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I have a 2008 G6 that I purchased in Oct of 2008 it had 13,048 miles on it when I bought it.
I am having a HUGE mileage issue with it, I am getting about 11MPG in the city. I never really kept track of what I was spending in Gas until recently.
I was filling up at about 1/2 tanks and getting about 93 miles. So when i questioned the dealer about it they basically said that it was not possible that the car was getting that kind of mileage and they wanted me to fill it, Mark down my mileage and then drive it til empty go back to the same gas station & same gas pump and then figure out the miles used and figure it out manually. (Which is what I was doing anyway) But I was not letting it go to empty. I do not understand what the differance is if I fill at half tanks or full tanks as it is Miles driven to gal used.... SO to humor myself I filled the car drove it until 1/4 tank of gas and hmm... got the same about 11 MPG in the city.
When I returned to the dealer, They basically said again that it is not possible to get that mileage and not have a check engine light on, or to have a black tail pipe. They did book the car in and said that they checked the converter, ect. They said that they called GM and that there are no known issues. I was asked again to pick the car up and drive it until the gas LIGHT COMES ON and then go back to the same station and refill the car. I am not sure how doing this is going to make any differance. I took the car back and I am still on that tank of gas, I am at about 111 miles and have burned approx 3/4 tank of gas. so it will not be long before my gas light is on I'm sure

I do not drive my car hard, nor do I put alot of miles on it but they are all city, Nor do I idle for long periods of time besides being in a drive thru ect. I have owned the car for 2 years and have put about 4970 Miles on it.
They did call me yesterday and said that they have no idea and are at a loss at the moment. I told them that I have NO PROBLEM dropping the car off for them to take and have someone drive it for a tank of gas and they could see for themselves. He said well it may have to come down to that.

ANYONE HAVE ANY INFO OR IDEAS ON WHAT COULD CAUSE THIS?
Any Info would be greatly appreciated... :)
 

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Moderator Member Thingy
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Leak in the gas tank? Damn I don't know what to tell you.
 

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Nope no leaks, Thought that maybe someone was stealing it... So I bought a Locking gas cap.. ughhh...:)
 

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I did have it in and they showed nothing... Said that fuel (Something) cant remember what thatwas but it was normal as well... I will post my latest reading tomorrow when I fill the car up from empty...:)
Thanks Guys
 

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O2 sensor? sensor thinks theres more oxygen in the exhaust than there really is so it dumps more fuel to increase the power output.

Mike
 

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Discussion Starter #7
Gas Consumption

O2 sensor? sensor thinks theres more oxygen in the exhaust than there really is so it dumps more fuel to increase the power output.

Mike
Hi Mike...
Thanks for the info... I just read the last service repair tha I have and it says the folowing... Scanned vehicle - No codes, Checked T-Stat operation over 196' F Good. Checked Fuel Trim +/- 2 Good. Checked Bulletins for this concern non found. No Fault found at this time.
I have to add to this, I wa there today as I had to Fill my car today and I mustsay the nembers still SUCK...

I got 249.7 Kms and used 50.991 Ltrs of gas. Which works out to 20.4 Ltrs per 100k.
converting that to MPG is 155.11 Miles to 13.47 Gal of fuel used = 11.5 Mpg... They said that they have no idea...they did tell me today that they have checked the O2 Sensors and were fine but i dont see them mentioned on the work order unless the are covered under thefuel trim or something.

I am SO AT A LOSS and it is costing a small fortune to run the car... I even said tothem today again.. Give me a car and you can take mine and the forman or whoever can drive it for this tank of gas ans they will see... All they say is well it may come down to that.... UGHHH>...

Do you or anyone know about a fel chip that is in the main computer board? i was told that it is possiable that there is a chip in there for a 8 cyl and thatthe car isactually buring the fuel and that is why the check engine light would not come on.. DOnt know enough about it to know if that is true or not..?

Thanks for all the help!!!!
 

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Make sure there's no leaks in the air intake tube between the MAF sensor, and the throttle body.
Leaks can cause the PCM to incorrectly calcuate the fuel charge.
 

