This deals with the importance of the non-return valve (sometimes called the PCV valve)and the pressure control valve.
The crankcase ventilation system on the AAN is operated mechanically and is not controlled by the ECU. The only connection (so to speak) is the ECU controls boost (and vacuum) depending on the throtttle position and the crankcase ventilation system function changes with boost/vacuum it sees.
Under boost, the oil vapour-laden crankcase air goes one way; under vacuum it goes the other. Unfortunately, the diagram and the text in the VAG143 document are in conflict.
Suffice it to say, if there is a rip in one of those hoses or the PVC (non-return) check valves is stuck open, you can have problems.
Personnally, I replaced both valves last year when I did my first FMIC.
The Pressure Control Valve in the above diagram is called a “pressure regulating valve” in ETKA and has a 034 129 101A part no. (Note: 101A has been superceded to 101B and apparently its less expensive) (Ref: Item 3, Kate’s photo album 103-89)
The non-return valve in the above diagram is a check valve, aka positive crankcase ventilation valve (PVC) or in ETKA, “bleeder valve”. Its a one-way ball-type check valve so it needs to be installed in the correct orientation. PN is 035 103 245A. (Ref: Item 10, Kate’s photo album 103-89)
There is also something called a “flame deflector plate” ((Ref: Item 5, Kate’s photo album 103-89) (PN 035 103 477A) located just downstream (towards the engine) from the pressure regulating valve. This thing is supposed to stop flames travelling in these hoses – might just be a means of condensing oil vapours – no idea. Here is a photo:
