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WTH?

4K views 29 replies 15 participants last post by  Belo 
#1 ·
I have a 2009 G8 GT with 93,000 miles on it. No codes showing and running fine except for one thing....I'm only getting 5.6 mpg city and 16 hwy. What could be the culprit? Latest OEM parts added were a radiator, upper radiator hose, radiator cap, battery, brake pads all the way around. Non OEM was a new full headliner.

Guys can you point me in a direction that I can resolve? :-(

BTW, no leaks in any of the lines on a visual inspection, no loss of power. Running Sunoco 93 octane always.

Thanks
Ras
 
#2 ·
Is the car all stock other than what you wrote above?
 
#5 ·
Are you calculating MPG's by just reading the cluster display or are you tracking your miles driven divided by tank fill up's and taking an average? 6 MPG is very low even for city driving. No mechanical issues with the car?
 
#6 ·
I'm using the the calculator that comes with the car that shows you average speed/mpg. When driving I can actually watch the fuel needle drop substantially. I've used this this since I've had the car and there were no issues up until now. My visits to Sunoco are all too frequent compared to before. Thing is I'm not even goosing (foot to the floor) the vehicle. It's a beautiful G8 I get compliments on it all the time. Has never been run in the snow and rarely in the rain.
 
#7 ·
i'd verify using actual math in gallons and your miles traveled first to rule that out. You should very easily notice 6mpg on your wallet though. You'd be burning through gas ~3x your normal rate.
 
#9 ·
Thank you for responding Belo.

Like I said in my previous post, I can see the fuel needle dropping as I drive and my visits to Sunoco has increased tri-fold. I've had this car for way to long not to notice a huge change in mpg. It now eats up 3/4 of tank like its nothing. I did the mileage and from 3/4 of tank, usage miles was 47...ugh. No leaks, no codes coming up on my DBBII. When this started happening I thought someone or thing had punctured my gas tank so I looked under it after a fill ,a short run, let it sit for a minute and look under........Nothing! This is driving me carazy and my pocket too.
 
#15 ·
You're welcome. At the very least would inspect a spark plug from each side. Check the air filter and for vacuum leaks as well.
 
#16 ·
Will do as per your instructions. The air filter doesn't even have 1,500 miles on it yet. I'll check for vacuum leaks, pass side plug, and driver side plug. If the plugs look funky what type do you suggest for my block? I had replaced the air filter with a high flow but it just created many issues so I reinstalled a new OEM type.

Thanks for taking the time to respond.
Ras
 
#21 ·
Have you cleaned the MAF? Make sure the plug is tight? Perhaps something got bumped while doing the radiator hoses?
 
#22 ·
Hopefully spark plugs as that would be an easy fix. But for the mileage to dive like that, I would expect misfires but none are detected. My best guess is that the engine is stuck in cold warmup mode, so monitor the engine temperature Readout. Could be the sensor or the thermostat stuck open.
 
#23 ·
also check short and long term fuel trims, it wont set a light until + - 15%, at 10% fuel increase from just a vacuum leak would be substantial
 
#25 ·
Do you possibly have a stuck caliper piston or slide that's dragging a brake pad (or multiple)? It's a heavy, powerful car so you might not notice some drag. My wife's forester had stuck caliper pistons on multiple occasions and aside from a gas mileage drop and some infrequent vibration, you couldn't tell anything was off. The ultimate "tell" was that I went around the forester and felt the face of the wheels after driving and the stuck caliper corner had a hot wheel. On a separate note, when recently replacing my front brakes on my G8 GT, I noticed that one of the caliper piston boots had a small tear so it will probably rust and stick sometime soon.
 
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#26 ·
I can’t believe no one has mentioned this yet, but it sounds like an injector or two are stuck open. If you have a backup vehicle, I’d pull all 8 injectors, and send them out to be flow tested and cleaned. And they’ll let you know if any are bad. I drive my Caprice like a madman and get 11.9 with DOD disabled. 5 mpg would be average for a turbo LS running e85.
 
#27 ·
I am really sorry you are experiencing this, the Pontiac G8s are already hungry little beasts without this happening. My knee jerk reaction was also to configure mileage on paper, only because my issues are currently electrical and sensor malfunctions. I had Stryker up to a 123mph* yesterday and my needle didn’t sink as bad as yours. There are so many brilliant suggestions here. Is there a way for me to save this thread? I need to know all these possible issues to look at.

*Sabrina is a bad person, don’t be Sabrina. Any slight irregularities on the driving surface could cause catastrophic results, including death.
 
#28 ·
You mentioned new brakes.

We're they installed correctly? Correct parts? Brakes dragging? Emergency brake adjusted correctly?
 
#29 ·
Id be looking at something that controls fueling. Do you have cats? If its burning that much fuel they must wreak. Cleaning the MAF is a quick cheap check. The fact there are no codes would tell me that whatever is wrong, the pcm thinks its normal or correct. Like a bad upstream O2 sensor giving incorrect feedback, dirty or leaking fuel injectors. I had a dying air fuel sensor in my cobalt ss and it would run normal, id go down a hill and then the sensor started reading full lean, added 30% fuel, mileage dropped way down, then it came back .

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