What are you guys getting for gas mileage?

98 Lumina 3800, now when it is 10 degrees 18-19 short trip. 25 long trip, good weather 29 hway. at 135k now.

my 99 pontiac GP 3800 only can get 23.2 mpg should get better than that I would think kinda got me stumped. whats your Lumina got for gearing? my GP has the 3.29 regals have the 3.05 I think

'99 Regal LS (112K miles). Gets 22 - 23 in city. 28 highway. Over 450 miles on 17 gallons on a long distance trip.

The Regal LS has RPO option F83 which is 3.05:1 gearing.

Lumina LTZ has 3.29's, mileage difference with these diff ratios would be negligible, I also have aggressive sno's on in the winter.

Heck its 35*F here the only way I could get above 24 is if a strong wind is pushing me along. here is my post on the pontiac forum:

Mileage issue when I fist got to car was good been declining lately Its got a new Air filter plugs wires coil packs (terminals rusted) fuel filter new tires as of yesterday (plus one rim) new upstream Borg O2 sensor. PCM update. The only thing this car does on take off when say about 170/180*F you can feel the engine hold back slightly barely notice jerky acceleration. TCC locks up good and so on. Dang thing just should get better gas mileage than what it does. Even going 65 I can't hit 28MPG when I first got the car it was 30mpg. The only thing I can think of is maybe the cat is getting plugged. Gas I get it from a couple different stations so that shouldnt be an issue.

'01 Regal LS, 29.8 highway, 24.2 city.

Heck its 35*F here the only way I could get above 24 is if a strong wind is pushing me along. here is my post on the pontiac forum:

Mileage issue when I fist got to car was good been declining lately Its got a new Air filter plugs wires coil packs (terminals rusted) fuel filter new tires as of yesterday (plus one rim) new upstream Borg O2 sensor. PCM update. The only thing this car does on take off when say about 170/180*F you can feel the engine hold back slightly barely notice jerky acceleration. TCC locks up good and so on. Dang thing just should get better gas mileage than what it does. Even going 65 I can't hit 28MPG when I first got the car it was 30mpg. The only thing I can think of is maybe the cat is getting plugged. Gas I get it from a couple different stations so that shouldnt be an issue.

How many miles are on your car? Is the CAT convertor the original? At approx. 82K miles the gas mileage on our Regal LS got as low as 18 mpg city. The acceleration would surge (up and down) and feel jerky. GM's CAT warranty had expired. I replaced the CAT with a Magnaflow Hi-Flow CAT off of EBay. Cost was around $60.00 with shipping. Had a mom & pop muffler shop install it for $40.00. The gas mileage went back to what we were used to. Also, if you still have the u-bend get it deleted at the same time.

If you have access to a vacuum gauge (0 - 30 hg), run a vacuum test to see if you have excessive exhaust backpressure. You can tap into or tee in the lines off the throttle body feeding the purge solenoid. Run the rpms up to 1000 - 1500 and hold steady. If backpressure is abnormal the reading will gradually drop towards zero instead of stabilizing or reading steady. Another method is a low pressure test at the forward 02 sensor where a 0 - 5 psi gauge is connected. Backpressure reading with this method should not exceed 1.25 psi.

yes its the orginal cat 81K on the car its self. so how many inches of vac are we talking here? 60 bucks not bad for a cat. So what are you getting for gas mileage right now with your buick

also with a sanner what should your map reading be if you would bring the engine up to 1000-1500 RPM.

This what you got?
http://cgi.ebay.com/ebaymotors/GM-Magnaflow-Cat-Catalytic-Converter_W0QQcmdZViewItemQQcategoryZ33629QQitemZ7 987682574QQrdZ1QQsspagenameZWD1V

yes its the orginal cat 81K on the car its self. so how many inches of vac are we talking here? 60 bucks not bad for a cat. So what are you getting for gas mileage right now with your buick

also with a sanner what should your map reading be if you would bring the engine up to 1000-1500 RPM.

