Picked up a 2006 gmc envoy Xl gas mileage...not so good

deerslayer1985

Well-Known Member
Just picked up a 2006 GMC Envoy XL for a great price. It is in really good condition. I knew the guy who owned it. It has 185,000 miles on it, but has been babied, garage kept, and maintenance performed regularly on it. Just the usual brakes, oil change, air filter. Never had any issues from what he has told me. I have found out it doesnt do nearly as good on gas as my 2002 trailblazer did. I constantly averaged 22-23 mpg in my trailblazer. I have ran a couple tanks through the envoy, and Im only averaging around 14 mpg. The only differences between the envoy xl, and trailblazer I have beside the obvious size difference is....my trailblazer didnt have the spare tire underneath. It didnt have the big can muffler on the back I removed it. I cant imagine those 2 things doing much if anything at all for gas mileage. Do the Envoy XL's really suck that bad on gas, or is there anything I can do to increase it. Even if only slightly?
 

Mooseman

Moderator
A few factors may affect MPG. Gearing, weight, overall engine condition and maintenance, plugged cat or exhaust, brake drag and tires. We've had people report changed MPG with a tire change. And I've had a plugged cat that affected MPG negatively. You'd have to do a backpressure test for that.

My '07 SWB has had a consistent 17 MPG combined. I think that different trucks have different personalities and just have different MPG.
 

Blckshdw

Moderator
FWIW, my 05 TB EXT also gets around 14 mpg. I'm running 31.5" tires, on 3.42 gears with a 2.5" front lift, and a tune. I also drive it, like I have somewhere to be, other than where I'm currently at :biggrin:
 

gmcman

Well-Known Member
My buddy has an 03 XL and he says he can manage 20 MPG on a hwy trip if he keeps it under 70.

What tires are on the vehicle? Can you get a vacuum reading at idle, @ 600 RPM?
 
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deerslayer1985

Well-Known Member
They are stock tire size. 245/65/17 I think. I hae the foseal obd scanner. will that tell the vacuum reading? I have driven it really easy to get that 14. It wasnt all highway, but pully 50/50 driving. I would coast as much as I could and take off easily. I do notice there is a more noticable delay in the throttle on the envoy xl compared to my trailblazer. It seems like I push the throttle down andit takes a second or so before the vehicle reacts and accelerates. I cleaned the throttle body, and it didnt change anything really. It does idle smooth and doesnt miss or anything.
 

Blckshdw

Moderator
My 14 mpg avg is all city, mostly my 30-45 minute commute, which is bumper to bumper most mornings, and red light to red light in the afternoons. On the highway, if I can keep the RPMs under 2500 while cruising, I can get about 20 instant mpg

The throttle response you describe sounds like mine without the tune. Had the transmission replaced last spring after having the tune for 5 or 6 years, and the dealer wiped my tune back to stock. The throttle lag was painfully evident on the way home.
 

Sparky

Moderator
make sure it is running at proper temperature.
 

gmcman

Well-Known Member
Not doubting you but have to ask, are you going by the DIC for the MPG or gallons used? If the DIC, are you resetting each time?

Few things that come to mind, do you know how old the plugs are? What does the air filter look like? Is the MAF meter clean?

Take it for a drive after sitting overnight for a few miles, avoid heavy braking but pull over and see if you have excessive heat from the brakes. Should be luke warm but not smoking hot.

As mentioned earlier regarding the CAT, a backpressure test would help rule it out.

To check the vacuum, I disconnect the evap hose from the TB, be mindful of the locking tab.
 

TequilaWarrior

Well-Known Member
A few things that have been mentioned here I'd like to expound upon.

