Idle air control valve

[Mike McKown]

The idle air control valve has three small coolant hoses running to/from it. One into the intake manifold water jacket, one to the heater return line and the other connects to the intake manifold plenum. Doesn't matter it looks like the valve has coolant flowing through it and possibly vacuum acting on the coolant.

I don't know how this thing is set up but I know it holds the idle speed up when the engine is cold and then drops it as the engine warms up, maybe by sensing coolant temperature.

Here's the question: If there is a problem in the described system, is it possible for it to discreetly suck coolant into the intake manifold and run it through the engine?

The idle air control valve doesn't seem to be working by the way but the engine will idle down slow.

If the answer is yes, I have other details.

[JohnGunn]

There are only two coolant system hoses on the ISC, one in and one out. The hose you identify as a coolant hose going to the plenum is actually part of the circuit that allows air to bypass the throttle body. When the engine temp is less than 60C(140F) air is allowed to bypass the throttle body to increase idle speed. That process is not controlled by a sensor, but by "thermo wax" which expands as it is heated by the coolant and acts to gradually close the bypass valve, until it is fully closed at the above temp. At that point, with the bypass valve closed, the idle speed control screw, alone, controls the amount of air allowed past the throttle body at idle.

This last statement is not entirely accurate, for as you have no doubt noticed there is an electrical connector to the ISC. This allows the computer to control the bypass valve in its effort to maintain a constant idle speed when load is added to the engine and it slows down. To prevent that the computer senses from the air vane flow meter that the change has not been the result of less air flow and pulses the IDC to allow enough bypass air to keep the idle speed at its previously established level.

To my knowledge there is nothing which selectively adjusts the flow of coolant through the ISC.

I have tested several of these units to observe if they closed correctly. In that time I have never seen anything which would suggest leakage from the coolant circuit into the bypass air section. Of course, anything could happen, but I would think what you describe would not be a common problem with these units.

John Gunn
Coronado, CA