Very High Idle ISV Problem

[JShadzi at aol.com]

*sounds* like a bad idle switch, have you confirmed that it works?

Javad

>Listers,
>
>My 90tq decided to act up on the way to dinner Friday night, challenging me
>to parallel park with a 3000 rpm idle.  The engine is an MC from an '86
>5ktq.  The throttle valve is fully closed and the idle returns to normal if
>the ISV hose is pinched shut.  I opened the ISV and it wasn't sticking open
>but I cleaned it anyway.  It seems that the pressure differential over the
>ISV is sufficient to open it.  While it's idling at 3000 rpm the vacuum is
>about 4 inH2O higher than regular idle at over 22 inH2O.
>
>I don't have a clear picture of the idle air bypass layout in my head but is
>it possible that the screw-adjusted bypass is blocked causing vacuum to open
>the ISV valve?  I tried to blow and suck through the ISV (smart, I know) and
>it is very much closed but when the engine runs/idles there is definitely
>air passing through it even when it's disconnected (electrically).  I won't
>mention that I got an oily piece of junk caught in my throat when I tried to
>perform CPR on the ISV.  Oops, too late.
>
>My ISV isn't receiving any signal but I don't think that's my problem.  If
>the connnector is disconnected on an ISV it should stay closed, right?  I
>went through the Bentley ISV diagnostics (which aren't for an MC but most
>details are similar) and everything checked out leading to the
>recommendation of replacing the control unit.  Once again, I don't think
>that's related to my problem since having no signal usually leaves the valve
>closed.
>
>Summary:  The ISV moves freely and is normally closed but when the engine
>runs, it is apparently open, even though it is disconnected, and causes the
>engine to idle at 3000 rpm.
>
>What's going on?????
>
>
>
>
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