I just bought a 2007 Malibu LS 2.2L 4cylinders, with odometer 130K, and have replaced the air filter, engine oil, oil filter, spark plugs, according to the records of maintenance left in the car. Just a few days ago, I got the second chance to refill the tank up fully and calculated the MPG of last half month, and the result shocked me with the little number less than 17. What's the wrong with this? This car runs smoothly but accelerates not fast enought.

Either a dirty Mass Air Flow (MAF) sensor or a dirty throttle can cause that. If that's the case, do this video. https://www.youtube.com/watch?v=NWCKvU2FmDc


It sounds good. Do I need to change the air fuel ratio sensor?

sometimes they go bad, BUT it's best to have a pro test them with his scanner data rather than guess with such an expensive part.