Originally Posted by waybomb

Get a can of gumout carb cleaner. Spray it around the intake manifold while running. If you get a change in speed, you have vacuum leaks in your air distribution hoses. About a hundred bucks for everything and a few hours to change them.

Could be worse too - may need to change round intake manifold seals. Big job.

If that is not the problem:

While running, pull the ovp. If idle goes up further, not the ovp.

While running, pull the idle speed control located under the passenger side carpet on the metal piece attached to the floor, near the top, small black box. If idle goes up, not the control.

All that's really left then is the idle control valve, located on top of the engine just behind where the thermostat is.

A two wire connector is attached to it, and a 10mm head nut holds it in place.

With wires disconnected, should read about 4-5 ohms. Remove the valve. Shake it, it should rattle a bit. If not, clean the internal parts with that gumout you used earlier. Will take quite a bit of cleaning.

If that doesn't fix it, then find a known good valve and swap it out, before you spend a couple of hundred bucks on a new one.