Gas Mileage

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
I never thought I'd be one posting about bad gas mileage....LOL!! Those kind of threads always kind of annoyed me. But I havent been able to find out why my mileage is suffering...

I used to get 20.5mpg on the highway on average. I only drove my truck 7000 miles last year, so I dont know exactly when it started to suffer. I remember going up north with the Envoy in July and realizing my mpg sucked but thought it was a 1 time anomaly - not so apparently. Ive been driving the truck a lot more lately and its definitely not a 1 time thing. :frown:

I replaced my oxygen sensor (original) and my city mileage went up 1.5 mpg but my highway mpg suffers the same as before though. I tuned my truck up at the 100k mark with Delco iridiums (of course!), I use synthetic 5w/30 and only get gas at the same top tier gas station (have a gas credit card that gives me a discount). I replaced my air filter AND removed my spare tire in the last months - No difference in mpg at all. :mad: Im averaging 16.5mpg in the city and 17mpg on the highway. No CEL light, no pending codes.

Air in my tires are all at 36psi each. I have Destination A/T's. Wondering if the A/T tires are my problem? Because at this point, thats the only thing left I havent changed...lol
 

jham

Member
Nov 20, 2011
1,334
MichEnvoyGuy said:
I never thought I'd be one posting about bad gas mileage....LOL!! Those kind of threads always kind of annoyed me. But I havent been able to find out why my mileage is suffering...

I used to get 20.5mpg on the highway on average. I only drove my truck 7000 miles last year, so I dont know exactly when it started to suffer. I remember going up north with the Envoy in July and realizing my mpg sucked but thought it was a 1 time anomaly - not so apparently. Ive been driving the truck a lot more lately and its definitely not a 1 time thing. :frown:

I replaced my oxygen sensor (original) and my city mileage went up 1.5 mpg but my highway mpg suffers the same as before though. I tuned my truck up at the 100k mark with Delco iridiums (of course!), I use synthetic 5w/30 and only get gas at the same top tier gas station (have a gas credit card that gives me a discount). I replaced my air filter AND removed my spare tire in the last months - No difference in mpg at all. :mad: Im averaging 16.5mpg in the city and 17mpg on the highway. No CEL light, no pending codes.

Air in my tires are all at 36psi each. I have Destination A/T's. Wondering if the A/T tires are my problem? Because at this point, thats the only thing left I havent changed...lol


have you serviced you transmission and diff's. Since it started in July we can rule out the switch to winter gas
 

RayVoy

Member
Nov 20, 2011
939
Maybe a brake dragging????????????

Sounds like something maybe adding additional resistance.

Maybe a brake, maybe tires out of alignment?

Stop on a hill, place in NEUTRAL, see if it roils easily down the hill.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
jham said:
have you serviced you transmission and diff's. Since it started in July we can rule out the switch to winter gas

Yes, I just did my rear axle in June and a trans filter/fluid in July. I was getting the truck ready for a summer vacation trip we took it on. My front axle leaks here and there, so I top it off every month. I use Amsoil 75/90 in both axles and Walmart brand Dex VI in the trans (I change the trans fluid/filter every 20k miles or so).

RayVoy said:
Maybe a brake dragging????????????

Sounds like something maybe adding additional resistance.

Maybe a brake, maybe tires out of alignment?

Stop on a hill, place in NEUTRAL, see if it roils easily down the hill.

Hmmm... this makes a little sense. My lower control arm brackets are the originals and are badly worn out. I just got my new ones last week and was planning on replacing my drivers side axle seal & the brackets tomorrow. Maybe they are messing up my alignment? :undecided: :undecided:

Wooluf1952 said:
Is the temp where it should be?

Yeah, right at the 210 mark, or a small hair before it. I replaced my t-stat in 2009. It actually warms up very quickly too (love my Envoy for that) :biggrin:
 

TangoBravo

Member
Dec 5, 2011
208
Yeah that sure is really low, I would follow the suggestions and check for drag. Im lifted 3" run bigger tires and have 162,000 miles and still get 20-22 mpg on highway. However I keep my TB very well maitained as it sounds you do as well, I run Amsoil products in everything including my trans. Also I Seafoam my engine every oil change which is usually done between 15-20 thousand depending on oil analysis results.
 

Dozer22

Member
Dec 17, 2011
29
ive been trying to figure out the same thing ... idk what rear end i have but 11.5 -12.5in the city is terrible i mean thats driving gently too. I could sure use some advice on how to improve even slightly . 16-17 highway @ 70mph is that normal too ?. oh since were at it on the codes in the glove box which ones refer to axle colorcode etc. if someone had a chart that'd be awesome
 

TangoBravo

Member
Dec 5, 2011
208
Dozer22 said:
ive been trying to figure out the same thing ... idk what rear end i have but 11.5 -12.5in the city is terrible i mean thats driving gently too. I could sure use some advice on how to improve even slightly . 16-17 highway @ 70mph is that normal too ?. oh since were at it on the codes in the glove box which ones refer to axle colorcode etc. if someone had a chart that'd be awesome

GU6 = 3.42 GT4 = 3.73 GT5 = 4.10

As far as how to get better fuel mileage, there really is alot too it. IMO keeping up with the maintenance on your vehicle is critical as soon as you get behind on maintenance you loose. Also I believe in using quality products on anything that moves. I use full synthetic fluids on everything in my TB add that with being up on every aspect of maintenance and I get great gas mileage. So that's what I do and it seems to work great for me. Any questions feel free to ask.
 

Dozer22

Member
Dec 17, 2011
29
thanks for the quick response . idk how the previous owner took care of it but i work at a dealership and we took it in on trade and it only had 58,400 on it when i got it. 1 owner . it just hit 60 k so i was going to replace the plugswith acdelcos and was wondering what else i should do...are those codes for all gm vehicles or trailblazer specific

just checked my code so its a 3.42 rear axle could that aslo effect the milage.
 

djthumper

Administrator
Nov 20, 2011
14,956
North Las Vegas
Dozer22 said:
thanks for the quick response . idk how the previous owner took care of it but i work at a dealership and we took it in on trade and it only had 58,400 on it when i got it. 1 owner . it just hit 60 k so i was going to replace the plugswith acdelcos and was wondering what else i should do...are those codes for all gm vehicles or trailblazer specific

just checked my code so its a 3.42 rear axle could that aslo effect the milage.

You shouldn't have to replace the plugs until 100,000. You may want to take a look a them and check out the condition.

The 3.42 should give you better highway mileage.
 

Dozer22

Member
Dec 17, 2011
29
djthumper said:
You shouldn't have to replace the plugs until 100,000. You may want to take a look a them and check out the condition.

The 3.42 should give you better highway mileage.

Im getting between 15-17 mpg on the highway @ 68< >75mph
 

TangoBravo

Member
Dec 5, 2011
208
Dozer22 said:
thanks for the quick response . idk how the previous owner took care of it but i work at a dealership and we took it in on trade and it only had 58,400 on it when i got it. 1 owner . it just hit 60 k so i was going to replace the plugswith acdelcos and was wondering what else i should do...are those codes for all gm vehicles or trailblazer specific

just checked my code so its a 3.42 rear axle could that aslo effect the milage.

3.42 will give you the best mpg's on highway not sure about stop and g traffic I live in wyoming and we dont have stop and go traffic. As far as the codes go I believe they are GM standard. The plugs should be inspected, that will tell you alot about how your vehicle is running but they should not need replacing untill 100,000. You could also check the airfilter and maybe switch to an upgraded one. I use a wix air filter frim Amsoil works great.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
Im think Im going to replace those LCA brackets Wednesday and then Im going to bring it in for alignment.

