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Old 04-13-2010, 10:31 AM   #7
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
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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