How does the ISV work?

[Robin]

Anyone know?

[GSBellew]

In which way?

As in, are you asking how it stabilises the idle or are you asking how it works internally?

[Andy]

It contains a bi-metallic strip inside it that closes off the valve inside it as it warms up - that's why it's bolted to the water pipe at the rear of the block. The two wires to it are wired straight to the ignition circuit, and power a heating element inside it.

When the engine's cold, the valve will be fully open and give you a raised idle. As the engine warms up it'll close up, dropping the idle. At that point its down to the ECU to regulate idle speed by playing with the ignition timing.

[vwmk3jon]

What would happen if the ISV was unplugged?

[PeteG40]

if open would be shit when warm

if closed would be shit on start up

[vwmk3jon]

So shit when warm, would this cause the engine to cut out?

[cheys03]

Certainly make it run lean if it was stuck open when engine was warm.
If you suspect the ISV then you can crimp it's hose to determine if it's operating correctly.