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Re: Crankcase pressure relieved, Thanks to Guy and (archive)

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Posted by David F on April 02, 2001 at 22:48:22:

In Reply to: Crankcase pressure relieved, Thanks to Guy and posted by Gilbert on April 02, 2001 at 21:42:02:

That's the way it should be. You are introducing a large vacuum leak when you remove the cap. There are two large, unrestricted vacuum ports connect directly to the crankcase (one at each throttle housing on bottom side).

others who offerred advice. Especially Guy for sending the diagrams.
Alas, it was more time than I had to fiddle with, but I finally understood how it all operated. But there was no obvious fix, so I took it to the dealer. The car was running so much crankcase pressure that it was leaking a quart of oil per 100 miles all over everything. It had a previously had a little positive pressure, and I took it in to have the PCV valves changed. I didn't notice for a couple of days about the oil, then saw (and smelled) the leaks, then tried to remove the oil filler cap and found it had 30 pounds of pressure or more. Blew the cap right out of my hand. But still ran fine.

I finally got it to the shop and the mechanic said it was the oil separator. He cleaned it and everything is fine. But now I'm puzzled again. I tried to take the oil cap off with the motor running to ensure it had positive pressure. I could hardly get it to turn, and, when it came off, it seemed to have 30 pounds of negative pressure. With a big whoosh it sucked a ton of air into the crankcase, then abruptly died. Put the cap on, runs fine. Take cap off, big suction, engine dies immediately.
I have often taken the oil cap off to watch the oil spraying on the cam, and the negative pressure was very minor, which I thought should be normal.

Is the big negative pressure and the quitting of the engine if I take the oil filler cap off the way it should be?

Puzzled in Tampa.
Lin Gilbert
1990 750iL
210,000 miles

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