About Idle Air Control Valves

Comments by Dan Thompson

I got a pretty good explanation of how the IACV works from Clark at JWT the other night (I myself am trying to hunt down a cold-idle gremlin).

Basically, as we already know, the IACV regulates idle speed, raising and lowering it as needed. It does this by opening and closing a "trim" valve to allow more or less airflow into the intake manifold when the throttle is closed.

The adjustment we make on the IACV is for setting the "base idle". By unplugging the TPS (and the IACV in some cases) we tell the ECU to completely close the trim valve in the IACV. When this happens, the idle airflow comes completely from a needle valve - the very one we are adjusting when we turn the screw on the IACV body.

If everything in the fuel and intake system is operating properly, the idle speed will not be able to drop below this setting. A few things can cause the base idle speed to go out of adjustment.

First, the needle valve can become clogged with dirt or carbon, reducing or completely blocking off airflow through the needle valve. This would reduce the airflow through the needle valve and cause the ECU to rely partially or fully on the opening of the trim valve in the IACV to maintain target idle. This would mean that the idle could possibly fall below 750 before the ECU could intervene.

Second, the trim valve within the IACV could be rendered partially or totally inoperable, either through blockage or failure. If this valve is blocked open, idle speed would be high unless the needle valve's opening was reduced to compensate. The result of this may be the inability for the ECU to compensate for all idle conditions - base idle would be set for Y percent trim valve opening, leaving only X-Y percent of the operating range of the trim valve for ECU adjustment. If the trim valve is not blocked open but the passage to this valve is partially clogged, it would require the ECU to open the valve farther than normal to allow a given amount of airflow to pass. This would also reduce the amount of compensation the ECU can apply to idle speed.

If Peter's base idle speed is set correctly, I'd say that one of the conditions in the second instance is the cause.

To sum it up, there are two idle speed adjustments to control idle on our cars. First is the manually adjusted base idle setting, which determines the minimum airflow through the IACV (and minimum warm-idle speed). Second is the ECU's control over the "trim" valve within the IACV, which compensates for hot and cold idle and engine loading (power steering and A/C operation).

Comments by Vito Biundo

There is one other factor in this equation, the IACV-Air Regulator.   This valve is in parallel with the IACV-AAC valve.  This valve is what supposedly makes the car idle fast when the it's cold.  The FSM gives the following description of the IACV-Air Regulator.

"The IACV-air regulator provides an air by-pass when the engine is cold for a fast idle during warm up. A bimetal, heater and rotary shutter are built into the IACV-air regulator.  When the bimetal temperature is low, the air by-pass port opens.  As the engine starts and electric current flows through the heater, the bimetal begins to turn the shutter to close the by-pass port. The air passage remains closed until the engine stops and the bimetal temperature drops."

If there's a blockage in this valve or a malfunctioning valve, will the IACV-AAC open further to maintain a high idle during warm-up?  Another question I have is can the engine idle at 800 rpm when it is cold (lets say 32F)?

The diagram of the valve shows the mechanism for opening and closing the valve is on a slide plate.  Perhaps this valve malfunctions during deceleration.