I do have the 4.10 gears however, Ive owned the truck since 2004 and this is the worst gas mileage Ive ever gotten (on average). :sadcry: Im hoping its the alignment throwing off my mileage. Keep you guys posted!
 

djthumper

Administrator
Nov 20, 2011
14,956
North Las Vegas
Dozer22 said:
Im getting between 15-17 mpg on the highway @ 68< >75mph

That is about what I am getting. Of course I am lifted...
 
Dec 5, 2011
578
Central Pennsylvania
I'm surprised no one mentioned it yet (unless I missed it). How's your fuel filter? On '02s we're supposed to change them every 30k - says so in the manual. Also, how's your throttle body looking? When's the last time it was cleaned (actually removed and cleaned).

Do both, with a PCM reset (total cost around $20 depending on the fuel filter and tb cleaner you buy) and see what happens (don't forget to let your HVAC control header do a recalibration so you don't wreck your HVAC actuators)..

I get absolute crap for mileage, but I know I need plugs, fuel filter, and air filter (106k miles with original plugs, fuel filter and air filter were done at 70k). I just haven't done them yet. Same with the wife's (we have matching Bravadas). I smile whenever I climb to 14 mpg, most days I'm in the 13s lately. I do have a 12 mile commute to work, on crap country hilly roads - so my mileage will never be great.
 

Matt

Member
Dec 2, 2011
4,030
Dozer, I also have 3.42's and I get about 22 on a 150 mile trip at 70-75mph to the in-laws cabin...better on the return trip home. It's got quite a few hills when we hit PA.

Go through the list...throttle body, check your plugs, try and get a real reading on the engine temp...don't rely too much on the gauge...your thermostat may have had the carrot.
 

TangoBravo

Member
Dec 5, 2011
208
Menthol said:
Man, if all those things don't improve your mileage, next step is to change your right foot :rotfl:

No doubt man.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
Well, well, well...you'll never guess what I discovered today :undecided:

During a checkup of my truck, I saw that one of my tires looked low. I checked it - 29psi. Actually, all 4 were at or right around 29psi. I had set them to 36psi about 2-3 months ago when I first started trying to track down the mpg issue. With the temperature swings since October, Im sure thats where my psi went. Tire pressures can swing ~1psi per 10f degree temperature swing.

So back up to 36psi per tire it went!! Im on "E" and that does not stand for exceptional, so when I gas up tomorrow I will see if my mileage improves any (and Im sure it will some). Lets see if I get the rest of my mpgs back.
 

Dozer22

Member
Dec 17, 2011
29
Menthol said:
Man, if all those things don't improve your mileage, next step is to change your right foot :rotfl:

nice definately true. if i drove like i stole it all time id expect bad milage, but ive been easier on this vehicle than others i've had. Im gonna try and check all the things mentioned in this thread and see if that helps at all. now i remember reading something on the OS about the thermostat . my gauge is always 1-3 lines to the left of straight up and down
 
Dec 5, 2011
578
Central Pennsylvania
Has your cruise control worked recently? I'm asking because cruise will not engage if the tcase is bound. A bound tcase will shoot your mileage in the butt.
Also, is your Torque Converter Clutch locking up properly? Lack of TCC lockup will cause your mileage to go down as well.
TCC lockup only occurs over 45mph, after reaching operating temp (so stuck tstat will prevent lockup), and while not engaged in 4wd or awd.
Test your TCC lockup by getting to a steady speed - say 55 or so. LIGHTLY touch the brake pedal while holding the gas pedal steady, if the RPMs don't go up by a few hundred RPM then TCC lockup isn't occuring.

When I picked up my bravada the awd tcase was bound, the tstat was tuck and I had no cruise and no TCC lockup. After the 2x drain & fill fix for the tcase, I still had no TCC lockup, but had cruise. A tstat change fixed that - now I need plugs, fuel & air filters and will finally have it all licked.
 

lynch55

Member
Dec 14, 2011
42
TequilaWarrio said:
I'm surprised no one mentioned it yet (unless I missed it). How's your fuel filter? On '02s we're supposed to change them every 30k - says so in the manual. Also, how's your throttle body looking? When's the last time it was cleaned (actually removed and cleaned).

