Gas Mileage is decreasing

[rt9bil]

So this spring when I put my 20's back on I was getting 16.1 miles/gallon. Over the past 4 month its been dropping...today I checked it and its now at 14.9 miles/gallon. Anybody know what the problem could be??? It's an '02 4.2L w/ 52K.

[Blulytes]

Its all those hills in N-town that are killing you!

Have you done all the normal checks? Filters? Change of driving habits? I'm always pretty consistant... at 13.8!

I have a pretty heavy foot on 76 and 422!

[rt9bil]

Its all those hills in N-town that are killing you!

Yeah I hear that. The air filter was recharged about a month ago...but other than that I haven't changed anything else.

[Kahunakai]

using more a/c lately?

[ylab]

Ah, but was that 16.1 with the 20s or from before they were installed? Larger tires usually result in a slight loss in fuel economy, since they weight more and may have a higher rolling resistance due to the increase in width. (Some brands also have a higher rolling resistance than others).

Assuming that the 16.1 was with the 20 inch tires, have you checked their pressure recently....is it the same as back in the spring?

Could their be more brake drag than in the spring, perhaps some rust build up or warpage on the rotors?

Have you changed gasoline brands recently.....I've noticed differences with different brands.

Check the throttle butterfly, mine gets incredible gummed up and I clean it down good every few months with some carb cleaner and run some injector cleaner through at the same time. (If the vapor can gum things up like that, image what the injectors look like!) Prestone & Chevron cleaners both work great.

Didn't you post about putting on an exhaust a couple of months ago? .... If so, could you be hitting the gas harder from a stop due to a slight loss in low end torque? The mid-high end power increase is sometimes accompanied by a low end (below 2500-3000 rpm) decrease in torque if too much back-pressure is take away. (Totally free flowing isn't usually the most efficient in normal driving).

Those are the things which come to mind off the top of my head anyway, hopefully it helps. I've actually noticed my Envoy getting slightly better mileage lately....either the Borla is helping or my foot has gotten slightly lighter with the increase in gas prices.

[rt9bil]

Ylab thanks for all the info. I checked a few things out...first off, my tire pressure was low in both front and back driverside tires. Second, the crap that has built up on the throttle butherfly was nasty!! So I cleaned that up and then dumped some injection cleaner in the tank also. We'll see what happens. As for the exhaust, I've had it about 3 months now and I also added drilled and slotted rotors and ceramic pads at the same time. So hopefully theres nothing wrong w/ them. I've been thinking about my driving habits and realized that they have changed drastically since May. I used to do alot of highway driving and now i'm doing more city driving. So I figure thats probably the main cause.

[OurZoo]

I'd make it a habit to clean out the multiple airflow sensor whenever you recharge your air filter. That probably wasn't the main reason for your lack of fuel economy as you noted. But as you found out, that oil and other crap does get into your system.

[autotech]

just for informational purposes for all of you readers out there. the envoy (I can't speak for the TB's) do not have a Mass Airflow Sensor. They are a speed-density setup. in '06 they added the MAF Sensor.

As a tech, I usually suggest cleaning the throttle bore and butterfly and performing an induction service every 12-15k. You should also have the injectors flushed out a minimum of every 30k. But that is just my opinion. :m2:

[dpm6836]

How exactly do you clean the throttle bore and butterfly?
Thanks.

[ScarabEpic22]

OK, now youve got my attention, what is done to clean the injectors? How invloved of a process is it, and are there any special tools needed? I feel I am pretty competant when it comes to these trucks, but there are some things Im not going to stick my hands into.

TIA

[homertime]

I think some of this stuff would make a great article.

I unfortunately don't know how to do it!

Anyone out there with some automotive experience want to help out?

Also make an article on changing your own oil?

[TheAlchemist]

Greetings guys... this is my first post here...

The wife recently got a Saab 9-7x (4.2i) and we're seeing right around 17.9 mpg doing around the town driving. Not too bad considering the weight and size of the vehicle and the performance as well.

Anyways, run seafoam or a gm top end cleaner through to clean out the top end and your mileage should go up a couple mpg. I do this every 10k in my Trans Am and find it helps out a lot.

[Blulytes]

Welcome to the site... thanks for the info!

[dh4645]

i was getting 16.1 mpg avg, then put on my 22's and now it's 15.9.
i am pretty happy with that

[CarbonEnvoy]

I just cleaned my throttle bore and butterfly last night with some carb cleaner. There was quite a bit of gunk in there at 15,000 miles.

I was a little apprehensive since I've never done it before, but it wasn't that difficult. The biggest pain is removing the resonator on top of the engine. On my '04 it there were only 2 bolts on the intake side securing it. There was also a hose on the bottom near the front that needed to be unhooked, and a sensor wire on the front left side that pulls off.

Once the resonator is off it's pretty easy to get in there. It was a little tricky holding the butterfly open to clean the bore. I'd recommend not using paper towels since there are too many edges that could tear off a piece that could fall into your engine. Also need to be careful not to scratch anything. I just used a clean cotton rag, turning to a clean part until I saw no more gunk on it.

