced the pressure regulator about eighteen months ago for hard starting but felt the rest of the assembly looked okay. Like you, I'm in no hurry to spend the $425.00 (priced yesterday) to buy a new one. I think I'm down to having to do it, though. I had to replace the catalytic converter recently and I don't want to have to do it again due to the fuel mixture being screwed up. I have 187,000 miles on my truck and this is it's only issue.

Looking forward to your future posts.

Hi, thanks for posting. That a real possibility, When I get some time my next step is to get a distibutor body and check the rotor/post relationship. I may pull off the plenum to do it too as I have to replace my distributor body anyways.

Looking forward to you doing it first, lol, and let us know.

Anyways does yours only do it between maybe 5 - 20 % throttle and between 5 - 25 minnutes of running?

Light throttle and early into my trip yes. After everything else, the exhaust, MAP sensor and correct thermostat (it had a 165 Deg for some reason so I put in the 195), I was getting a Bank 1 sensor 3 O2 SES light so I replaced it also. It seemed to run great today. But by reading your posts, you know how that goes. Guess we'll see.

It got cold here, mine ran perfect for 2 days. Thanks.

i have a 92 jimmy 4.3l vin w - and i was wondering if you had solved the problem yet - i am getting the same thing as you - tried 4 different brands of premium wires and they all leaked or showed glow spots at night - the coil arcs to its metal bracket - it has been a frustrating problem for me too - i had the same thing happen on an f150 but ford sucks anyway and i turffed it to the junkyard - please post back and let us know what you find.

i have a 92 jimmy 4.3l vin w - and i was wondering if you had solved the problem yet - i am getting the same thing as you - tried 4 different brands of premium wires and they all leaked or showed glow spots at night - the coil arcs to its metal bracket - it has been a frustrating problem for me too - i had the same thing happen on an f150 but ford sucks anyway and i turffed it to the junkyard - please post back and let us know what you find.

Why are you replacing the wires when the coil is arching?

Replacing the coil would be the smrt thing to do when the coil is failing right?

Here's where I am at.

I have ran it enough now with the lap-top and it is getting bad enough that I can very easily duplicate the problem and make it act up.

I'm confidant to say that in order to act up:

The system must be in closed loop, the timing must be over 32 deg, it must not have been running for over 30 minutes.

I allowed 1 more good mechanic to listen to it who is quite familiar with rotor/post issues and misfires and backfires and cross-fires. He then consulted another and they both recommended (after hearing I used champion plugs) to use the factory plugs.

My question was why do the plugs work perfect until closed loop?
Answer was exactly as I stated in post 30 . " a lean mix with advanced timing is harder to fire"

My next question is why after 30-40 minutes of elapsed time does it again run perfect?

Answer "No idea" So anyways , going against my own judgement I am deferring to the experts and have purchased new delco plugs and will install them today and be happy and confused if it's fixed, and if it isn't just kick myself another time for wasting more money and labor.

i have a 92 jimmy 4.3l vin w - and i was wondering if you had solved the problem yet - i am getting the same thing as you - tried 4 different brands of premium wires and they all leaked or showed glow spots at night - the coil arcs to its metal bracket - it has been a frustrating problem for me too - i had the same thing happen on an f150 but ford sucks anyway and i turffed it to the junkyard - please post back and let us know what you find.

I've had 3 diffferent "good" coils on this and they all have that light show going on by the bracket and when misfiring they all jump to the bracket.

I've seen many cars at night appear as though it had bad wires, usually you can see light right thru the wires if it's dark enough and the wires are stock.
I'm going to say that both things are normal , and when sparks actually jump out of the components and snap to a metal peice then that's a problem or a symptom of another problem downstream like plugs or my problem (whatever it is)

If you have a 1995, use Standard AC Delco plugs gapped at I believe 0.045.

What is your gap?

thanks for the responce 543bc... i am still wondering if aftermarket stuff is just borderline acceptable for newer vehicles - even the msd wires would show glo spots - maybe its because this only my second efi vehicle ,but on anything older ive owned like a 79 olds when the wires were glowing they got replaced and problem solved - on the other hand the hei coil was hidden under the cap so you wouldnt see any arcs if they were occouring - maybe it was ? on the other hand the old in cap coils were protected from the elements . as far as plug gaps go - standard ac delco - .035 rapid fire - .045 ( theory is tha rapid fires are capable of carrying the voltage across a wider gap ) ive tried thyem and they do improve acceleration but not with the current problem we are having .

so ,,, 534bc - did you kick yourself yet ...lol i would be really surprised to if the plugs solved the problem `- i feel confident in guessing that they did not help .. :banghead: