Alan --

Two things come immediately to mind -- first, the throttle switch, which tells the FI ecu ("electronic control unit") that the throttle is at idle position (essentially closed) so the ecu can take control of idle speed. If the switch (or throttle linkage) is misadjusted or misbehaving, the ecu may not take control of idle.

Second is the CIS motor ("constant idle speed"), which is also called the "idle air valve." This is what the ecu controls. This computer-controlled air valve bypasses a small amount of air around the (closed) throttle while the ecu monitors idle speed. If the idle increases, the valve is driven closed slightly. Likewise, if idle drops, the ecu opens the valve slightly. The ecu monitors idle speed and opens-closes the valve to maintain this speed.

This CIS motor can sometimes be revived by washing through it with carb cleaner. The accumulated gummy varnish can cause it to stick, so idle speed varies wildly. Washing out the crud sometimes restores the valve to functionality.

However, sometimes it's the motor portion, and generally this is not recoverable. You must install a new valve (around $200-250 for a new one, I think).

The CIS motor is mounted under the intake manifold, and not as difficult to replace as it appears. It should take under an hour to complete.

Don Foster (near Cape Cod, MA)