Thread: M44 PCV help
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Old 01-15-2014, 11:01 PM   #3
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I don't want to vent to the atmosphere. From all I've read, BMW motors are designed to have a slight negative vacuum at all times. Somebody said something about low piston ring tension, and needing the vacuum, but I don't fully understand the concept behind that.

I also tested this on my X5 at school, since I left the 318 back home. It has a working PCV, which was replaced last summer, and it wasn't blowing air out of the oil cap when I removed it. People assured me that everything was working right in that case.

Now, I've been reading up on PCV and substituting in a catch can. When not venting to the atmosphere, the catch can system seems to create a lot of vacuum. Stock spec is 10-15 mbar, but this is enough to keep the cap stuck on the valve cover. I know too much is bad for an OEM M54 set up as it can suction oil from the pan and hydrolock, but a catch can eliminates that possibility. How much vacuum is too much for our engines, and when will it be bad?
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