Temperature: if the thermostat is faulty OR the temp sensor is faulty and the engine is either not reaching temperature or the PCM is not seeing the engine as reaching temperature then the Torque Converter Clutch will NOT engage. Granted, this usually only engages above 25 and at a low engine load but it WILL make a difference in MPG. If your temp gauge is not reading STRAIGHT UP (or just literally a hair to the left) then either the thermostat or the temp sensor need replaced. If you're going to replace either, you might as well change both as it is a PITA to do one. Bite the bullet and do it once.
Plugs: This can't be overstated. At 185,000 miles the plugs from the factory I would expect to be completely shot. It's seems whatever IS in the engine are not the plugs installed at the factory - BUT, what are they and how old are they? These engines REALLY only like ACDelco 41-103 plugs. They're iridium tipped and a tad expensive. If you're replacing worn or incorrect plugs the correct replacements will pay for themselves fairly quickly.
WEIGHT: I'm not sure the exact weight difference, but the XL is definitely considerably heavier - obviously that's going to have a mileage impact. Ditch the spare and the resonator like you did on the TB. I ditched my spare because the wheel was shot and I can't remember the last time I actually used a spare tire. The weight shed will total less than 100 lbs which won't make up the difference between the TB and the XL, but they will be a little closer in weight.
GEARING: The rear differential ratio will have a huge impact on mileage. I had two identical '02 Bravadas. However "mine" has a very low gear ratio (towing gears) compared to "my wife's" Bravada (since traded). Hers had what we used to refer to as "airplane gears". Hers regularly got 20-22 MPG while mine only rarely breaks 16. It was evident on the tachometers that their gearing was different. Compare the TB and the XL tachometer readings at the same speed.
TIRES: Are the tires the RIGHT SIZE on both vehicles? Incorrectly sized tires will throw off the math used to calculate MPG. If either are not the correct size tire you cannot compare the MPG readings from both DICs.
O2 sensor, cat, exhaust manifold: I had 0 codes for my O2 sensor but my manifold was cracked all to heck. I just went through everything and replaced the exhaust manifold, em gasket, intake gasket, throttlebody gasket, valve cover gasket, O2 sensor and CPAS. While I haven't had any highway traveling to do since it sure seems like I picked up a few MPG.
CPAS: unplug the CPAS. If you see oil in the connector, replace it. If the CPAS "spins" when you try to turn it in the block, replace it. Mine threw no codes but both spun and had oil in the connector. It ~seems~ I have a little more power since replacing it.
Throttlebody cleaning: Did you disconnect the battery and remove the throttlebody and clean it OFF of the car or just spray and wipe? The PCM needs to relearn the throttlebody's flow behavior after a cleaning that's why we disconnect the battery.
 
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deerslayer1985

Well-Known Member
The tires are the correct size. I looked that up this evening. I did take the throttle body off when I cleaned it. I pulled the fuse, and unhooked the battery negative. It didnt seem to change any. The gas mileage before was the same as after. I calculate the gas mileage at fill up, but it is within decimals of what the envoy is reading. Also my envoy doesnt have instant economy like my trailblazer, but like a trip mpg meter. The temp is straight up. It isnt popping any codes, and has no stored codes. I checked and the air filter looks like new. I wasnt able to check the plugs yet, but will for sure tomorrow or the next day. I can get all the readings from my foseal obd scantool, and in car doctor. If that would help with some readings you guys are talking about, because i not real sure how. It seems to idle really smooth and has good power, but it does have a delay when pressing the throttle. How do I see which gear ratio I have?
 

Mooseman

Moderator
Look at the RPO codes in your glove box. Look for one of these codes:

GU6 - 3.42 axle
GT4 - 3.73 axle
GT5 - 4.10 axle

And for your info only:
G80 - Whichever axle as listed above, equipped with the Eaton Locker mechanism inside. If this codes isn't present, it's an open diff.
 

northcreek

Well-Known Member
I may be missing something but, nowhere in this thread can I see if we are talking about the 4.2 or 5.3 engine....:confused:
 
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deerslayer1985

Well-Known Member
Its a 4.2

I found GU6. So a 3.42 axle. Wouldnt that be better for gas mileage? Atleat highway gas mileage? I will try to see if I can get a reading of all the info from my scanner, and see if anything looks off to you guys.
 

TequilaWarrior

Well-Known Member
Its a 4.2

I found GU6. So a 3.42 axle. Wouldnt that be better for gas mileage? Atleat highway gas mileage? I will try to see if I can get a reading of all the info from my scanner, and see if anything looks off to you guys.
3.42's should give better highway mpg, but might be a little sluggish to accelerate compared to say a 3.73. 3.42's might even be a little less efficient climbing hills as well.

I didn't notice if you mentioned - is it a 4x4? If it is, are you experiencing "crow hop" - a shuddering feeling when making tight turns almost like the car is trying to drive "through" the turn.
If so, you could have a bound up transfer case which would reduce your mpg, prevent TCC lockup from occurring and may prevent cruise from working (I don't remember on that one).
If it's a 4x4, are you sure the transfer case fluid was changed every 50k miles? Not changing it every 50k miles WILL cause the clutches in transfer case to stick/bind. The fix is simple. Change the transfer case fluid TWICE driving between changes. I drove probably a few hundred miles. Only use GM Auto-Trak II, sold in liters not quarts. It should be about $9/liter at the dealership, I know of no other reliable source or substitute. You'd need a total of 4 Liters. Change the fluid - drive - change the fluid again. This should free up the clutches in the transfer case if so equipped and if that's part of the problem.
 
