idle control valve/ idle stabilisation unit.

[slamdunk]

How does the idle stabilisation unit work? I understand that it has a motor that drives against a spring. The shaft of the motor is connected to a butterfly valve that regulates the volume of air flowing into the engine, hence changing the position of the valve regulates the engine speed. Right? If the valve fails, presumably it does so with the valve closed, hence the engine wont idle. Does this not mean that the engine wouldnt start unless some throttle was applied, opening the butterfly valve in the throttle body? I am trying to understand how ISU failure causes the engine to die intermittently. Also, bypassing the valve suggests that the idle speed is essesntially unregulated and would result in a very fast idle.

Confused,

Slam

[California Dreamin]

ISV compensates for load (alternator/power steering etc) basically sensing and compensating when the engine revs decrease below idle. It also compensates during the warm up cycle.
However....with the engine upto temperature and not under load the valve shouldn't be doing anything...infact, disconnecting the ISV at this point should have no effect to the idle.

The engine should be setup to idle WITHOUT the ISV...all the valve does is to add additional air to increase revs when needed so is not responsible for an engine that won't fundumentally idle. It also does not effect starting. When you say it could be responsible for the engine intermittently dieing, well yes, in so much as there wouldn't be compensation for load and cold start so would have a tendancy to stall.

The ISV works in conjunction with the normal air by-pass idle process, it doesn't replace it.

Martin