danny951

Something has changed with my car regarding boost/vacuum. I'm not sure what is supposed to happen, but.. when I let off of boost (I do get full boost), both my stock and my aftermarket boost/vacuum gauge needles slowly fall down into vacuum. It will get all they way to 17-18 in/HG, but it takes several seconds. Prior to about a month ago, when I would let off of boost, the needle would shoot down to 18 in/HG immediately.

What should the boost/vacuum gauge needle do when I let of the accelerator while in boost?

schnellfahrer

It should shoot down to vacuum immediately.

Eyal 951

diverter/bov?

Tom M'Guinn

It takes several seconds? That's not right. Have you checked you bypass (BOV) valve and/or heard any turkey gobble sounds when you let off?

Laust Pedersen

The boost/vacuum measurement should be taken on the typical spot between the 3rd and 4th intake runner, which is after the throttle plate (i.e. independent on the condition of the BOV). The must be a rather large leak somewhere around the intake manifold (IM).

TB O-ring? Intake gaskets? Hoses attached to the IM? ISV malfunctioning?

Or just maybe it could be some hardened (dried out) oil in the vacuum line sensing the pressure.

Btw, the vacuum should be about 22 in Hg on overrun above about 2500 rpm.

Laust (with a good vacuum system and a dead turbo)

Tom M'Guinn

Can't disagree with this. I was thinking he may have been talking about situations where he let's off the throttle, but not completely. If it happens when he puts the clutch in a let's off the gas completely, the only thing I can think of is a kinked or restricted KLR line that is somehow holding pressure for a few seconds (with the aftermarket gauge T'd off the KLR line).

Even a leak around the intake would bleed off the boost, rather than holding it like that....I'd think.