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[ams]

.::Start here for cold/warm idle issues::.

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).

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 (Throttle Position Sensor) 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.

SOURCE article


Side note: I don't have a picture of a Subaru IACV but if anyone does, please post it up or email it to me and i'll attach it to this post.