This what you got?
http://cgi.ebay.com/ebaymotors/GM-Magnaflow-Cat-Catalytic-Converter_W0QQcmdZViewItemQQcategoryZ33629QQitemZ7 987682574QQrdZ1QQsspagenameZWD1V

Vacuum should be nothing less than 15 in.

I have two Regals, a '99 LS and '01 GS. LS gets 22-23 in city and 28 highway. The GS gets 20.5 in city and 29 highway.

On MAP I'd have to check the service manual specs.

On EBay link that is the Magnaflow Hi-Flow CAT but I don't recall if it was the same company but the price is reasonable. Hopefully you get the matter resolved.

Well the only information I can get you, is that my Regal LS with 3800 cc 2002 does in city 20 mpg hot, normal or on cold weather. On highway, on summertime its about 33 mpg, during winter 27-28 mpg. the mix of both its about 25-26 mpg!

I'll test the Vac today hopefully, How fast has everyone been driving that has posted?

I'll test the Vac today hopefully, How fast has everyone been driving that has posted?

On the highway 75 mph taching at 2100 rpms.

well no drop in vac runs 20 in hg at 1000-1500 had to unplug the purge vavle to steady out the gauge. any other ideas? I think at 75 I'm like 2300

well no drop in vac runs 20 in hg at 1000-1500 had to unplug the purge vavle to steady out the gauge. any other ideas? I think at 75 I'm like 2300

Sounds good to me. I wouldn't rule out the CAT out but I'd defintely hold off on it based on the results.

What about 02 sensor? What brand are you using? If you can get the highway mileage figures up that would be better than nothing. The 02 sensors are critical in PCM closed loop mode which is cruising speeds and idling after proper engine warmup. Keeping the air/fuel mixture ratio at the ideal 14.7:1. For open loop mode the 02 sensors are basically out of the picture and the MAF sensor plays the key role in fuel economy. A MAF sensor that is out of calibration could cause poor mileage but I would say mainly the city mileage. Other than that I have no other ideas. :dunno:

I know it must be frustrating but it sounds like you are taking the right approach and not giving up. :bigthumb:

new upstream borg O2 and the same old stock after cat O2. The surge when the car is warming up on take off around 2500 RPM is kinda strange. In town mileage of 18-20 I think is respectalbe but the hiway 23 MPG is BS :banghead: Might take it to a engine guy I know and let him have it as us tranny guys are dumb :lol:

new upstream borg O2 and the same old stock after cat O2. The surge when the car is warming up on take off around 2500 RPM is kinda strange. In town mileage of 18-20 I think is respectalbe but the hiway 23 MPG is BS :banghead: Might take it to a engine guy I know and let him have it as us tranny guys are dumb :lol:

Is the Borg Warner 02 sensor OEM? Hey, it is good to have knowledgeable tranny guys especially with the history of the 4T65E. Just a suggestion but send GMMerlin (Kevin) a PM. Maybe he can help you with his bag of tricks. Just tell him Bob referred you. He is a good man and should help once he gets to your email. Good luck!

Threads have been merged due to an accidental deletion by troy1. All is now well. Procedd to enjoying the thread :)

Thanks tman I KNOW I BROKE IT :chair: How about deleting some of the dulicates up at the top of Pg 2! #16 & #17

Pulled the Vac line off the fuel pressure regulator to see if fuel was being sucked in thanks to a ruptured FPR but that dont seem to be the answer either. I am serious thinking about just buying a CAT.

troy, I deleted the second one, but you can do that yourself on your own posts. hit the edit button on the bottom right of the post and hit delete in the minisection there, and then click the new delete that popped up.

Proceed to the next post, nothing to see here, move along.

Thats what I thought I was doing before IC my mistake now. :ylsuper:

93 Regal. I'm getting about 22-24 in city and 28-30 Hwy. That is average between summer and winter. At 120 km per hour my rpm is 2100-2200. It also seems the faster I go the better milage I get.

Car went to the shop tonight SO hopefully they find something I'll keep you guys posted.


How fast is 120km >> 80mph?

