I changed the air filter with a K&N oiled filter on my 2006 330i. After noticing that the MPG in city is even not as good as when I used the old filter which runs about almost 100k miles..... Few days later I gave up on the K&N filter and replaced with my old filter, however, the MPG doesn't go back. Now I have a almost 15~16 MPG in city which is unusually low. Last year, I can have a almost 20~22 MPG in city.
I don't know which part goes wrong with my car, is it because the MAF sensor got damaged by the oil or because there is some sort of leaking in the intake system?