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Discussion Starter #10
Gas

Not that i am aware of.. had the car fully serviced not to long ago and they checked brakes ect... I dont feel any restrictions... But could have them look...
 

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Has this dealer hooked it up to any diagnostic equipment?
A screwed up torque converter can potentially cause poor gas mileage, does the car feel sluggish at the throttle, or when shifting?

If it was running that rich, you would be able to smell it.
 

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Discussion Starter #12
Just what wthe service invoice says as far as I know... "Scanned vehicle - No codes, Checked T-Stat operation over 196' F Good. Checked Fuel Trim +/- 2 Good. Checked Bulletins for this concern non found. No Fault found at this time."
I dont know how they check the fuel Trim?

The car does not feel sluggish or when shifting anymore.... I say that as they have already replaced the torque converter.. it used to chug really bad on the freeway and shift REALLY hard when going from 50k to 60K...

I wonder if it could be related? They told me for the longest time that was normal as well... and that it was the TCC kicking in... But then after taking it out a couple more times they felt that there was an issue and investigated...
and that is wat they did to stop it...
 

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I just read your first post didn't see where you had posted the scanned values.
It appears that it's not fuel related, as the fuel seems to be on target @ operating temps. I would investiage the torque converter further, especially since you now mentioned that you had issues with it in the past. Think of your torque converter as the link between the flywheel and your tires, if it slips excessively your gas mileage will definitely see a drastic drop. It's like driving a manual trasmission car and riding the clutch. (not a good thing)

Always remember that hard shifts are better than soft shifts. Soft shifts = more friction which = more heat as the clutches slip with one another till full engagement. Soft shifts also reduces transmission life. Your dealer should be able to check your torque converter, they need to make sure that it's locking up properly, and at the right speeds.
 

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Verify that your torque convertor is transmitting power properly. You don't want excess slippage there.

At idle, in Drive, briefly apply and hold the brake.
While the brake is fully depressed, the push the throttle to the floor, until the revs stop climbing. The engine should groan steadily for a bit.

Your tachometer should show about 2400rpm, at the most, during this test. If you see 3000, 3200, 4000rpm, then you've got a slipping torque converter. If you can only raise it to 1200rpm or so, you have a converter that is too "tight" or low engine output.

With the brakes fully pressed, the car shouldn't move when you hit the gas. But do it in a safe place just in case. Don't hold the brake and gas down together for more than 10 seconds.

Only do this with the engine and tranny fully warmed up. That's the only test of the torque converter that you can do yourself... besides just feeling for it's operation.
 

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Discussion Starter #15
Gas

Thank u for the info. I will try this tomorrow.
i had the car in since my last post. They had the car for 4 days and a tech took the car home, back and forth... I told them that I wanted them to drive the car the way I do in the city. They claimed that the driver drove city with a bit of driving on a parallel rd to the freeway and that he put 60kms on it each way which was 120k a day.
need less to say they got 400 kms on a 1/2 tank and they said that was normal... They were sitting at 7.8l per 100 k. and the avg speed was 35kms.
I found that interesting and said that it is impossible for them to be driving it in town as I have NEVER got that EVER. They told me that the cause is the "shorter in town Trips" Because, I do not drive to work everyday and that when I do go out I am going to say 15- 20 kms. The other thing they claimed is to much idling. Well I can tell you I am not in the HABIT of sitting in my car and letting it r
I took the car back and tried driving a little different, Every time I started going out I tried to use the hwy as much as I could. Gas was a bit better but the 35miles per hr started dropping. and the consumption started rising. Then I thought, This is crazy. I am adding extra time to every trip and going out of my way to try and get "good In town Gas consumption".
I since filled the car and started driving it back the way I "normally do" and I am back to the crappy numbers, In city 250kms on a full tank. and the avg speed has dropped to 25.
I had my mother try a little test. She drives her 98 Malibu 6 cyl the same way that I drive my G6. Actually she takes shorter trips. Only going to the food store and back every other day and she is getting 500kms on a tank which is 56.8 ltrs.
So there theory of it being because of Shorter driving tips is a load of BS.
I was wondering if it is possible that driving between 50k and 60k if the TCC could be burning an access amount of fuel?
I know that when I had the transmission issue that it was hitting hard and chugging around 50kms and if I was on the hwy and pressed down on the gas it would chug. That's when they replaced the transmission, But I do not have any of those issues now as far as chugging or " Hitting hard when it shifted around 50k" but am wondering if it is not consuming to much gas when being driven in that range?

