curyfury

From my understanding, the BPV should open when you shift gears, so the excess pressure bypasses the throttle plate. If this is true, then whats the big deal if it leaks? I can understand if a BOV leaks, that is leaking pressure into the atmosphere, but a BPV still vents the air into the intake manifold. Where's my misunderstanding?

Cup-a-Chow

I'll try, but I'm not 100% sure.
You're making boost when you're on the throttle. You let off the throttle and you're still making boost.... But now your throttle body is shut, so there's a lot of "compressed" air b/w your turbo and throttle plate. This can cause your turbo to slow down or spin backwards.
The BPV basically releases this pressure, by venting that "compressed" air back into the intake (not intake manifold, intake) before the turbo. The difference b/w BPV and BOV is that the BOV releases that air into the atmosphere. The BPV releases that air back into your intake.
The reason why putting a BOV on an S4 makes it run rich is because the MAF has already metered that air which gets blown out into the atmosphere by the BOV. Your computer thinks that you have X amount of air, when you actually have less, so you have too much fuel and you run rich. The BPV's aren't affected by this because the released air goes back into the intake AFTER the MAF, as in b/w the MAF and turbos.

curyfury

Assuming the BPV is a normally closed valve, whats the big deal if it leaks? What exactly is happening to the car when the BPV leaks?

Turning Japanese

You're not getting as much boost

JCJiffy

At idle and part-throttle cruising, the diaphragm/piston in the BPV is held open by vacuum. When on boost, the valve should be shut tight. If it can't keep a perfectly tight seal, then that's when it leaks. When it leaks, you will lose boost. If you don't get the boost levels you're supposed to see, then you won't have the power you're supposed to originally have.

So when you lift during hard acceleration, the valve opens and allows the air to bypass the closed throttle plate to get back into the intake tract.

curyfury

If it leaks, how will you lose boost? The boost is still entering the intake manifold.

You said, "When on boost, the valve should be shut tight." If the valve is tight, all the boost goes to the intake manifold through the throttle plate. If it leaks, doesn't it still go into the IM but around(aka, bypass) the throttle plate and still into the IM?

I still don't understand.

WYSIWYG

it leaks/releases it BEFORE the turbo

curyfury

Say what?