5-Series (E12,E28, E34, E39, E60)Chat relating to the BMW 5-Series of all generations. Specific models include: BMW 518, BMW 520, BMW 520i, BMW 530i, BMW 528i, BMW 530i, BMW 518i, BMW 524d, BMW 525i, BMW 525e, BMW 528e, BMW 540i, BMW 535i, BMW 520d, BMW 525td, BMW 525d, BMW 530d, BMW 525i/xi, BMW 530i/xi. (BMW 5-Series Forum)
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.
You probably have a vacuum leak somewhere. Check the intake boot and hoses around the intake manifold area. If you cannot find anything use carb cleaner sprayed on different suspect areas until you hear the engine improves. since now you've found the area of the leak, dive deeper and fix it. Also, check which code the is stored in the system for the engine light to be on.
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