There's tons of postings about "MY IDLE GOES UP & DOWN! WHAT DO I DO?" It's tempting to yell & scream & tell people to search, but hopefully this will be more useful than flaming...
Here's what to check:
Look for (& fix) vacuum leaks. They can be anywhere...
Get ALL the air out of the cooling system. Some years have a bleeder screw near the upper radiator hose. You also have to squeeze the hoses & work the air over to the radiator cap. Air likes to hang out in the heater core too, so have the temperature slider set to full-hot when you do anything.
Idle Air Control Valve (IACV)
is bolted on the back of the intake manifold. Clean it out with throttlebody cleaner.
Fast-Idle Thermo-Valve (FITV)
is mounted under the throttlebody (up thru '95). If your car has it, clean it out too. Sometimes it's guts get loose, so you can drain some coolant & open up the cover. Tighten & put it all back. (Beginning in '96 the FITV's job was built into the IACV.)
After everything else is fixed, set the base idle speed like this.
- Warm up the engine & turn off ALL accessory loads.
- Unplug the elecrical plug from the IACV, hold the revs up so it doesn't stall. Gently let it drop to idle.
- Adjust the idle screw (above the TB bore) to get 480 rpm.
- Turn off the engine, plug in the IACV, pull the ECU fuse or battery cables to reset the ECU.
- Start the engine & let it idle for several minutes without touching the throttle.