Board logo

Dump valve
luke2152 - 17/4/15 at 11:08 PM

So I have a used forge diaphragm one that I brought 2nd hand. It actually functions fine to dump the pressure when thottle closes but leaks on idle. Leaks so much I can block the inlet and engine runs just fine through the BOV. No idea if its incorrect spring, diaphragm or something else.
Looking at these options for replacement:
A. OEM diverter valve from saab or audi or whatever. Can these be used vented to atmosphere? - I worry they may have some leakage by design.
B. Used branded diaphragm one similar to what I have but hopefully not buggered.
C. New chinese diaphragm one
D. Used branded piston one (don't see any chinese ones in this style).
I'm not bothered about the sound and don't need to recirculate as I'm using speed density

rachaeljf - 17/4/15 at 11:21 PM

It's quite normal for a dump valve to be open at idle, it is pulled open by the high manifold vacuum at idle. Recirculating valves are needed by FI systems that use a MAF as the dumped air will have been measured by MAF, making the engine run rich.

Normally the turbo keeps a positive pressure in the inlet tract in front of the throttle plate so there is no danger of the engine breathing unfiltered air by sucking it through the open dump valve.

Hope that helps!

luke2152 - 17/4/15 at 11:30 PM

So it would be normal to be able to completely block the turbo inlet and feel no suction and the engine continues to idle happily on air sucked in the BOV? Seems excessive but I'll be happy if I'm wrong

luke2152 - 17/4/15 at 11:32 PM

So it would be normal to be able to completely block the turbo inlet and feel no suction and the engine continues to idle happily on air sucked in the BOV? Seems excessive but I'll be happy if I'm wrong

coozer - 17/4/15 at 11:52 PM

Bleed valve!!

mark chandler - 18/4/15 at 06:40 AM

quote:
Originally posted by luke2152
So it would be normal to be able to completely block the turbo inlet and feel no suction and the engine continues to idle happily on air sucked in the BOV? Seems excessive but I'll be happy if I'm wrong


Yes, the turbo is not providing any boost as you have blocked it off and the vacuum is holding the valve open.

On my car at idle the piston BOV is partially open however air is being gently blown out as even at idle it produces a little pressure so no additional filtering required.

I set the spring to around 10psi, turbo boosts at around 8psi so it gives me some over boost protection.

luke2152 - 18/4/15 at 07:10 PM

Indeed you're right! I put a plastic bag around the BOV and the bag inflated ever so slowly so the leakage is actually coming out not going in.