93 Regal. I'm getting about 22-24 in city and 28-30 Hwy. That is average between summer and winter. At 120 km per hour my rpm is 2100-2200. It also seems the faster I go the better milage I get.

yeah i wasn't sure if I was crazy of not but at 115-120 km/h, its where I get the best gas mileage.. is that normal?!?!

Driving at 100-105 km/h, my rpm is about 1600-1700 but I get 26 mpg;
Driving at 115-120 km/h, my rmp is about 2000-2100 but I get 32 mpg.

Is it explicable?

Well got the car back they found no problems they checked 19 things other than the sensor data. So buy a CAT take a chance or maybe a rear O2 sensor which is in spec. It would be interesting to see if the vac gauge drops out on the road at 80. If the CAT was just starting to go bad would it show up under a load before sitting in the parking lot looking at the vac gauge?

Well got the car back they found no problems they checked 19 things other than the sensor data. So buy a CAT take a chance or maybe a rear O2 sensor which is in spec. It would be interesting to see if the vac gauge drops out on the road at 80. If the CAT was just starting to go bad would it show up under a load before sitting in the parking lot looking at the vac gauge?

You forgot the banghead smilie..lol. Seriously, if I were in your position I'd probably run that vaccum test while on the road. I did a similar test with a fuel pressure gauge and taped it to the driver's side wiper.

There is one other test that can be run on the exhaust system to test for backpressure or CAT acting up but it is a real hassle. The dealers normally have the test fixture or check with any mechanic friends. One can be fabricated if you are willing to go through the trouble. The test is run with a 0 - 5 psi low pressure gauge. A connector (tee) is needed where bottom is male thread 18mm X 1.5mm pitch and the top is female 18mm X 1.5mm. It has to have a fitting to connect the gauge. The fixture is threaded into the front 02 sensor bung and the 02 sensor is threaded in at the top. Engine is warmed up and brought up to operating temperature. Then rpms are taken up to 2000-2500. The gauge should not read greater than 1.25 psi.

The CAT is still a possibility and under "poor fuel economy" it is near the top of the list in the GM service manual, especially with all the other tests passing. :dunno:

:banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :lol: Its all I do all day long where can you find the parts to fab up on of those testers? Do you think the intown mileage would stay up 18/19 and the hiway would fall if that CAT was bad? Its in the 30's around here but I dont think mileage should drop that much If I could get what you get 28 that 4.5mpg better I suppose I could nurse it around untill spring and see if comes up or just spend the 50 and replace the cat :dunno: The thing that gets me is I really dont seem to have a loss of power just mileage if the CAT was bad you would think my performace would suffer also

:banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :banghead: :lol: Its all I do all day long where can you find the parts to fab up on of those testers? Do you think the intown mileage would stay up 18/19 and the hiway would fall if that CAT was bad? Its in the 30's around here but I dont think mileage should drop that much If I could get what you get 28 that 4.5mpg better I suppose I could nurse it around untill spring and see if comes up or just spend the 50 and replace the cat :dunno: The thing that gets me is I really dont seem to have a loss of power just mileage if the CAT was bad you would think my performace would suffer also

Check these out on Ebay to give you an idea. Little different. GM recommends a 0 - 5 psi gauge whereby these are 0 - 15 psi. Also, based on what they claim you can plug into the 02 sensor bung without having the 02 sensor physically installed. Testing methodology is still the same.

http://motors.search.ebay.com/back-pressure-tester_Automotive-Tools_W0QQbsZSearchQQcatrefZC6QQcoactionZcompareQQ coentrypageZsearchQQcopagenumZ1QQfcclZ1QQfclZ3QQfg tpZQQfposZ79935QQfromZR2QQfrppZ50QQfsooZ2QQfsopZ2Q QftrtZ1QQftrvZ1QQlopgZQQsacatZ34998QQsadisZ200QQsa prchiZQQsaprcloZQQsargnZQ2d1QQsaslcZ2QQsbrftogZ1QQ sofocusZbs

Normally bad CATs will degrade performance but I guess it depends how far gone it is. On our '99 Regal LS the first indication was gas mileage. The performance appeared to be good but after the CAT was replaced it was much better. You could feel the difference in the seat of your pants. Luckily it progressed to generating a CAT efficiency DTC error code.

