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The idle control valve (sometimes called the CIS motor or Idle Air Valve, IAV) is an electrically-controlled valve that bypasses a small amount of air around the throttle butterfly. It's controlled by the FI computer. This permits the computer to control the idle speed.
Three things (commonly) can cause the valve to malfunction. First, the valve itself can be NG. The brushes and commutator burn through. On an '84 car, probably with high mileage, this is most likely the cause. The only remedy is a replacement valve.
Second, the idle switch might be defective or misadjusted. This can also occur if the throttle body is varnished up so that the throttle does not fully close. The throttle switch, which "clicks" (turns on) as the throttle returns to idle position, tells the FI computer to start controlling idle. If the switch never clicks, the CIS motor is never engaged.
Third, a crumbling wiring harness might have defeated either the switch or the motor.
When I bought my '86 the idle was not controlled, even with a new motor. It turned out that the PO's meathead mechanic had simply adjusted the throttle mechanical stop to compensate for a bad motor and varnished TB. The result was that the switch never clicked. Even worse, the throttle had been adjusted open, so I had a 2000 rpm idle. A thorough cleaning and complete readjustment of the linkage and throttle stop restored it to perfect (and properly controlled) idle.
Don Foster (near Cape Cod, MA)
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