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.
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.
On the 1FZ-FE engine, idle air control is by a stepper motor type IAC valve located in front of the air intake chamber. Intake air bypassing the throttle valve is directed to the IAC valve through a passage. The stepper motor consists of four coils, a magnetic rotor, valve shaft and valve. When current flows to the coils due to signals from the ECM, the rotor turns and moves the valve shaft forward or backward, changing the clearance between the valve and the valve seat. In this way, the intake air volume bypassing the throttle valve is regulated, controlling the engine speed. On this engine, there are 125 possible positions to which the valve can be opened.
On the 2RZ-FE, 3RZ-FE and 5VZ-FE engines, a rotary solenoid type IAC valve is located in front of the intake air chamber and intake air bypassing the throttle valve is directed to the IAC valve through a passage, controlling the engine idle speed. The ECM operates only the IAC valve to perform idle-up and provide feedback for the target idling speed.
On the 2UZ-FE engine, the idle speed is controlled by the Electric Throttle Control System (ETCS). It does not use an IAC valve. ETCS is composed of the throttle motor to operate the throttle valve, the magnetic clutch to connect the throttle motor with the throttle valve, the throttle position sensor to detect the opening angle of the throttle valve, the accelerator pedal position sensor to detect the accelerator pedal position, the ECM to control the ETCS and the one valve type throttle body. The ECM controls the throttle motor to make the throttle valve-opening angle properly for the target idle speed. If the electric throttle control system malfunctions, the magnetic clutch separates the throttle motor from the throttle valve as a safety disconnect. The ECM shuts down power for the throttle motor and the magnetic clutch and the throttle valve is closed by the return spring. The throttle can still be controlled by the accelerator pedal through the throttle cable.
Except 1FZ-FE Engine
- Using an ohmmeter, measure the resistance between terminal +B and other terminals ( RSC and RSO ). It should read between 17.024.5 ohms.
- Connect the positive lead from the battery to terminal + 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.
- Remove the IAC valve.
- Apply battery voltage between 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, it may be necessary to replace the IAC valve.
- Install the IAC valve.
REMOVAL & INSTALLATION
- 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.