I will however try the converter test tomorrow and advise.
If anyone else may have some other suggestions that would be great.

Thanks
 

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i have that same problem, however my gas mileage isnt as bad but its not good. Dealer said everything is running fine O2 sensors etc. found a small leak in one strut and starting to see wear on my axle seal but nothing fuel economy related. i dont know what to do either. my car can be sluggish at times too. let me know what you find out.
 

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o and i tried the torque convertor test it would only rev to 200k rpms. not sure what tht means for my car tho in your previous post didnt quit say. What did you get when you tested yours?
 

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Thank u for the info. I will try this tomorrow.
i had the car in since my last post. They had the car for 4 days and a tech took the car home, back and forth... I told them that I wanted them to drive the car the way I do in the city. They claimed that the driver drove city with a bit of driving on a parallel rd to the freeway and that he put 60kms on it each way which was 120k a day.
need less to say they got 400 kms on a 1/2 tank and they said that was normal... They were sitting at 7.8l per 100 k. and the avg speed was 35kms.
I found that interesting and said that it is impossible for them to be driving it in town as I have NEVER got that EVER. They told me that the cause is the "shorter in town Trips" Because, I do not drive to work everyday and that when I do go out I am going to say 15- 20 kms. The other thing they claimed is to much idling. Well I can tell you I am not in the HABIT of sitting in my car and letting it r
I took the car back and tried driving a little different, Every time I started going out I tried to use the hwy as much as I could. Gas was a bit better but the 35miles per hr started dropping. and the consumption started rising. Then I thought, This is crazy. I am adding extra time to every trip and going out of my way to try and get "good In town Gas consumption".
I since filled the car and started driving it back the way I "normally do" and I am back to the crappy numbers, In city 250kms on a full tank. and the avg speed has dropped to 25.
I had my mother try a little test. She drives her 98 Malibu 6 cyl the same way that I drive my G6. Actually she takes shorter trips. Only going to the food store and back every other day and she is getting 500kms on a tank which is 56.8 ltrs.
So there theory of it being because of Shorter driving tips is a load of BS.
I was wondering if it is possible that driving between 50k and 60k if the TCC could be burning an access amount of fuel?
I know that when I had the transmission issue that it was hitting hard and chugging around 50kms and if I was on the hwy and pressed down on the gas it would chug. That's when they replaced the transmission, But I do not have any of those issues now as far as chugging or " Hitting hard when it shifted around 50k" but am wondering if it is not consuming to much gas when being driven in that range?

I will however try the converter test tomorrow and advise.
If anyone else may have some other suggestions that would be great.

Thanks
Hmmm. I know how the dealership can be, when addressing problems that are not easy to solve. The first thing out of their mouth is always, "it's your driving style" or "they're all like that" or "it's characteristic of the vehicle."
I went through a similar nightmare with Honda a couple years back. But I stayed on them with a vengeance until they corrected every single problem.
As long as it's not to your liking, keep bringing it back. I assume that you are under warranty??
Unfortunately, you may have to do a little leg work yourself, to get them headed in the right direction.

Let's check some things. You have the 3.5 V6 right?
Does your radio have the trip computer included?
What does the display show your AVG MPG @ right now?
Using the INSTANT MPG, track what your mpg is at various (steady) speeds.

At a steady 35mph, 50mph, 60mph, 70mph? I've always been very impressed with the 3.5's highway economy. At 65 you should see, atleast 26mpg on the display. 28-30MPG is what I've always seen indicated at those speeds.

In spirited, city driving. I *always* get 17 or 18mpg indicated by the trip computer. This is on *every* recent GM 3.5, 3.8, 3.9V6 I've driven. If I am more sparing with the throttle, I may see 19mpg indicated for city driving.
So I can see how city mpg could be a little low. But on the highway, these engines, with their healthy low end torque and tall gearing, really shine (rarely falling below 26 MPG AVG).

