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Idle Air Control (IAC) valve, checking
The IAC valve (more commonly known as the Idle Stabilizer Valve, or ISV) is bolted on to the side of the intake manifold, and lets the ECU let a bit more air into the mixture if the engine is loaded down under idle -- this prevents the engine from stalling under certain conditions.
The first thing to do (because it's easiest) is to make sure that the ISV is not electrically faulty. To do this, disconnect the harness connector from the ISV:
Now check the resistance across the two terminals. You should get a value between 7 and 11 ohms. If you do not, you need to replace the valve.
If that was OK (probably it was), you should clean and lubricate the valve. To do this, remove the three 10mm bolts that hold the valve on to the intake manifold, and gently pry the valve off of the manifold. Be careful not to lose the gasket. Then visually inspect the sliding part of the valve (arrow) for signs of scoring or scratching.
If there is excessive scratching, you might need to replace the valve; in any case you should try lubricating the valve first. Take your favorite cleaner/lubricant (i.e. WD-40) and spray a lot in the two large holes. Let it sit for a bit, then shake it a bit and drain the lubricant. Repeat once or twice until the valve looks clean inside, then re-attach the valve. Tighten the bolts to 10Nm (7 ft lb).
If that does not work, you can check to see if the ECU is activating the valve. To do this, push back the rubber boot from the harness connector (leave the connector on the valve):
Probe the two connections with a multimeter while the car is running; at idle you should see a little activity. If you don't see anything, try switching the air conditioner off and then on -- the engine should lag a little bit, and then the ISV should activate to bring the idle speed back up.
If you don't see any activity, you can check the wiring between the ISV and the ECU. ISV connector pin 1 corresponds to ECM connector D, pin 11. ISV connector pin 2 corresponds to ECM connector D, pin 7.
If the wiring is OK and the ISV does not activate, you might need to replace the ECM. If the wiring is OK and the ISV appears to activate, but the engine still stalls, and you saw excessive scoring on the sliding part of the valve, you might need to replace the ISV.
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