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High Idle and Vacuum boost


Well-known member
Jun 20, 2004
Kent - industrial suburbs of Seattle, WA
Quoting EgonOlsen:
I just checked the Biss screw. its all the way down. I took it up just a notch to check and the Idle got even higher.

So thats prolly not it, it cant go lower than it already is.

Boost leak check that car. You either have a vacumm leak at manifold to head gasket, leak at injector to head insulator, leak from any of the vacuum hose that branches off the manifold or any of the TB gaskets or an internal leak at the TB assembly that lets air bypassing the TB plate. I just dealt with the same problem with a car here, even with the BISS set all the way down the lowest rpm I could get was 1200 rpm. Sent the TB to for a quick rebuild (it was leaking massively from the shaft seals as well) and when I got it back its still doing the same idling high thing. I took off the lower portion of the TB and install a block off plate and the rpm went down to the normal range. Must have problem with the ISC pintle sealing its bore or the FIAV internal bypass leak.


Well-known member
Jan 25, 2010
Bunker Hill, WV
make sure the idle switch isn't holding the throttle open a touch more than it should... this is a common stop-gap fix for a bad ISC system that I have run across...

if you need an intake and idle checklist let us know

good luck!


Well-known member
May 4, 2010
Denmark, Europe.
ok so far so good, or bad. Havent found the problem yet.

I put in another ISC black 1. made no change whatsoever.(Imeasured both the new and the old 1, the old one have a funny read on 1, but the new measured same resistance on all pins.)
I took off the EGR and found a can homemade block off there, made a new block off and put it on again. Made no difference either.
I had the throttlebody off and put 2 new gaskets on. Made no difference.

I have bought a pipe and stuff to make a boostleaktester.

1 Question. This "sensor" or what it is on the EGR valve, is it ok for it NOT to be plugged in? and if no, where does it plugin to? mine was already off.

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