Did you even read this thread? Your answer is right here:
Quote:
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Originally Posted by blue2000s
In order to reduce oil contamination, foaming and moisture build-up and minimize pressure build-up, the car's crank case is under a slight vacuum by a connection with with intake manifold. The AOS sits in the middle of this connection to collect most of the oil that is pulled from the crank case and returns it to the engine. It is completely normal to hear or feel a vacuum when opening the oil cap.
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'01 Boxster S, 51k miles
'05 Mazda 6 Grand Touring Wagon
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