[Fwd: Very High Idle ISV Problem]

[John Sherrow]

Christopher Flanner wrote:

> Please help me to get some basic ISV understanding.  The engine is an MC
> from an '86 5ktq.  A spring keeps the ISV closed when unpowered.  Does the
> solenoid help to keep the valve closed when powered or does it help to keep
> it open (I know it varies in a range)?  I assumed the valve stays closed by
> itself since I've heard of people disconnecting it and having an acceptable
> idle when warm.
>
> Thanks,
> Chris

Chris I posted this a while back hope it helps.

 Airflow passes thru the MAF (metered air) and enters the intake manifold at
the point where the "Michelin
 Man" hose connects to the intercooler. If you remove the MM hose you will see
the throttle plate and a
 cast air channel which allows air to bypass the closed (at idle) throttle
plate and enter the other end of
 the intake manifold via the ISV. This is why the MM hose has the tear drop
shape.

 With the throttle plate closed (foot off the gas) the throttle switch directs
the ISV controller (marked as
 Idle Volume Control on my fuse panel, gotta love the translation) to regulate
air flow into the intake
 manifold using the ISV. The ISV contains an air regulator disk controlled by a
solenoid. With current
 applied to the solenoid the disk will move toward the electrical connector
providing a path for air to enter
 the intake manifold. With no power applied a return spring returns the disk to
the shut position.

 It was suggested that a vacuum leak could cause the problem. It was not clear
to me how a leak could
 cause the engine rpm to increase. It seemed that the increase in rpm was
likely due to an increase in
 metered air and the engine computer was responding accordingly. I felt a
vacuum leak would cause
 unmetered air to enter the intake manifold resulting in an idle stumble.

 As suggested I cleaned the ISV and then tested it with a 12 volt/half amp
power supply. The disk moved
 freely and snapped shut when power was removed. One notable difference was
that the replacement ISV
 would not pass air when I blew into the inlet side of the ISV. The problem ISV
would allow air to pass.

 I believe the return spring in the failed ISV was fatigued from years of
fighting the solenoid and no
 longer has the spring force to seal against engine vacuum thus resulting in
the increase in "metered" air
 during idle.

>
>
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