The ECM controls idle rpm with the IAC (idle air control) valve. The idle rpm is programmed into the PROM. To increase idle rpm the ECM moves the IAC valve out allowing more air to pass by the throttle plate. To decrease rpm it moves the IAC valve in to reduce air past the throttle plate. A scan tool will read the ECM commands to the IAC valve in counts. Higher the counts the more the air is being allowed to pass the throttle plate (higher idle). Lower the counts the less the air is being allowed to pass the throttle plate (lower idle).
- Warm the engine to normal operating temperature. This is important ... the car needs to be in closed loop.
- Connect scan tool. Verify it's in closed loop.
- Put the car in park and wait a couple seconds for it to settle into a regular idle.
- Look at the IAC counts (# and not % in TurboLink™)
- If the IAC counts are between 10-40, then it's pretty close. Otherwise, do the following adjustment procedure, with a target number of around 20-25 counts.
- The Minimum Air adjustment screw is on the dirver's side of the throttle body on the throttle linkage arm (often times misunderstood as the "idle adjustment screw". Pull off the gray rubber cable protecting the screw head.
- Turn the screw in VERY small amounts (1/4 turn or so) to adjust the Minimum Air setting. Turning it counter clockwise closes the throttle blade and will normally raise the IAC counts. Turning it clockwise opens the throttle blade and will normally lower IAC counts.
- Carefully monitor the IAC counts while making small adjustments to the Minimum Air setting screw. Be sure to allow the car to "settle" a bit after each adjustment a couple of seconds.
- Once you have IAC counts adjusted to within your target range, adjust the TPS as described HERE.
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