It seems like replacing the CAT outright is the cheaper and most hassle free route to take, especially if you get a hi-flow type. Hopefully that alone may help improve your mileage.

120 km/h is 75 mph. Eventually I want to drive at 140 km/h to see if the milage goes down. It is very strange that the milage gets better the faster the car goes. It would be nice to hear why this occurs.

You forgot the banghead smilie..lol. Seriously, if I were in your position I'd probably run that vaccum test while on the road. I did a similar test with a fuel pressure gauge and taped it to the driver's side wiper.

There is one other test that can be run on the exhaust system to test for backpressure or CAT acting up but it is a real hassle. The dealers normally have the test fixture or check with any mechanic friends. One can be fabricated if you are willing to go through the trouble. The test is run with a 0 - 5 psi low pressure gauge. A connector (tee) is needed where bottom is male thread 18mm X 1.5mm pitch and the top is female 18mm X 1.5mm. It has to have a fitting to connect the gauge. The fixture is threaded into the front 02 sensor bung and the 02 sensor is threaded in at the top. Engine is warmed up and brought up to operating temperature. Then rpms are taken up to 2000-2500. The gauge should not read greater than 1.25 psi.

The CAT is still a possibility and under "poor fuel economy" it is near the top of the list in the GM service manual, especially with all the other tests passing. :dunno:

You said the CAT was near the top of the list what other things are at the top with no SES. oh by the way I ordered a downstream delco O2. oh I got 24MPG today 137miles round trip 5.7 gals at fill up thats on 55 & 65 mph roads 20 miles of 80mph on the interstate 25*F.

You said the CAT was near the top of the list what other things are at the top with no SES. oh by the way I ordered a downstream delco O2. oh I got 24MPG today 137miles round trip 5.7 gals at fill up thats on 55 & 65 mph roads 20 miles of 80mph on the interstate 25*F.

02 sensor(s)
EGR valve
ECT (Engine Coolant Temperature) sensor out of calibration - thermostat
Dirty or clogged fuel injector(s)
Spark plugs and/or wires
Torque Convertor Clutch (TCC) operation
Vacuum leak
Fuel pressure
Air intake system leaks
MAF sensor
Ignition control, timing, and knock sensor
A/C in full time operation
Engine compression

It doesn't differentiate with or without SES light.

02 sensor(s)
EGR valve
ECT (Engine Coolant Temperature) sensor out of calibration - thermostat
Dirty or clogged fuel injector(s)
Spark plugs and/or wires
Torque Convertor Clutch (TCC) operation
Vacuum leak
Fuel pressure
Air intake system leaks
MAF sensor
Ignition control, timing, and knock sensor
A/C in full time operation
Engine compression

It doesn't differentiate with or without SES light.

EGR valve >>>>>>>>>>>>>>>Checked operated correctly

ECT (Engine Coolant Temperature) sensor out of calibration - thermostat

Checked new 195*f stat>>

Dirty or clogged fuel injector(s) >>LTFT/STFT were ok super junk added to fuel tank

Spark plugs and/or wires >>NEW NGK plat. plugs/delco wires/delco coil packs

Torque Convertor Clutch (TCC) operation >>I would of felt dumb :grinyes:

Vacuum leak >>>>>>>>>>>>visual and propane check

Fuel pressure >>>>>>>>>>>>In specs

Air intake system leaks >>>>Tested with propane around UIM

MAF sensor >>>>>>>>>>>>Checked dont know what they did to it

Ignition control, timing, and knock sensor Did not check timing/Ignition module

A/C in full time operation>>>>Not in full time operation

Engine compression >>>> Did not check

O2 sensor (s) >>>>>>>>Said were OK
HUH:banghead:

Got a new toy on the way! backpressure tester! So it should be no more than 1.25psi BEFORE the cat?

Troy

Got a new toy on the way! backpressure tester! So it should be no more than 1.25psi BEFORE the cat?

