Idle Air Control Valve. This electric valve is driven by the computer to bypass a controlled amount of air around the closed throttle plate to keep the engine idling at the proper RPM. It's mounted on the back of the throttle body. It looks kind of generally like a cylinder about 3 inches long with a 2-wire plug at one end and two water hoses out the bottom (to preheat the air for better running in cold weather).

To clean it, unbolt from the throttle body and spray carb cleaner into the air passages thus revealed. Probably going to find a lot of black gunk in there after 300,000 miles. Also go ahead and take the intake hose off and clean the throttle plate and throttle body while you're there. You don't necessarily have to disconnect the water lines from the IACV unless you want to take it completely out of the car.

 

These pictures may help you locate and identify the EACV (IACV).

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