Also recharged my K&N and already had a bottle of Techron in the tank. Hope to see a difference in the next couple fill-ups!

[02NYenvoy]

Carbon Envoy, I did the same with mine a couple months ago, after that I started to get rough idle constantly, there was a lot of buildup, I used Valvoline Syn throttle body cleaner with a terry towel to clean it out. I went to the dealer to see if they can find the fault not telling them what I did, the first thing he said was maybe the throttle body is dirty and it needs cleaning. Then he ask, did you try cleaning it yourself? :no: :yes: :( . Well yuh damage the sensor!! :duh: , if you force open the butterfly, a lot of damages can be done to the electronics.....anyways, I have extended warranty so he's ordering a replacement throttle body...that's just an experience I'm sharing with the forum... ;)

Taking in the Envoy for throttle body replacement tomorrow 9/9/05, I will ask the tech how exactly to clean the darn thing.....update soon.

[ENVOY2LOW43]

Thanks for the info...almost cleaned mine today :duh: ... I'm gonna wait for your feedback. :yes:

[CarbonEnvoy]

02NYenvoy,

Dang, that's not what I wanted to hear. I was really careful with the butterfly, but I don't know if I'll clean it myself again. I guess I'll wait to see if your new throttle body fixes the idle problem.

For the record, my Envoy ran really, really good today. No rough idle to report either. Not sure how much I can attribute that to cleaning the throttle body, the K&N recharge, or the 40 miles of driving with Techron in the tank.

Good luck with your service experience and thanks for the warning...

[rt9bil]

Some D@mn good info. in this post. Thanks guys!

[02NYenvoy]

I will get some more positive info for you guys tomorrow about cleaning the TB. I trust this dealer that's why I drove 1hr to Greenbrook, NJ (where I bought my voy in Jan'05) for them to do any service on my Envoy.

Left the Envoy in the parking lot, drop the keys in the service slot and left....when I got home I realize that I didn't lock the car :duh: Called Onstar right away, within 10mins they called back ..."your car is now locked" :) worth my $17 per month subscription already!

I will check on the TB work tomorrow and report back to the forum.

[Envoy4ever]

This is my first post, I like it here, there is so much inforamtion... :eek:

I had an '02 SLT with +/- 60K on her. I was getting a avg of 19.6 mpg!! :yes: BTW everything was stock. Unfortunately my wife had a roll over two weeks ago and it was totalled :( She is fine :)

Good news is that three days later, I had her twin in my driveway :D For me it was incredible, the only differences are the grill, the radio and the wheels!!


To this one I reseted the DIC and started getting around 15 mpg but is has risen. On tuesday (09/06) it was at 18.5 and today (friday) is at 19.0 :) Again I do have everything stock and found out tonight that the air filter is crap, so I'm going to change it tomorrow, it should further increase mpg's.

VERY Impotant I have changed my driving habbits since the price of gas went through $3.00. Interistingly enough, I found, what I think, is the sweet spot for increasing mpg. What I do is very simple, my daily communte is about 37 mi on I93, so I get up a little early (to avoid rush hour traffic) and I go on cruise control set @ 65 mph, it is giving me an inst mpg of 18-25 mpg. My ride is more or less flat , no steep hills or anything. So I guess 65 is the compromise between speed and fuel economy. Any thoughs? :undecided

[Uncle Bill]

I have an 05 TB, 4.2. I live in colorado at6500 ft. With 1600 mi on eng I average 18.5 to19.5 MPG. 16 mi to work w/about 12 of it hwy. Wnen I get to3000 mi I am going to get a K&N or Airaid and switch to synthetic oils. I hope to get 20 mpg average or better. BTW I run 60 mph on hwy. Got any suggestions for better MPG?

[ieatglue]

I have an 05 TB, 4.2. I live in colorado at6500 ft. With 1600 mi on eng I average 18.5 to19.5 MPG. 16 mi to work w/about 12 of it hwy. Wnen I get to3000 mi I am going to get a K&N or Airaid and switch to synthetic oils. I hope to get 20 mpg average or better. BTW I run 60 mph on hwy. Got any suggestions for better MPG?
You could also replace your stock muffler and remove the spare tire

[itchyfishnv]

i clean my throttle body every oil change, i use a long screw driver to hold it open, while i spray and scrub with a toothbrush, and then start it, let it run and repeat the process.. you can see all the gunk goin out the exhaust. oh yeah do this when the truck is hot. The tb spray will eat away at the gunk alot better when its hot. After i assemble everything i usually take it out and open it up a little bit.