Troy

Great. From what I understand no greater than 1.25 psi but that is an upper end tolerance. A lot lower would be better. The Regal will have a different result because it is single exhaust all the way out to the tailpipe whereby your GP GT has the split duals after the resonator.

Let me know how the test fixture works out because I might get one. I'd like to do some testing for more exhaust modifications in my GTP. Good luck!

Great. From what I understand no greater than 1.25 psi but that is an upper end tolerance. A lot lower would be better. The Regal will have a different result because it is single exhaust all the way out to the tailpipe whereby your GP GT has the split duals after the resonator.

Let me know how the test fixture works out because I might get one. I'd like to do some testing for more exhaust modifications in my GTP. Good luck!


Will do I got it through a local parts store if I like it I'll give you the brand name.GP should have less PSI thanks to the dual exhuast I would think. If its above 1.0 PSI the CAT is going in the scrap pile.

Edit for type-o 2.0 change to 1.0

Since I changed the CAT, at average speed of 65 mph, my car is doing 34 mpg on the road and 27 mpg in the city :D :D :D

Since I changed the CAT, at average speed of 65 mph, my car is doing 34 mpg on the road and 27 mpg in the city :D :D :D

:bigthumb: Man, I love success stories.

Thats great! I hope I find out whats wrong with mine soon 22-24 sucks for all hiway.

also is 1.25psi based on the regal Bob?

Thats great! I hope I find out whats wrong with mine soon 22-24 sucks for all hiway.

also is 1.25psi based on the regal Bob?


I hope so too.

That figure is directly out of the GM Grand Prix Service Manual.

Good luck!

Well got the gauge today backpressure barley comes off of zero, needle shakes alot while testing. One thing I did notice was I have oil sitting down on the lower intake by the injectors on the firewall side of the engine. UIM/LIM problem? or just the valve cover leaking. Took a trip out of town this weekend going there couldn't get to 24.5mpg @ 80mph reset the trip computer with out filling (not completely accurate) on the way back dumb thing never got less that 26.9mpg @ 80 mph all the way. usally it drops like a rock to 22/24 at stays there. Keep inmind that I do not rely on the trip computer just using it as a "gauge" and I always calulate it out. My average is probably 18/21mpg but thats alot of intown driving. Tried using the whole tank I tried some BG 44 fuel system cleaner @ $18 a can. I'll I have to say is HUH!!

Troy

Well got the gauge today backpressure barley comes off of zero, needle shakes alot while testing. One thing I did notice was I have oil sitting down on the lower intake by the injectors on the firewall side of the engine. UIM/LIM problem? or just the valve cover leaking. Took a trip out of town this weekend going there couldn't get to 24.5mpg @ 80mph reset the trip computer with out filling (not completely accurate) on the way back dumb thing never got less that 26.9mpg @ 80 mph all the way. usally it drops like a rock to 22/24 at stays there. Keep inmind that I do not rely on the trip computer just using it as a "gauge" and I always calulate it out. My average is probably 18/21mpg but thats alot of intown driving. Tried using the whole tank I tried some BG 44 fuel system cleaner @ $18 a can. I'll I have to say is HUH!!

Troy

Was the gauge a 0 - 15 psi? It sounds OK to me even with the fluctuation.

When you get that oil sitting in those spots it is normally a valve cover gasket even though it may look like the source is the manifold. I've had to replace the valve cover gaskets on both the GTP and Regal LS for similar plus seeping down the block. The valve cover gaskets start heading South around 70K miles.

Did you ever get and try the 02 sensor? 26.9mpg @ 80 mph doesn't sound that bad now. That's about 457 miles on a tank of gas which is pretty close to normal. :dunno:

I've got it but no time to install :frown:! Filled the car up today after running that can of junk through it 293.1 miles 14.16 gals 20.6MPG 106miles are hiway. Maybe I got some bad gas this all happed after Katrina and it gummed stuff up :screwy: I think the gauge is a 0-5 psi its got marks a 0 1.25 2 & 3 but it shakes so rapidly kind of weird. I guess I'll see what this tank does. right now its 21.6. It does not seem that I have to have my foot into it as much.

