RE: surging idle still lingers

["Buchholz, Steven" <Steven.Buchholz@kla-tencor.com>]

> My ISV is good, replaced it when I first got the car due to a surging
> idle. I did the pull plug test on it and the idle didn't change. I pulled
> the idle controller, idle shot up to ~2200, put back in, the idle went
> back down to the surging. Took apart the idle switch. A replacement Bosch
> unit in working condition. Made sure of its adjustment and clicking
> switches.
> 
I may be misunderstanding what you are saying, but if the idle speed does
not drop significantly when you disconnect the ISV, then what may be
happening is that you have air bypassing the throttle plate somehow (TB Idle
screw, throttle plate not fully closing, ???) that is setting the idle speed
too high.  I don't have the specs for duty cycle on the ISV, but if the idle
speed with the ISV disconnected is fairly close to the value that the
controller is trying to set, then the duty cycle is going to be fairly low
and it may be that the ISV really can't operate well in this regime ...

> I've had vacuum leaks on my old kh turbo and I was wondering if on the mc
> it'll cause surging idle. The reason I ask is because of the cast in
> passage in the manifold for the wg and other things. I've taken some
> things apart, but have not been able to find the leak (if any), i still
> have more lines to go.
> 
... a vacuum leak causes unmetered air to enter the intake tract, which
tends to screw up the mixture.  While this could cause some sort of surging,
I would expect that the symptom would be more like a rough idle or stalling.
The throttle plate bypass valve (coast valve?) does the same thing ... it
basically allows unmetered air into the intake tract.  

HTH!
Steve Buchholz
San Jose, CA (USA)