Horrible Fuel Mileage

I have a 2002 GMC Envoy SLT. The fuel mileage is absolutely horrible. I get about 12.5 miles to the gallon and I owned an 05 ext trailblazer before that got 16-18mpg. I have replaced the plugs, cleaned the intake and throttle body, changed the air filter, replaced both the upstream and downstream oxygen sensors, replaced the fuel filter, changed the oil, replaced the thermostat because it was stuck open, and replaced a few of the coil packs because they needed replaced. I have used sea foam in the gas tank as well as in the top end. I still get 12.5mpg!! I know this can’t be normal, what else can it be? I do on occasion get a P0440 Evaporative System leak however it runs fine. Gas cap has also been replaced. What else could that be and could that be the issue as far as fuel mileage as well? Unrelated I replaced the rear sway bar links because they were broke, as well as the rear lower trailing arms because they were broke as well. The shocks are relatively new in the rear as well as the springs. Tires are inflated properly. 35psi all the way around. Rated for 32 psi on the door panel but I like to run at least 35 psi. The brakes were also recently done all the way around including rotors and the left rear caliper was replaced as well. What am I missing? I should get at least 16mpg I am sure of that!
 
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0

02envoySLT

Member
I would check your cat. converter. If thermostat was not working too long, the cat can be damaged.
How would I go about doing that? I’ve done cat replacements before not too hard to do snd not too expensive. But not sure how to check it? It doesn’t bog or anything has full power, usually when the cat is damaged it has a loss of power.
 

budwich

Well-Known Member
I would get some running data on your fuel trims to see what the system is operating at. You have changed a lot of "stuff" without much impact so it is likely those things weren't a factor. Another likely culprit is an intake leak. Do some leak testing around intake areas with some carb cleaner or equivalent to see if there are changes in the engine rpm... go from the results.

As mentioned, if you have been running rich for a while (because of thermostat issues), its possible that could cause cat problems which may cause poor mileage among other things.... but you indicate little cat symptoms. You can do a rough check with a thermal gun to see if the cat is functioning.
 

TequilaWarrior

Well-Known Member
How are your brakes? Dragging breaks can definitely negatively impact fuel mileage. It should be obvious if that's the problem as the brakes themselves will be excessively hot.

What differential do you have vs. the one you had in the other vehicle?

12.5 MPG is "bad", but it's only 10% less than what I average around town. Also, doesn't the EXT have a V8 in it? These 4.2s drink a lot of gasoline.
 

Blckshdw

Moderator
I average about 13 around town with my 4.2 EXT
 

Mooseman

Moderator
Another symptom of a clogged cat is low power. It might be only partially clogged so a temperature check might not tell you anything. An exhaust backpressure test will tell you for sure.
 

Mike534x

Well-Known Member
Not sure if it helps much, but my old 02 Envoy averaged 12.8 MPG consistently. No matter how conservative my driving habits were, I would get 232 miles each fill up. I did drive around through most of the winter with the thermostat being bad, which I'm sure clogged up the cats without hindering performance. I get around 10-11 city with my current Envoy (V8), but I do drive with a bit of a lead foot haha. Highway comes out to be about 19-22 on most days with 16.5 being my average. My 05 Ext had a V8 as well, and I would get 13-14 in the city, but then again it had the DoD/AFM which helped immensely. I know my current Envoy's city MPG is bad due to the passenger side cat failing, so while I don't have power issues it is slowly affecting my MPG.
 

gmcman

Well-Known Member
A restricted cat is surely a possibility.

How cold is it where you are? Many hills?

Are you letting the engine warm up for a lengthy time considering the winter months?

What tires do you have? When was the last time you checked the pressure, was it during a cold morning?

Low MPG's in winter are attributed to one or more of the following:

Lower tire pressure due to the colder ambient temp. I know you said it was 35 PSI, but didn't specify a cold or warm tire.

Type of tire

Are you driving in A4WD instead of 2HI?

Winter blend fuel

Lengthy warm up times.

Cold air is very dense, more energy required to fight mother nature, especially above 60-65 MPH.

A non- Low-Rolling-Resistance tire + long warm up times could EASILY account for a 1 MPG loss per each factor, especially when cold.
 