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deerslayer1985

Well-Known Member
No it drives very smoothly in turns and everything. Yes it is in 2hi. I honestly dont know if the fluid has been changed in it or not, but it turns quite easily, and going through all the 4wd selecters seem to work fine and it even steers fine with 4wd on. I've done monstly 50/50 driving maybe even 60 city 40 highway, and it is quite hilly around my parts. So that could be it too. I dont think it is anything like the brakes dragging or transfer case issues. The car honestly runs really good. Its just the mileage isnt near as good as my 4 year older and smaller trailblazer. Im not sure what ratio is in it, but I should check. I have changed the spark plugs in my trail blazer cleaned the throttle body the same way. I removed the big can muffle, and put just a tip. I removed the spare tire. I just cant imiganine that making up that much difference. It may just be the difference in weight of the vehicles along with possibly a different gear ratio. I also noticed the 2006 horsepower and torque ratings are higher than the 2002...possibly that could make it less fuel mileage?
 
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deerslayer1985

Well-Known Member
I've been calculating the gas mileage with the onboard meter, and my calculations at the pump. They are usually within .5 mpg. usually the onboard gas meter is lower than the calculated equation. Its shows like 13.6, and I calculate 14, and its shows 14, and I calulated 14.5. Well I did a fill up yesterday, and the onboard gas meter showed 13.7, but my calculations showed 16.8 mpg. Can the meter be that far off?
 

Chickenhawk

Well-Known Member
I would repeat what others have said about a plugged cat. Can you give us your coolant temp reading when warm? These trucks are very sensitive to correct coolant temps, and running for even a few months with temps in the 185 to 195 range will plug the cat. Also, if it ever had a bad misfire before you owned it, even running for a few minutes will plug the cat.

Aside from giving us the coolant temp readings when cold, how well does it accelerate on the highway? Fast to accelerate on on-ramps? Climbs hills easily?
 

gmcman

Well-Known Member
Are you resetting the AVG MPG at every fillup?

I would remove your upstream O2 sensor and take a pressure reading. Mine was about 1/4 PSI around 2,000-3,000 RPM on stock exhaust. I had a Walker exhaust for a short time and it measured 1/2 PSI and I lost a couple MPG.

With that being said, you really should get a back pressure test done to further evaluate your low MPG's. That's just one base that needs to be covered especially not knowing the history.
 
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deerslayer1985

Well-Known Member
Yea I am resetting everything at every fill up. The last time the on board gas mileage meter showed a little over 3 mpg's less than what the actual math showed. Im guessing around 16.8 mpg with 50/50 city/highway driving would be pretty close to accurate. I'm not sure how you get a pressure reading on the exhaust. I will try to get coolant temp readings in the next day or 2. It seems to accelerate pretty good. Doesnt seem quite as quick as my trailblazer, but the envoy is bigger.
 

gmcman

Well-Known Member
What gears does the TB have? A 4:10 to 3:42 would be quite a jump.

Any exhaust shop can perform a pressure test. I bought a Winters gauge that is 0-5 PSI, then drilled and tapped an O2 plug.

Remove upstream O2 sensor, install pressure gauge into sensor bung. Check pressure at idle and perhaps 2K RPM.

Autozone may even have one to loan.
 

coolasice

Well-Known Member
I would repeat what others have said about a plugged cat. Can you give us your coolant temp reading when warm? These trucks are very sensitive to correct coolant temps, and running for even a few months with temps in the 185 to 195 range will plug the cat. Also, if it ever had a bad misfire before you owned it, even running for a few minutes will plug the cat.

Aside from giving us the coolant temp readings when cold, how well does it accelerate on the highway? Fast to accelerate on on-ramps? Climbs hills easily?
funny you say that.. mine runs 193 almost constantly without any issues. I get lousy gas mileage, usually 14-15 in summer and 18 highway... just the curse of the 4.2... And yes... i did change thermostat and sensor...
 

budwich

Well-Known Member
as mentioned by "t...warrior", a cracked exhaust manifold is a good possibility (may not may not be "visual" as even might be a broken bolt). Depending on the severity, it may not throw any codes because the "detection range / limit" on the "fueling / air" may not get hit. However, an "ingress air leak" will show up as the system attempts to overcome a "sense of lean" by adding more gas to "fix the ratio" which will slowly eat your mileage figures.
Monitoring trims may provide some evidence therein.
 

gmcman

Well-Known Member
I get lousy gas mileage, usually 14-15 in summer and 18 highway... just the curse of the 4.2
Mine is about the same and I have the 4:10 gears. As far as highway, depends on speed, tires play a big part also.

Wanted to add to your comment with a few thoughts I've come to learn in 18yrs of ownership.

If I cruise at 60-65, 20MPG is not unheard of, bump that up to 70-75, then 18 would be nice.

Low-rolling resistance tires are a factor and can hurt MPG. I sold a pair of Kumho Ecsta's I had on the Envoy back in 2008-09..ish with 17K miles on them as my MPG's went down 2-3 on the highway. Went back to LRR Michelins which fixed that.

