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Hello,

Today I found some time to do some maintenance. Changed plugs, air filter and did a pressure test. I filled the system with 10 psi. No leaks found, but something questionable.
I blocked off the breather connection on the y-pipe with a cap, but much air was coming out of the pcv breather itself. I confirmed this by holding my finger on it. Taking my finger off caused a psssshhhhh...
I didn't remove the oil filler cap at this point.
Is this normal, or do I have a broken one-way valve?

nobody

When you say air coming out of the PCV, do you mean via the pipe that connects to the y-pipe?? If so, then you should replace the PCV as they are supposed to be one-way AFAIK.

That's why you remove the oil filler-cap to stop the hissing noise due to build-up of pressure in the crankcase returning through the breather pipes. It's normal, caused by the blow-by of the piston-rings etc into the crankcase. That's what breathing is all about, returning to the inlet system to be re-burnt through the cylinders.

The pcv-valve (Positive Cranckcase Ventilation) controls the pressure in the crankcase. For example, to stop the breather system reversing under boost/overrun conditions which could pressurise back through the breather system.
This breathing contains oil mist, which is the cause of much concern to the B7 RS4 boys who have FSI Engines injecting fuel direct to the cylinders. The oil mist is coking-up the valves. Our engines are washed with fuel which is mixed with the breather mist prior to burning.

When pressure testing, remove the filler-cap and then listen for hissing (leaks) If you leave the cap on, you will pressurise the crankcase and create a hissing noise as the air is blown through the breather system and out via the disconnected pcv valve. That's also why you remove the pcv valve in the first place, to stop the spider hose being subject to pressure.

Aaah, missed the bit where you said that you didn't remove the oil filler cap. DOH!

Thanks for the explanation guys. I did just pressure it to about 7-10 psi. So the good news is that the system is absolutely leak-free. No hissing

you want to go up to 15psi

mikeyquattro wrote:you want to go up to 15psi

Correct - The TBB (Throttle Body Boot) is a very common leak point and only opens up at near 1 bar.

Also, a leaking or stuck diverter valve will not show up on a pressure test as both sides are pressured on this type of test.

I doubt that.. The intake system isn't designed to get under such high pressure, and the RS4 doesn't have a TBB like the S4. It's metal over there.
Diverter valves are new Forge 007p

Try searching the site for this subject.....

both my leaks were found by testing up to 15psi and they were the O rings leaking beetween the metal TBB and the alloy pipes coming from the IC's