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Discussion Starter · #1 ·
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.
 

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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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have you checked the EGR valve there is a small pipe that comes off that .. that may be loose?

im sure that may be the cause.. as that would generare the hissing sound

Good luck - keep us posted on your progress

Izzy
 

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Discussion Starter · #5 ·
Update:

I thought I would update everyone and thanks for the help. I took it to the shop to have it repaired since I was going out of town for a few days. Here's the story.

The primary problem was the crankcase vent valve. It was replaced along with the connection line which was starting to crack.

As a result of the problem though, all of my spark plugs were fouled and had to be replaced, as well as one coil from the #5 cylinder.

I hope this helps anyone else with similar problems, and thanks to everyone who replied.

DG
 
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