Idle Air Control (IAC) Setup and Tuning Guide

IAC

IAC: What is it, why do we use it, and how do we tune it?

Today, we want to go over the Idle Air Control (IAC) and how it works. The IAC is the equivalent of a controlled vacuum leak that works to control idle speed. Using a carefully engineered passage and a stepper motor, idle speed is constantly monitored by the ECU and corrections are made using the IAC motor. It closes when RPM rises and opens when it falls too low, maintaining a stable, consistent idle.

During cold starts, extra air is commanded during warmup instead of holding open the throttle to keep it running like the carburetor of years past. Another useful feature of the IAC is bumping up the idle when extra loads like the A/C or cooling fans kick on. When the IAC fails, it can result in issues such as stalling, unstable idle, or low or high idle conditions. Some of these issues can have multiple causes, but drivability can absolutely be affected by a failed IAC.

Our IAC motors are 4-wire stepper motors with precise control for opening and closing. Each direction is controlled by a pair of wires with a high and low signal. The tip on the end of the IAC is what seals inside the throttle body when the IAC is closed.

Basic IAC Operations

1. Key ON: ECU resets the IAC to a known position.
2. Cranking: ECU retracts the plunger to allow additional airflow for cold start.
3. Running: ECU adjusts the plunger in small “steps” to maintain the target RPM.
4. Throttle Closed: IAC controls idle by allowing bypass air around the throttle blade.

How to Set the IAC

Setting the IAC on your Aces kit can be done in a matter of minutes with just a piece of tape and a screwdriver. The goal is to get the idle close to the desired target without the IAC interfering, so once control is returned to the system, it is not working overtime.

Bring the engine up to a temperature of 165°F. Once the engine has reached temperature, shut it down. Using a piece of tape, block off just the IAC port.

Restart the engine and observe the idle speed:

• Too low? Open the front set of blades on the throttle body until the idle gets close to the desired target.
• Too high? Close the front blades slightly until the desired RPM is achieved.

Once the throttle blades are set, remove the tape and check the IAC count on the handheld. It should be between 10 and 25. Reinstall the air cleaner, shut the hood, and go for a drive.

The IAC is now properly set and will work behind the scenes to maintain the RPM you want.