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the diaphragm is a membrane inside the AOS. the vacuum from your engine at various RPMs will open and close what is more or less a valve inside the AOS. when the diaphragm goes bad, the AOS is always open & sucks in liquid oil in addition to the vapor it's supposed to clear. this oil is fed directly to your throttle body, where it then burns and smokes.
another common failure on the AOS is the rubber bellows that connects the bottom of the AOS to the crankcase. this bellows cracks. when it does, it causes a vacuum leak that can make your car run funny & throw codes. it also causes some oil leakage.
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