What exactly goes wrong with an AOS?
 

I'm in the process of working on what may be an air/oil separator related issue. My question, though, is what goes "bad" on one of these? I'm aware of the bellows and J-tube connectors that can crack and leak, but assuming that the AOS itself is not physically broken, what causes it to need replacing? My reading seems to indicated that it's essentially just a plastic container with a series of baffles and chambers inside. Granted, it's not terribly expensive, but why can't this unit just be cleaned out thoroughly and re-installed? Is there something else I'm missing, like some moving part or internal filter media?

Apparently, the diaphragm goes bad....

Can you elaborate?

I was afraid you'd ask.
Just what I've read.

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.