These are things you may have to do (but your dealer should have already):
Jack up each wheel and make sure it spins freely, without drag.
Check your tire pressure.
I'd replace the fuel filter.
Run a quality fuel system treatment through the gas tank.

Did this car ever get good mileage? Give it a good listen for any strange sounds from the engine and tranny. Both should be relatively smooth.

You say the dealer got 7.8lpk? Whoah! I just converted that... 30.1 mpg!! No way. In the city? That's just bs, for sure.

I got my best results with the dealer by taking the service technician on a ride with me and pinpointing each of my problems. The ride was NOT over until he confirmed that he could see/hear exactly what I saw and heard.

Verify that your trip computer is somewhat (they are rarely perfect) accurate. Pick out a test loop (near the dealer). At the beginning of the test route, reset the computer. At the end, note the mileage. We already know that you're gonna get less than the claimed 30.1 mpg the dealer claimed.
Repeat the procedure, but this time with the dealers tech driving. If there's no problem, his numbers should be considerably higher than yours. And that's with a similar driving style, driving normally, not trying to artificially raise the mpg.

But if he get's bad numbers too, pin him down on it and stay on them until they fix it to your satisfaction.

The other thing though... there's gonna be variation in every engine design. In a perfect world all G6 3.5's would get the same mileage under the same conditions. But in reality power, mpg and driveability can vary by a considerably large margin from engine to engine while being in perfectly good shape. Your particular engine may just be one that is gonna deliver low mpg. It happens.

But 11mpg is outside of any margin that could be considered normal. That's extreme. Did it get good mileage before? Ever?

Did you get this car used? Beware that there is a good chance that a used G6, may have been a RENTAL CAR in it's former life. That in itself opens up a whole can of worms. Rental cars get regular service, but they also have a hard service life.
They come off the delivery truck and go straight to the rental lot with, often, less than 2 miles on the odometer. Then they are rented and immediately taken on 500+ mile freeway trips non stop. Or they get thrashed around town. A NEW engine needs a proper break in, if it is to produce respectable mpg, power, and reliability over it's lifetime. Long trips with steady rpms is not good for a virgin engine. High rpm, high throttle is not good for a virgin engine. But this is exactly the kind of service these fleet units see. Even a regular used car may not have been treated well before you received it. Just something to thing about.

So many factors can effect your mpg. Yes, it's gonna be hard to find out what the problem is. But the dealer should acknowledge the problem AND be most helpful in resolving it.

Keep us posted.
 

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After reconnecting my battery (I replaced several of the under-hood wire loom covers) my consumption meter was reset. After a week of pure city-only driving, I'm running about 15 mpg. This is nearly 15 l/100k. On an economy run, I have been able to get 33 MPG / 7 l/100k driving for 62 miles (100k) setting the cruise for a constant 61 mph (basically 100k). The consumption dropped a bit at 67 mph by about 2.5 MPG.

This is with a G6 GT with the 3.5 l engine. The GT has lower gearing and thus the consumption is higher than the G6 non-GT or the 4-cylinder engine. With a typical tank of fuel, I average around 18.7 MPG (12.8 l/100k) as most of my driving is city. I usually behave pretty well, but we have a lot of 'smart streets' that are actually pretty stupid, so I spend a lot of time idling.

~ MattInSoCal
 

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Discussion Starter #20
Verify that your torque convertor is transmitting power properly. You don't want excess slippage there.

At idle, in Drive, briefly apply and hold the brake.
While the brake is fully depressed, the push the throttle to the floor, until the revs stop climbing. The engine should groan steadily for a bit.

Your tachometer should show about 2400rpm, at the most, during this test. If you see 3000, 3200, 4000rpm, then you've got a slipping torque converter. If you can only raise it to 1200rpm or so, you have a converter that is too "tight" or low engine output.

With the brakes fully pressed, the car shouldn't move when you hit the gas. But do it in a safe place just in case. Don't hold the brake and gas down together for more than 10 seconds.

Only do this with the engine and tranny fully warmed up. That's the only test of the torque converter that you can do yourself... besides just feeling for it's operation.
I tried this test today and got 2000rpm... :)
 
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