ipsodox

My PCV valve (2008 A3 2.0T) is bad (i think..) and I'm deciding if I want a catch-can or not. But before I make that decision I'm just trying to understand how this damn system works.

First, how many actual valves are there in this system? PCV valve and breather valve? Are those the same?

Second, here is my half-understanding of the two PCV paths/modes:
No boost: The vacuum in the intake manifold pulls air through [tube name here], through the PCV valve, through the crank case mixing with blowby vapors, through the breather tube, from the [some clean air location].

Boost (Still confused on this one): High pressure in the intake manifold closes the PCV valve (and breather tube valve?). Low pressure at the turbo inlet is used to ventilate crank case. Is clear air brought into the crank case in this condition? Or is the blowby pressure so high that this isn't needed?

This PCV Valve, connects to 3 ports on the valve cover. Which port is which?

And it seems that a catch-can goes in addition to the PCV valve, but the kits I've seen have a plate that replaces the part above. How does that work?

And finally, what causes the huge negative pressure that prevents the oil cap from being removed under idle when the PCV valve (?) is bad? Doesn't a bad PCV valve mean it's stuck open, which is how the valve is normally during idle? Huge vacuum would seem to me that the PCV is venting properly, but the inlet/breather is clogged: large suction with no input = vacuum.

Sorry for all the questions...I just don't want to replace the wrong parts...

(Who makes cars that don't have valves, followers, and transmissions that go bad consistently? :P)

ipsodox

And why are these "PCV valves" completely different from the ones on ECS?

http://www.thepartsbin.com/catalog/?...&mk=Audi&md=A3