rough idle, reduced power, oil cap suctioned on
2002 525i. Check engine light is on and the car runs rough at idle with reduced power during accleration. There is a "hissing" noise coming from the engine compartment that does not change pitch with RPM's I noticed when I take the oil filler cap off while the engine is running the hissing is diminished and the engine smooths out slightly. When I do take the oil filler cap off it is "suctioned" on and there is a rush of air into the engine when I pry it off. Earlier posts suggested MAF sensor but had a mechanic tell me that if the oil filler cap is difficult to remove that it's probably something else, I just can't remember what he said it might be though.
Thank you in advance.