The purpose of the Idle Air Control (IAC) system is to control engine idle speed while preventing stalls due to changes in engine load. The IAC assembly, mounted on the throttle body, controls bypass air around the throttle plate. By extending or retracting a conical valve, a controlled amount of air can move around the throttle plate. If rpm is too low, more air is diverted around the throttle plate to increase rpm.
During idle, the proper position of the IAC valve is calculated by the ECM based on battery voltage, coolant temperature, engine load, and engine rpm. If the rpm drops below a specified rate, the throttle plate is closed. The ECM will then calculate a new valve position.
The IAC only affects the engine's idle characteristics. If it is stuck fully open, idle speed is too high (too much air enters the throttle bore) If it is stuck closed, idle speed is too low (not enough air entering). If it is stuck somewhere in the middle, idle may be rough, and the engine won't respond to load changes.
See Figures 1 and 2Excluding 1FZ-FE Engine
- Using an ohmmeter, measure the resistance between terminal +B and other terminals (RSC and RSO). It should read between 17.0-24.5 ohms.
- Connect the positive lead from the battery to terminal +B and negative lead to RSC, then check that the valve is closed.
- Connect the positive lead from the battery to terminal +B and negative lead to RSO, then check that the valve is open.
- If the valve did not perform as indicated, replace the IAC valve.
See Figures 3 and 4
- Remove the IAC valve.
- Apply battery voltage to terminals B1 and B2. Then, apply ground to terminals S1, S2, S3, S4 and S1 in sequence. Check that the valve moves towards the closed position.
- Apply ground to terminals S4, S3, S2, S1 and S4 in sequence. Check that the valve moves towards the open position.
- If the valve does not function as indicated, replace the IAC valve.
- Install the IAC valve.
REMOVAL & INSTALLATION
See Figure 5
- Drain the coolant to a level below the throttle body.
- Remove the throttle body.
- Remove the screws and the IAC valve. Discard the old gasket or O-ring.
- Using a new gasket or O-ring, attach the IAC valve and secure the mounting screws.
- Install the throttle body with new gaskets.
- Fill the cooling system with the proper water/coolant mixture.
- Start the engine, top off the cooling system. Check for leaks.