Air pressure is key, 30 PSI I would lose a MPG or 2, I've always kept these at 35 psi and all my tires wear even and regain that lost MPG.

With these heavy platforms and the 4.2, everything matters, coolant temp, good tires, proper pressure, proper plugs, fuel trim, unrestricted exhaust, and no heavy foot.

With 3:42 gears and a steady cruise at 65 I would think 20-22 would not be hard to achieve.
 
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deerslayer1985

Well-Known Member
I'll have to do some checking. My exhaust manifolds being cracked I dont really see that. I mean wouldnt I hear exhaust leaking. This thing is quiet as can be.
 

TequilaWarrior

Well-Known Member
I'll have to do some checking. My exhaust manifolds being cracked I dont really see that. I mean wouldnt I hear exhaust leaking. This thing is quiet as can be.
"Probably" is the best answer I can give. Most exhaust leaks have that typical rhythmic "tick" but occasionally they can leak in such a way that the "tick" is extremely quiet. AND - it being timed with the firing of the cylinders is almost completely lost in engine sounds. It also doesn't help that the valve train in these seems to be quite noisy even when everything is close to perfect.

It is very possible that your mileage is "normal" for that vehicle. This would both be good and bad. Good, because there's nothing needed to "fix" it. Bad, because there's not much you can do to improve it.
 
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deerslayer1985

Well-Known Member
I figured the mileage would be worse on this than the trailblazer...being as it is the xl version, but didnt figure it'd be that much difference. I'm still gonna try to check the exhaust pressure, but the thing runs like a top. The only thing I dont like is the delayed throttle response. My trailblazer has the same thing, but not quite as bad. It was like floor it, and a half second later it goes. The envoy seems like a couple seconds after flooring it. Its very odd, and I dont like ti when I go to pull out in a blind turn and then I see a car come around the corner. I wish there was a way to fix that. I'll prolly also do a full tune up, get rid of the big can muffler, and maybe remove the spare for the city commute. All that might help a touch. Even a mile per gallon or 2 adds up over the course of a full tank.
 

Blckshdw

Moderator
The only thing I dont like is the delayed throttle response. My trailblazer has the same thing, but not quite as bad. It was like floor it, and a half second later it goes. The envoy seems like a couple seconds after flooring it. Its very odd, and I dont like ti when I go to pull out in a blind turn and then I see a car come around the corner. I wish there was a way to fix that.
Getting your PCM tuned can help with that. :twocents:
 

gmcman

Well-Known Member
Also having a bluetooth OBD adapter and an app like Torque, we can see what the fuel trims are, spark retard, o2 sensor health, etc.
 
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deerslayer1985

Well-Known Member
Where do you get the pcm tuned at? I have an obd reader, and I have the app incardoc free. I just need to figure out how to get everything to go here....lol
 

Mooseman

Moderator

He's also a member here @limequat
 

gmcman

Well-Known Member
Make sure your fuel pressure is good, not sure what yours should be but I believe it's in the neighborhood of 50 psi.

I would take it out on the highway, clear the MPG reading, and drive it for about an hour and see what you come up with.

Drive at a steady cruise of about 60-65 and see what you get.

If you're at 14 -15 on the highway at 60 to 65 then there are other areas to address before you get a tune IMO.
 
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deerslayer1985

Well-Known Member
Alright I have did 5 consecutive tanks of gas. I reset the mileage, and mpg calculator every time on the dash. It is always in the 13's.....13.1-13.8 mpg. I calculate it by how many gallons it takes to fill up and how many miles I have driven as well. The dash usually shows a little low. Here is where its weird though. The first 2 tanks were 13.8 mpg calculated by me, and in the mid 13's on the dash. That is with 87 octane. I had a lot of cents off. Enough to where it made no sense not to get high octane. So I filled up with 93 octane. I drive the same thing everyday, So the mpg should always be about the same. Which it is....except with the 93 octane. With the 93 octane is when I got 16.83 mpg, but the dash showed 13.7. I've did 2 more tanks of 87 since, and the following tank was 14 mpg, and 13.8 on the dash. the following tank was 13.8 mpg, and 13.1 on the dash. Are these suppose to run on high octane, or what? It still doesnt outweigh the cost of 40 cents more per gallon, but kinda weird.
 

Blckshdw

Moderator
Are these suppose to run on high octane, or what? It still doesnt outweigh the cost of 40 cents more per gallon, but kinda weird.
Not by default, no. But the PCM can be tuned for it. I tried this with my first tune, and didn't notice enough of a difference in MPGs to be worth the cost difference, so I had them switch it back to 87
 

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