The idle air control (IAC) valve on a 1999 Volvo V70 regulates the amount of air coming in and out of the throttle body in order to raise and lower the engine idle speed. The power train control module, or PCM, controls this valve. As the vehicle ages, carbon accumulates on the inside of the valve, preventing it from properly responding to the PCM's commands. Sensor-safe aerosol cleaning solvent is the only chemical that can safely clean the IAC valve without damaging its electronic components.
Stop the vehicle on a flat surface and shift the transmission into park. Turn off the engine and set the parking brake. Open the bonnet and let the engine cool down for one hour.
Locate the IAC valve. It is on the left side of the throttle body, on top of the intake manifold. It is made from black plastic and shaped like a roll of quarters. It protrudes approximately 1 inch from the throttle body.
Disconnect the IAC valve's electrical connector and remove the two Torx-head screws holding it to the throttle body using a Torx screwdriver. Gently twist the IAC valve back and forth while pulling it away from the throttle body until it's free.
Set the IAC valve on a clean shop rag and spray it with sensor-safe aerosol cleaning solvent. Use another shop rag to scrub inside the valve. Spray it once more with the cleaning solvent and blow out the valve with compressed air. A compressed air hose hooked up to an air compressor is an ideal air source.
Reinstall the valve into the throttle body. Reinstall the Torx-head retaining screws and reconnect the valve's electrical connector. Start the engine and verify that it operates properly.