Chevy Aveo Gas Mileage, Good after tune up but now declining Options

[whitefish]

Hi Everyone,

I'm getting nowhere on the chevroletforum site with this, so I thought I would ask here.

I recently read a post on ways of improving fuel mileage on Chev Aveo's. As per the post, I replaced my air cleaner (which was a fairly new OE GM air cleaner) with a K&N filter and a set of new spark plugs. The old stock air cleaner looked okay, but the spark plugs were BADLY eroded - stock Ac Delco non-platinum/iridium with about 70k on it at the time. Mileage jumped from about 350km per tank (mostly highway miles) to over 500km per tank (sometimes over 550km).

My mileage is starting to drop again to around to an horrible 350 to 380km per tank. I cleaned the K&N air cleaner (as per instructions and replaced filter oil) and checked the plugs. Some slight erosion, but still in spec (0.044 inch gap). The spark plugs were pretty black (sooty) and smelled of fuel, so I think the car is running rich. MAF or oxygen sensor?


My only other thoughts are that mileage would be dropping (now down about 20%) due to the change in temperature or winter grade fuel - I did the tune up in summer when the temp was about 25C and now its about 5C.

Any ideas....?

Thanks guys.....

[whitefish]

I'm going to pull the O2 sensors tonite to bench test them and try to test them installed (found a good video on how to do both).

If I don't find any problems, then I'll try Deep Creep (Sea Foam). If that still doen't work, then I'm taking it to the GM dealership in my hometown to hook her up to a Tech II to find out whats wrong - I trust these guys more. Like you said, if its an intake gasket, etc. then its on warranty.

I pulled the plugs again last night and they were the right color (greyish with brown) as apposed to the black sooty color that had last time.