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Mektek

Well-Known Member
My DIC shows an average of 17.9 mpg that's nearly all city driving. I do make an effort to drive efficiently. Also I inflate tires to 45psi front and 40 rear - that helps a bit.
 

gmcman

Well-Known Member
My DIC shows an average of 17.9 mpg that's nearly all city driving. I do make an effort to drive efficiently. Also I inflate tires to 45psi front and 40 rear - that helps a bit.

Do you reset your DIC after each fillup?
 

Chemman

Active Member
02envoyLT, assuming all is good and you do not have a CAT or other problem, try this - remove the fuses for the PCM and let the vehicle sit for 30 - 60 minutes, then put the fuses back in. This lets the PCM forget the learned driving parameters, and allows it to relearn, and hopefully improve your gas mileage. While it does not always work, it does work often enough that it's worth a try.

Good Luck!
 

Mooseman

Moderator
remove the fuses for the PCM and let the vehicle sit for 30 - 60 minutes, then put the fuses back in.
If you do this, you MUST clean the throttle body.
 

KEEBZ489

Well-Known Member
was teh milage better before ? or youre just comparing the two vehicles ? doesntt the rear diff ratio affect MPG ?

my 05 tb ext use to get 200 miles to a tank with local driving , now taht i switched jobs and have a commute i get 325 to a full tank
 

gmcman

Well-Known Member
I'm not sure what the average interval is. I don't reset anything after fuelling.

Some years have both "INSTANT MPG" and "AVERAGE MPG", the latter being a resettable option.

The reason I asked is I've had a couple friends that didn't know to reset this function, one on a different manufacturer altogether. He said it would always read the same, just wasn't aware it could be done.

INSTANT MPG is just that, a coast will net 70+ MPG easy.
 

TequilaWarrior

Well-Known Member
was teh milage better before ? or youre just comparing the two vehicles ? doesntt the rear diff ratio affect MPG ?

my 05 tb ext use to get 200 miles to a tank with local driving , now taht i switched jobs and have a commute i get 325 to a full tank

Different differential ratios will DEFINITELY affect MPG. It's not necessarily a geometric difference ( the ratio of the ratios ), but it will be affected.
 
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02envoySLT

Member
I do reset the dic every fill up! Throttle body has been cleaned, brakes should be good as they were done before. I do however get a constant squeal when backing up only. The ext has the 4.2l i6. I drive in 2hi. I live in Ohio so temperature is variable but right now it’s around 32-12°. I only got 180 miles on my last tank. I just filled up before writing this. My power is fine gets plenty of it. 35 psi is cold pressure. I will give pulling the pcm fuses a try. If I can find which ones they are.
 

Mooseman

Moderator
Fuses 10 & 28
 
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02envoySLT

Member
any difference? im in same or similiar boat...
It appears to he about the same still. Maybe a little better a .2 miles or soo. But nothing much. Although it hasn’t been that long either. I keep track by the DIC and by figuring it out the standard way pencil and paper.
 

gmcman

Well-Known Member
An exhaust backpressure test should hopefully answer more questions.

Could also be a very lazy upstream O2 sensor.
 
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02envoySLT

Member
An exhaust backpressure test should hopefully answer more questions.

Could also be a very lazy upstream O2 sensor.
I will see if I can’t get a pressure gauge to test back pressure. Shouldn’t be a lazy o2 sensor because I just replaced them both less than 800 miles ago. Less than a month actually. That’s what I thought it was at first as well after the tune up. What should I get for pressure when I do the back pressure test?
 

Mooseman

Moderator
The exhaust on these trucks are very good and should be close to 0. Shouldn't be more than 2-3 PSI at 2500 RPM.
 

Redbeard

Well-Known Member
When you replaced the upstream o2 sensor what brand did you use? Several years ago my upstream O2 sensor went out and the fuel mileage dropped significantly. I don't like throwing parts at problems, but I believe I would change out the upstream 02 sensor with a new a/c sensor. These platforms seem to not like other brands installed on them so I would stick to the oem parts.
 
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02envoySLT

Member
When you replaced the upstream o2 sensor what brand did you use? Several years ago my upstream O2 sensor went out and the fuel mileage dropped significantly. I don't like throwing parts at problems, but I believe I would change out the upstream 02 sensor with a new a/c sensor. These platforms seem to not like other brands installed on them so I would stick to the oem parts.
I use Bosch for both that’s all that was readily available at my local part stores. Bosch is what I typically always use for oxygen sensors. Other than that I don’t use Bosch for anything.
 