Do both, with a PCM reset (total cost around $20 depending on the fuel filter and tb cleaner you buy) and see what happens (don't forget to let your HVAC control header do a recalibration so you don't wreck your HVAC actuators)..

I get absolute crap for mileage, but I know I need plugs, fuel filter, and air filter (106k miles with original plugs, fuel filter and air filter were done at 70k). I just haven't done them yet. Same with the wife's (we have matching Bravadas). I smile whenever I climb to 14 mpg, most days I'm in the 13s lately. I do have a 12 mile commute to work, on crap country hilly roads - so my mileage will never be great.

:hissyfit:Drats! You beat me to it! I know cleaning mine helped a ton!:yes:
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
Dozer22 said:
nice definately true. if i drove like i stole it all time id expect bad milage, but ive been easier on this vehicle than others i've had. Im gonna try and check all the things mentioned in this thread and see if that helps at all. now i remember reading something on the OS about the thermostat . my gauge is always 1-3 lines to the left of straight up and down

That definitely sounds like a bad t-stat and a big waste of gas.
 

ItsOnVoy

Member
Nov 21, 2011
2,401
MichEnvoyGuy said:
Im averaging 16.5mpg in the city and 17mpg on the highway. No CEL light, no pending codes.

atleast you get that! I get 13MPG average :frown:
 

Jkust

Member
Dec 4, 2011
946
I don't know. The MPH sounds like the killer to me here. If I go above 61mph on the highway, my mileage tanks. Thrown in just a little city into the mix and say hello to 15mpg. I hate driving in the center lane with the slow crowd, but it is the only way I can eek out 20mpg's in the 9-7. I don't think that even the 29psi is going to make near the difference of just slowing it down. That's the deal I made with the devil to drive a truck based suv with a v8. Plus it is a SAAB so people just expect me to be a lefty, idiot.
 

fishguy1123

Member
Dec 5, 2011
310
ItsOnVoy said:
atleast you get that! I get 13MPG average :frown:

Just get out and push for a couple miles.... that will help a lil! :rotfl:

Maybe I missed something but are these mpg readings from the dic?
 

djthumper

Administrator
Nov 20, 2011
14,956
North Las Vegas
Dozer22 said:
nice definately true. if i drove like i stole it all time id expect bad milage, but ive been easier on this vehicle than others i've had. Im gonna try and check all the things mentioned in this thread and see if that helps at all. now i remember reading something on the OS about the thermostat . my gauge is always 1-3 lines to the left of straight up and down

Replace the Tstat
 

envoyjoe

Member
Dec 6, 2011
25
i am getting 10 to 11 mpg in town and 18 to 19 on the highway i have 3.42 gears and im runing BFG KM2 tires 245/70/17 31inch with full body armor and offroad front bumper :frown:
 

gmcman

Member
Dec 12, 2011
4,656
I have the 4:10's and get 16-17 city,about 20-21 hwy.

When you fill up again, add a can of BG 44K then change your fuel filter after that tank.

Put your spare back in.
 

Fire06

Member
Dec 18, 2011
7,223
Just out of interest what is your cat like. If it was plug it would be hard on your fuel economy. Or maybe a few of the problems mentioned. Good luck finding the problem.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
gmcman said:
I have the 4:10's and get 16-17 city,about 20-21 hwy.

When you fill up again, add a can of BG 44K then change your fuel filter after that tank.

Put your spare back in.

Well I have the 4.10's too and thats what I used to get for mpg's :sadcry:
 

gmcman

Member
Dec 12, 2011
4,656
MichEnvoyGuy said:
Well I have the 4.10's too and thats what I used to get for mpg's :sadcry:

I figured but threw that in for a reference. The cat being or beginning to get plugged is a good suggestion as well.

Try all the above mentioned and see what happens.
 

Dozer22

Member
Dec 17, 2011
29
all really sound advice . can someone elaborate on why a tstat being bad effects the gas mileage

also what does BG 44K do for the cat
 

Fire06

Member
Dec 18, 2011
7,223
If the truck does not get up to operating temp the fuel doesn't burn proper which then puts unburnt particles into the cat which then plugs it up. The cat then does not get up to operating temp to burn off the particles which then slowly gets plugged. It happens over a period of time so you would not notice it till it is real bad.
 
Dec 5, 2011
578
Central Pennsylvania
a stuck stat also causes the PCM to add more fuel and prevent the TCC from locking up to cause the engine to work harder and burn more fuel in order to help it reach operating temp. These things really want 210 degrees or so - less than operating temp and you burn more fuel, no TCC lockup, and the shift pattern changes.
 

Guenther

Member
Dec 7, 2011
65
On a new to me '04 Envoy SLT with 101,000 that I bought for my son a month ago I've done the following:

changed oil over to 5W-30 Castrol full synthetic
changed oil filter to K&N
changed transmission fluid
changed front differential fluid using Mobil 1
changed rear differential fluid using Mobil 1
changed transfer case fluid using GM fluid
changed both O2 sensors
changed brake fluid
changed power steering fluid
changed coolant using Prestone Dexcool
changed fuel filter using Purolator
changed air filter using Purolator
changed front brakes to ceramic
changed front brake rotors
cleaned throttle body (was bad because PO did it about 10,000 miles ago and wasn't bad)
t-stat was changed by PO 10,000 mile ago (temp goes up and down between 201*-207* at speed using Torque program)
Seafoamed engine twice
ran bottle of Techron through tank of gas
tires are at 36psi and tracks straight as an arrow

First off, thank the Lord for the specials that have been going on at Advance Auto. Between the 40% and 50% off codes I've been doing tons of maintenance to the Voy, '08 Sable with 110,000 miles and '09 Tahoe with 42,000 miles. Secondly, with all that I've done to the Voy the mileage is around 17mpg highway and 14mpg city. I thought it should be higher, especially with the numbers that some people are throwing out here and on the "other" site. Were are into the winter gas here in Iowa so I'm hoping that is what is keeping the mileage down. From what I've read there isn't really anything else I can do maintenance wise to help. If there is, I'm all ears.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
Guenther said:
On a new to me '04 Envoy SLT with 101,000 that I bought for my son a month ago I've done the following:

changed oil over to 5W-30 Castrol full synthetic
changed oil filter to K&N
changed transmission fluid
changed front differential fluid using Mobil 1
changed rear differential fluid using Mobil 1
changed transfer case fluid using GM fluid
changed both O2 sensors
changed brake fluid
changed power steering fluid
changed coolant using Prestone Dexcool
changed air filter using Purolator
changed front brakes to ceramic
changed front brake rotors
cleaned throttle body (was bad because PO did it about 10,000 miles ago and wasn't bad)
t-stat was changed by PO 10,000 mile ago (temp goes up and down between 201*-207* at speed using Torque program)
Seafoamed engine twice
ran bottle of Techron through tank of gas
tires are at 36psi and tracks straight as an arrow

First off, thank the Lord for the specials that have been going on at Advance Auto. Between the 40% and 50% off codes I've been doing tons of maintenance to the Voy, '08 Sable with 110,000 miles and '09 Tahoe with 42,000 miles. Secondly, with all that I've done to the Voy the mileage is around 17mpg highway and 14mpg city. I thought it should be higher, especially with the numbers that some people are throwing out here and on the "other" site. Were are into the winter gas here in Iowa so I'm hoping that is what is keeping the mileage down. From what I've read there isn't really anything else I can do maintenance wise to help. If there is, I'm all ears.

Well, 14/17mpg is what Im getting too and my Envoy is very well maintained. :frown:
 

Forum Statistics

Threads
23,444
Posts
639,434
Members
18,668
Latest member
Zee0719