Purpose & Problems of a Honda Idle Air Control Valveby Dan Ferrell
The computer on your Honda fuel-injected engine uses the Idle Air Control (IAC) valve to control idle speed. When you fire up the engine for the first time during the day, the engine is below operating temperature. The IAC valve lets air bypass the throttle plates to increase engine RPM and smooth engine operation. As the engine warms up, the air control valve begins to close. However, the IAC valve's internal components can wear out and develop problems.
The air control valve on your Honda is mounted on the throttle body or intake manifold. The IAC is a solenoid-operated valve, similar to an electronic switch, controlled by the computer. When the computer needs to increase engine RPM, it energizes a coil inside the IAC, which pulls a shaft to open an air valve. Loose or broken electrical wires, connectors, rust buildup, broken coils or weakened IAC components will prevent valve operation. When checking the valve, inspect the wiring and electrical connector. Make sure the wires are not damaged and the connector is free of rust and fully attached to the wires.
There are several things you can do to check the air control valve. With the engine off, unplug the IAC valve electrical connector. Then use a couple of jumper wires to send direct battery voltage to the valve connector. You should be able to hear the valve working. An alternative method is to check the valve windings' resistance using an ohmmeter. Either way, compare your results with the specifications listed in the vehicle service manual for your specific Honda model. If the valve is within specifications, unplug the electrical connector and remove the valve. Depending on your Honda year and model, the IAC valve may be attached to the side or bottom of the throttle body. It may be held in place by two or three socket-head screws that you can remove using an Allen wrench. Make sure to keep the gasket located between the valve and throttle body. Inspect the air passages in the valve and the throttle body for blockages. Clean any carbon deposits from the passages, if necessary, being careful not to damage the valve. Use carburetor cleaner on the throttle body to clean the passages. Not all air control valves can be cleaned. Consult your vehicle service manual, if necessary.