Idle Air Control

Idle Air Flow.:

Idle Air Control Valve.:

Operation:
Engine idle speed is controlled by the PCM through the IAC valve, mounted on the throttle body. The PCM sends voltage pulses to the IAC valve motor windings causing the IAC valve pintle to move In (toward the seat) a given distance (a step or count) for each pulse.

The commanded location (steps away from the seated position) can be observed as a number of counts, as displayed on a Scan tool. The pintle movement controls the airflow around the throttle valve, which in turn controls engine idle speeds:
^ Pintle Extended = Decrease RPM = Lower Counts.
^ Pintle Retracted = Increase RPM = Higher Counts.

The controlled or desired idle speed for all engine operating conditions is programmed into the PCM. The programmed engine speeds are based on coolant temperature, park/neutral switch status, vehicle speed, battery voltage, and A/C (if equipped).

Learn and Memory:
The PCM learns the proper IAC valve positions to achieve warm, stabilized idle speeds (rpm) in drive. This information is stored in the PCM keep-alive memories (information is retained after ignition is Off). All other IAC valve positioning is calculated based on these memory values. As a result, engine variations due to wear, and variations in minimum throttle valve position (within limits) do not affect engine idle speeds. This system provides correct idle control under all conditions. If power to the PCM is interrupted, the PCM has to relearn idle control. Until idle control is relearned, it may be necessary to partially depress the accelerator pedal when starting. This also means that disconnecting power to the PCM can result in incorrect idle control or necessity to partially depress the accelerator when starting, until the PCM relearns idle control. Engine idle speed is a function of total airflow into the engine based on IAC valve pintle position plus calibrated vacuum loss through access ones.

Minimum Position:
The minimum throttle valve position is set at the factory with a stop screw. This setting allows enough air flow by the throttle valve to cause the IAC valve pintle to be positioned a calibrated number of steps (counts), from the seat, during controlled idle operation. The minimum throttle valve position setting on this engine should not be considered the minimum idle speed, as on other fuel injected engines. Because of low internal friction, provisions for slight production variations, and the various altitudes production vehicles operate, the calibrated air rate at the minimum throttle position is too low to allow most engines to idle. The throttle stop screw is tamper resistant and should not be adjusted.

Reset Procedure:
The PCM resets the IAC valve pintle position only after a power is shut off. During the reset, the PCM commands the IAC valve pintle to move in to the seated position (to establish the zero count position), and then back out to the desired position.

The IAC valve also can be reset without driving the vehicle by using the following service procedure:
1. Disconnecting the battery.
2. Use the service tool (Saturn Tool) SA9195E and run the pintle in.
3. Use the portable diagnostic tester (Scan tool) and commanding the pintle in.

If the IAC valve is disconnected and connected while the engine is running, the resulting IAC valve counts may not correspond with the actual IAC valve pintle position. When servicing the IAC valve, it should only be disconnected or connected after the ignition has been turned off for at least 10 seconds. This allows time for the PCM to move the IAC valve to the 100 count position where it is parked while the ignition is off. If this procedure is not followed, the PCM will lose track of IAC valve position resulting in starting or idle control problems until the IAC valve pintle position is relearned.

IAC system problems may cause improper idle speeds, resulting in a DTC 35 (IAC Circuit Fault) or DTC 49 (Vacuum Leak). The DTC's 35 and 49 charts should be used to diagnose these problems.