Crankcase Ventilation Flow Diagram

[Fat Audi 80]

Here is a really useful diagram courtesy of UrS4boy.

[Fat Audi 80]

More here: http://forums.quattroworld.com/s4s6/msgs/22506.phtml

[UrS4boy]

20vt Positive Crankcase Ventilation (PCV) System with revised diagram, PNs and Links

Revised Link:

http://forums.quattroworld.com/s4s6/msgs/22506.phtml

[A80Avant]

Nice writeup. I have a question. I just replace the pressure regulating valve and the new one out of the box was in the open position. I am trying to get my head around how the valve is open and when it is closed...

As you say under vacuum (cruise) and idle, this route is open but the one way valve is also open and therefore vapours go to the inlet manifold.

Under boost the one way valve is closed and all vapours go to the maf. If the PCV valve is still open, then what is its use?

[Fat Audi 80]

wow

[UrS4boy]

I will add more info later today. In the meantime, and for the 11teenth time NO VAPOURS GO THROUGH THE MAF (in a stock system).

[Thuppu]

And I am going to rip everything off.

[A80Avant]

I did not say THROUGH, I said TO. There is a difference, but even so, although the hole on the stock turbo to maf hose is further down towards the turbo compressor, you do know that lots of oil vapours and sludge end up on the maf anyway. Im surprised you are playing with words when you know in practice what the deal is.

More to the point now, any specific idea when he PCV valve is open and when closed?

[A80Avant]

I'm only using the phrase 'goes to maf' just to simplify my writing, not suggesting that it actually goes to the maf!! Geez!

[newsh]

If the PCV valve is still open, then what is its use?

I think the PCV valve gets sucked closed when you shut the throttle, to give a greater vacuum to the crankcase, otherwise most of the vacuum would be applied to the MAF hose.

[newsh]

But I might be wrong

[UrS4boy]

I'm only using the phrase 'goes to maf' just to simplify my writing, not suggesting that it actually goes to the maf!! Geez!

How ever you try to change the words, the oil vapour from the cam cover and the crankcase does not go TO or THROUGH the MAF. They enter the MAF to turbo hose downstream of the MAF. As long as things are operating properly, i.e. the entire PCV system operating under a vacuum, then NO vapours would make it into the MAF and gum it up. However, if for some reason, the PCV system failed in a way that allowed boost pressure to blow through the system, then, yes, there is a possibility that oily vapour will make it into the MAF. But that is NOT how the system is supposed to work.

[UrS4boy]

I think the PCV valve gets sucked closed when you shut the throttle, to give a greater vacuum to the crankcase, otherwise most of the vacuum would be applied to the MAF hose.

I think one of the problems in this discussion is the over/mis-use of the term "PCV" to describe the two valves involved. As per VAG, they are called "bleeder valve" and "pressure regulating valve".

The bleeder valve is a one way ball valve. Under vacuum the ball is sucked upwards, out of its seat (but still contained in its "cage"). This allows the oil vapours and blow-by from the crankcase to be drawn into the intake manifold under intake vacuum.



In normally aspirated engines, that is the end of it. However, our 20Vts are different in that the crankcase and cam cover still need to be ventilated, even when the intake manifold is under boost pressure. This where the pressure regulating valve comes into play.


It contains a spring-loaded diaphragm. Under vacuum, when the bleeder valve is open, the pressure control valve is CLOSED with the spring pushing the diaphragm against an orifice. When the intake manifold is put under pressure (boost), the bleeder valve ball is pushed against its seat, closing that path for the blow-by and oil vapours. All is not lost since, at the same time, the pressure control valve is opened by the vacuum created by the higher flow going to the turbo AFTER the MAF, pulling on the diaphragm, against the spring pressure, lifting the diaphragm off the orifice, allowing the vapour and blow-by to be draw in to the unpressurized flow from the MAF to the turbo inlet.

When you let your foot off the accelerator pedal, the throttle plate shuts, creating a high vacuum situation. At that point the pressure control valve again closes under its spring pressure and the bleeder valve ball is lifted off its seat and opens.

I don't think I can explain it any better without creating more diagrams.

[Jamo]

Great summary, a lot of info is spread across in various threads excellent collation!

That one way valve was the cause of serious problems for Newsh and he has a write up of it.

[UrS4boy]

Great summary, a lot of info is spread across in various threads excellent collation! That one way valve was the cause of serious problems for Newsh and he has a write up of it.

You're welcome. If you or Newsh can point me to his write-up I can add it as a link.