This problem came up the other day, and I really haven't been able to figure out what it is. First off, I have an automatic 318is. When the engine runs for a few minutes, a whistling noise comes from the engine bay. It's really difficult to pinpoint exactly where it's coming from.
I first thought it was the pulleys but I've noticed that when i lift off the oil filler cap, there is a noticeable pressure coming from inside the engine. The noise also goes away when the filler cap is taken off. It must be a valve that is sucking in more air than it's supposed to, but which one could it be?
You have a crankcase leak and BMW's hate them. You will need to find where the whistling is coming from, that is your problem. If you think about it, it explains itself. When you have the oil cap on, the crankcase pressure is forced to come out of whatever leak there is causing the whistle. When you remove the oil fill cap the crankcase pressure goes away as you now have a huge leak. Put the cap back on and the whistling starts again. You can get a piece of rubber hose and put it to your ear and move the other end around until you find the leak.
would anybody be able to explain exactly how the crankcase vent valve works? does it constantly suck in air like a vacuum? does it change in strength when you accelerate? what is it supposed to do during idle?
i had to chase down an intermittent whistle on a 95 m3 auto. it would only be audible on initial acceleration, or (as i found out) when driving up onto the alignment lift. it turned out to be a rear main seal 'sucking in' air, which went away after i replaced the seal. trust me, the ccv is *MUCH* easier. i would do that first. (which is how we figured out it was the rms...)
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