Beacon

Well-Known Member
Have you calculated the fuel mileage yourself? I know my truck is always showing that I use more fuel than I actually use.
Edit: somehow scrolled past the answer.

Also, I agree with Redbeard, the brand of O2 sensor is critical for alot of vehicles, not just GM.

Edit: more of a curiosity, have you jacked the vehicle up and spun the tires?
 
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02envoySLT

Member
Have you calculated the fuel mileage yourself? I know my truck is always showing that I use more fuel than I actually use.
Edit: somehow scrolled past the answer.

Also, I agree with Redbeard, the brand of O2 sensor is critical for alot of vehicles, not just GM.

Edit: more of a curiosity, have you jacked the vehicle up and spun the tires?
Yes I have calculated it myself. Half a tank got me 88 miles this tank.
 

budwich

Well-Known Member
without looking at some of your running data, you don't know if you are chasing an "operation engine" issue or a vehicle mechanical issue. The "spin the wheels" suggestion is good for mechanical.
 
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02envoySLT

Member
without looking at some of your running data, you don't know if you are chasing an "operation engine" issue or a vehicle mechanical issue. The "spin the wheels" suggestion is good for mechanical.
I will jack it up and spin the wheels today. Should I jack it up on all fours? Or can I just do rear and then front? I was gonna start my jacking the rear up.
 

budwich

Well-Known Member
the purpose of the "effort" is so that you can readily spin a wheel and determine if it is spinning somewhat "freely" or not. Going from the result as to be a "rolling" issue which is "eating your gas" or something else.
 

Daniel644

Well-Known Member
well can't speak for OP's issue but I was getting absolute garbage mileage, used to get 14-14.5 then it became the weekend car and mileage dropped to around 13 MPG (as it's now just the drive from home to town and back, where I get better mileage on the drive from town to work and back) so some loss was expected there, but recently it had gotten much worse with the last tank being like 11 MPG, turned out the rear passenger side caliper was LOCKED UP BAD, ate the pad, shredded the rotor all over my Replica SS 22's wheel on that side, rebuilt the caliper, replaced all the brake lines, now to see how the gas mileage goes on the next tank, going a bit hot and heavy on this tank now that I've got the horsepower back that the thing had been missing making me think the engine was worn out, hell frankly I kinda wanna punt the truck on down the road to another owner and get a True SS now so I can have 50% more horsepower.
 

NJTB

Silver Supporter
I noticed this over the winter, fuel milage went way down. The vehicle is well maintained with all the filters and fluids changed on a maintenance schedule.
Around late October, early November, town driving, it was getting 13-14 MPG, which for this car, is poor, even for winter blend gas. Hooked up a scanner and the LTFT was +2, STFT bounced around +5 to -3, so I didn't see any real problems there.
Eventually, I chalked it up to significantly poor winter blend gas. Waiting for warmer weather to confirm.
 

Beacon

Well-Known Member
How long has the fuel mileage be bad? 4 or 5 months? How long do you idle before you start driving? I know some people in your climate will idle up to a half hour, so their car is nice and toasty before they start driving. I know this may not seem like much, but if you do that, then only drive 5 or 10 miles, it would drastically change your mpg.
 

TollKeeper

Well-Known Member
I know must GM products do not like BOSCH engine management products, and this includes O2 sensors. The brand of choice, for me, is NGK/NTK, Delphi, or Delco. The 4.2 engines are even more picky, if not the pickiest, of them all.

Still curious what his LT and ST fuel trims look like.
 

gmcman

Well-Known Member
How long has the fuel mileage be bad? 4 or 5 months? How long do you idle before you start driving? I know some people in your climate will idle up to a half hour, so their car is nice and toasty before they start driving. I know this may not seem like much, but if you do that, then only drive 5 or 10 miles, it would drastically change your mpg.

This is a huge issue. Idling for a warmup, 15 min a day, will easily drop your range by 20-30 miles over 5 days. Couple that with winter blend fuel and that could be a contributing factor to low MPG's.

The best fuel economy I can get is a brief warmup in the summer months and driving easy.
 

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