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Gas Mileage is decreasing

9K views 39 replies 24 participants last post by  quadhauler 
#1 ·
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.
 
#5 ·
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.
 
#6 ·
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.
 
#7 ·
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.
 
#8 ·
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:
 
#12 ·
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.
 
#15 ·
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!
 
#16 ·
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.
 
#18 ·
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...
 
#20 ·
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.
 
#21 ·
Gas Millage!

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
 
#22 ·
GAS MILEAGE

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?
 
#23 ·
Uncle Bill said:
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
 
#24 ·
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.
 
#25 ·
itchyfishnv said:
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
 
#26 ·
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.
 
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