[ieatglue]

i clean my throttle body every oil change, i use a long screw driver to hold it open, while i spray and scrub with a toothbrush, and then start it, let it run and repeat the process.. you can see all the gunk goin out the exhaust. oh yeah do this when the truck is hot. The tb spray will eat away at the gunk alot better when its hot. After i assemble everything i usually take it out and open it up a little bit.
Can you explain how to clean it, step by step, and if i still have my bumper to bumper warranty, is this covered too? :undecided

[tb_ky]

I cant seem to get above 17 in mine. It has nearly 100,000 miles so I should probably clean out the throttle body. Someone told me that my o2 sensor may be going bad, replace the spark plugs and get some some injector cleaner or something to add into the fuel. Im gonna do this and see if it helps.
How do you clean the throttle body anyways? I need some step by step instructions so i dont screw something up.

[tblazin18]

i was wondering about how to clean it as well. im getting 15.8 mpg right now with 22's. I also just replaced my air filter with a drop in k&n. but the mpg hasnt gone up yet. How long does it take to recalculate?? what is normal mpg for driving on 22s??

[Blulytes]

just reset your mpg on your DIC...
(press and hold the "return" button)

[ieatglue]

I get 12.7 MPG ... and that's after using fuel ingector :o

[ScarabEpic22]

I get 12.7 MPG ... and that's after using fuel ingector :o
But, you've got an EXT...

[tblazin18]

oh ok i'll go do that now. but is that good gas mileage for 22's?

[ieatglue]

But, you've got an EXT...
Oh, right...but that's a pretty big difference, oh well :D

[tblazin18]

ok i reset my mpg. but how long does it take to for it recaluate your avg.mpg.??? right now it keeps changing, is it suppose to?

[05redtb05]

In order for it to even out and give you a good estimate you will need to drive it for about 2 weeks. I got my tb about 3 months ago and it leveled out at 16.2 after about a month.

[quadhauler]

First post... I hate to complain, especially on my first post here, but I have to. :raspberry It caused me to search and find this forum.. one which I have found very helpful over the past 2 days until I could get to my e-mail and activate my account. Anyway, I looked at several different vehicles before choosing the TB in 2004. I looked at Expeditions (too big), Explorers (too small) and Armada's..... I really didn't want another rice burner. :worried: However, I think I may have made a wrong choice. It pretty much came between the Armada and the TB... the Chevy won. Three reasons: Price (rebate), Seat Comfort and Mileage. The Armada stickered at 13mpg city and the good ole TB 16... EXT with 5.3! Cool, I'm getting a Chevy. Wrong answer. Mine gets 13 on a good day and usually mid to high 12's. What gives? Why did it say 16 on the sticker and from what I can tell, everyone on here thinks that is okay. I know the TB is pretty big and has some power, but my 93 Ext Cab truck with 5.7 (and I drive hard) gets 19. Big difference in technology here.

Earlier, I saw where someone said the I6 needed the throttle body cleaned periodically... mine use to do better than 12.whatever... is the 5.3 the same? Thanks for the help and I look forward to being active on this forum. :cool:

[ScarabEpic22]

Give it time. As your engine breaks in, your mileage will go up. My 02 TB LT when I bought it new in Jan 02 was only getting about 12-14mpg avg in the city. Now, Im constantly in the 16-18mpg range, sometimes higher if there isnt a lot of traffic. Highway mileage goes up too, started at about 16-18, now its 20-21. I noticed it was getting better around 20,000mi. Heard it takes other engines (I have the I6), such as the 5.3L V8 different amounts of time to break in.

P.S. Welcome to the site! :D

[itstrick]

First post... I hate to complain, especially on my first
Earlier, I saw where so...............



--Welcome to the fourms--

[quadhauler]

Thanks for the welcome guys!

[EME]

A few times I've done a test to see how much gas I'd save if I were to drive a little more gracefully. My test for this is to always keep my tach under 2000. I'm typically a fairly agressive driver and hit the gas at the lights. The strage things was that I actually used MORE gas with the lower speeds!

What is the deal with that?

[quadhauler]

Well, I'm not sure how long it takes for a reflash to take effect and the PCM to get adjusted, but the past few tanks haven't been too bad. We actually saw a 14.75! Not too bad, but still not 16 like the sticker said. (I can never be just plain happy. :duh: ) However, I can definately tell it is running better. This past weekend we trailered with it. A 20' trailer (tip to tip) with 2 sport quads, 1 mini quad and 1 dirtbike... plus all the riding gear, tools and spare tires that you need with that. We drove from the Mississippi delta to the foothills of the Ozarks. I had it on cruise most of the way up at 72-74 and I could definately tell the TB was having no problem towing.... as compared to before the dealer reflash where a headwind was causing a downshift on flat ground. Anyway, with a little over a half tank used for this portion of the trip it did 13.8. I have to say that I am HAPPY with that. Coming home we filled up and I had a tendancy to get a little heavy footed, mainly due to the traffic on I-40 in the afternoon. Sometimes, it was 72 others it was 85... and a peak of 93. I know the mileage will or should stink on that, but I'll post it when the tank is done... should be today. No comment about the top speed.. dumb I know... but that d@#$ truck! :eek: