My dad has 2005 Impala SS. BONE STOCK. Over the past few month, the gas milage has decreased from the normal 19-21 he was getting to about 15-16 MPG and now to about 11-12 . This has never happened before, so I know its not because of a change in gas or the temperature changing or anything.
So, we have cleaned the tb and the TPS and IAC sensors, replaced the gasket, replaced the fuel and air filters, checked the spark plugs (they still looks new) check the o2s they look good and thorughbred chevy out here in PHX has had it hooked up to the tech II twice. they even did a cat pressure test and it tested well well below they high limit. they say NOTHING is wrong with it. Kinda really at a loss here. It idles fine until its warm, then gets a bit funny, has maybe 60-70% power when on it, and its getting 11.4 MPG now. HELP GUYS!!!
Once again, we have check the plugs, they look good, fuel filter is brand new, air filter is brand new. No codes for anything. Blower has never been off the car, everything has always been dealer service except for motor mounts and we changed his upper motor mounts a few months ago, after the MPG problems started. Oil changes in his garage every 4000 miles.
Im no professional but have you checked for a vaccum leak
I would be inclined to think it may have a vacumm leak as well.You mentioned blower (so it has turbo charger?)I am not fluent with the ss models.If so I would check charge air for leaks,bc if it has one and leaks it will kill the mpg.Just like big trks lose pwr,or mpg if a line leak,or seal oring sucks air in losing volume.
checked plugs? did u check the gap also? on my eco i was running the 2200 coil setup and had the coils and wires go out on me. it would rum but sluggish and i was averaging around 10mpg. might just want to go ahead and replace plugs wires and coils. i've came accross alot of coils failing and not throwing up any red flags. i even got into a arguement with the parts store manager that a coil was bad only on one side. he said it was impossible. i then asked then why did it run fine after i put the factory one back in? boy he was befuddled. thats the route i would go with. i know electricals can mimick fuel problems.
i agree with the coils and wires theory...
I agree, a vacuum leak could cause this, especially the boost control solenoid hose. Also, I've seen similar effects on the SC3800 from a faulty fuel pump ballast resistor. The supercharged 3800 has a two speed fuel pump, there is a fuel pump control module and ballast resistor. I don't remember any problems with the controller, but have seen cracked and burnt resistors. Sorry, I don't remember the exact location of the resistor, I think it was somewhere on the rad support in the Grand Prix. This issue may not set any codes, but will cause a huge lack of power and other driveability issues. Lastly, a faulty O2 sensor can't be ruled out just because no codes are set, they can have just enough voltage change to really kill fuel mileage without setting a dtc. Not having seen the vehicle, these are just guesses, hope this helps.