Oh its a STAR brand gauge

Well I'm almost convinced that I had dirty fuel injectors. Its been a few days now, going 70 the computer says 28.2 mpg which is excellent. I have not had a good long trip yet but the results so far are good. Also I did get the new downstream O2 installed also but the mileage was on the way up before that. So If anyone wants to try what I used it was BG 44K fuel system cleaner $18 a can. I will keep everyone posted for a while with my mileage. Just maybe it will help some else with crappy MPG.

Troy

Well I'm almost convinced that I had dirty fuel injectors. Its been a few days now, going 70 the computer says 28.2 mpg which is excellent. I have not had a good long trip yet but the results so far are good. Also I did get the new downstream O2 installed also but the mileage was on the way up before that. So If anyone wants to try what I used it was BG 44K fuel system cleaner $18 a can. I will keep everyone posted for a while with my mileage. Just maybe it will help some else with crappy MPG.

Troy

Probably more than that. The BG 44K also cleans all the carbon off the valves and combustion chambers. I've heard people swear by the stuff but most claim about a 1 -2 mpg improvement which in itself is better than nothing.

Our local Indian Tribe sells BG products at their convenience store chain. Better price than the dealers that carry it. I'll try some in my wife's Regal LS. Maybe I can get it up to 30 mpg highway. :dunno:

I never have understood how they can claim it removes carbon buildup. You gotta wire wheel that crap off most of the time. 3 - 4 + mpg for me hopefully it stays that way!!

Troy

I never have understood how they can claim it removes carbon buildup. You gotta wire wheel that crap off most of the time. 3 - 4 + mpg for me hopefully it stays that way!!

Troy

Yeah thats a good point Troy. I can see the soft carbon. Plus the carbon has to go somewhere? Out the exhaust via the CAT? I'll let you know how that BG 44K works for me.

Ok, today I got a DTC PO134 O2 sensor 1, bank 1 no activity. Anyone got the dianostic tree for the DTC or just a bad O2


Key on Engine off test:
Pink & black wires 11.93volts
Purple & Tan wires .38 volts

Ok, today I got a DTC PO134 O2 sensor 1, bank 1 no activity. Anyone got the dianostic tree for the DTC or just a bad O2


Key on Engine off test:
Pink & black wires 11.93volts
Purple & Tan wires .38 volts

I'll check the GP Serv. Manual but so far basic description says 02 sensor or 02 sensor circuit (before CAT) has developed an open.

Gee I wonder if this what was causing my crappy MPG:banghead: Crappy Borg O2 sensor from the get go?

Ok, today I got a DTC PO134 O2 sensor 1, bank 1 no activity. Anyone got the dianostic tree for the DTC or just a bad O2


Key on Engine off test:
Pink & black wires 11.93volts
Purple & Tan wires .38 volts


The pink is 02 sensor heater which should read around 12v. Black is ground. The purple wire is 02 sensor activity going back to the PCM. Voltage will vary from 1.0 v (rich) to .1 v (lean). The tan should be the PCM reference signal which is around .45 v. coming from the PCM. I recall the purple is where you tap into for 02 sensor activity for Autometer type scanner gauges. I'd bank on the Borg 02 sensor and replace it with an AC Delco AFS 109. Good luck!

Put it on the snapon scanner and said low activity. The guy I talked to thinks the upper intake has a vaccum leak and it just leaned it out to much.

Put it on the snapon scanner and said low activity. The guy I talked to thinks the upper intake has a vaccum leak and it just leaned it out to much.

It looks like you have a good grasp of the situation. Hopefully, the UIM/LIM job takes care of it. If not the 02 sensor. Only problem I found is they are not too readiliy available on EBay like the rear 02 sensor and not cheap through the dealer or AC Delco authorized parts stores. Ed Morad who lists on Ebay has a few front 02 sensors for cheap but used. Good luck!

Well did the UIM gaskets this morning started at 9 done by 12:15. I did find some antifreeze back by the 4 & 6 injectors not a lot but some. Most likely it was from when I took the bolt out to get the EGR sheild off. It didnt look like the old gasket was sealing well. Might swap out the MAF with a known good one.