What stops boost being lost through breather pipe

[newsh]

I checked the hoses to the blue valve at the back of the head and they are all ok also the vaccum hoses,I do not know how to check if the valve is working.

You should be able to blow through it towards the Inlet Manifold not the other way.

[220bhp]

I checked the hoses to the blue valve at the back of the head and they are all ok also the vaccum hoses,I do not know how to check if the valve is working.

[newsh]

There's a little blue and black plastic valve in the smallish pipe between the IM and the breather system, on the 3B its down behind the IM below the ISV, you need to remove the ISV to get to it. Its hidden in the join between 2 pipes of different sizes, from memory one about 20mm and one about 10mm. It looks quite strange these 2 pipes joining with 2 hose clips of different sizes. If this valve sticks open, which they do, boost pressure will be lost into the breather system and run round the tube and get sucked in at the MAF hose again. So like a boost short circuit..

While you've got the ISV out you can get to all the small vac pipes and look for leaks, if they're old they always leak at the ends where the clips are. You can sometimes get away with cutting a half inch off and replacing the clip with a small cable tie.

You'd get something similar if the DV (Diverter Valve) was stuck open or the diaphragm split, the output of the turbo feeds straight back to the input.

[220bhp]

Thanks for the replys its much appreceated,one more question what happens if one of these oneway valves dont work, what would be the syptoms.

[goodall]

crankcase should be completely sealed from the environment if the all seals and gaskets are good, eg sometimes you may notice when you take the oil filler cap of the engine may start to stumble, as this then allows the crankcase to be 'open' and allow unmetered air to enter the intake system.

When you suck or blow on the breather pipe all you are doing is changing the pressure of the crankcase, as a fairly large volume the capacity of your lungs wont make a huge difference

[UrS4boy]

What stops boost being lost through the metel pipe running from the maf hose to the crankcase emission system,should it be possible to blow through this pipe.

The crankcase ventilation system has a couple of one way valves that control the flow of the air from the crankcase. When the engine is under vacuum (idle, and light throttle while driving) the crankcase air goes directly into the engine at the throttle body. Under boost, a valve closes and the air is drawn into the engine on the suction side of the turbo, i.e. at the MAF to turbo hose. Sometimes these valves fail.

You are right about the unmetered air but somehow the ECU via the O2 sensor figures it all out.

This AAN-related post should help you:

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

[220bhp]

Stupid of me what I ment to say was what stops unmetered air being drawn through this pipe as I can both suck and blow through this pipe and am not sure if this should be the case. I assume that unmetered air is air drawn in after the maf.

[RoadKill]

its a brether for the crankcase , it should not be getting boost

[220bhp]

What stops boost being lost through the metel pipe running from the maf hose to the crankcase emission system,should it be possible to blow through this pipe.