fluctuating idle

i own a 91 passport optima(pontiac lemans). I have an idle fluctuation of about 200-300 rpm. the rpm bounces between about 600 and 900 rpm at idle. i just purchased the car and did some basic maintenace such as fuel filter, spark plugs, cap&rotor and 02 sensor. I also cleaned throtle body and intake well with carb cleaner. I'm thinking that this fluctuation may be caused by a faulty IAC valve. Any thoughts? Thanx.

could be IAC, but that's not what I'd check first. Reset the computer by disconnecting it for a while. Some say just a few seconds, but some of those little capacitors keep juice for a long time. If that doesn't fix it, chase down every gasket with a can of starting fluid. Spray short bursts of fluid on mating surfaces that might have a vacuum leak. Usually this idle fluctuation is caused by timing being advanced and retarded when its not required, which is why I said check the ECM. Also check your initial timing if its adjustable. It should be in the 91 model. The next place I would check is the Fuel Pressure Regulator. If its on its way out it could be supplying the fuel rail with incorrect pressure. The fuel pressure would rise and the engine would compensate by reducing pulsewidth. The cycle continues. Often if its the FPR, the idle fluctuation is different depending on when the engine is warm or cold.

Lastly, check the IAC. Usually, if it fails, it just sticks in one place. It doens't fluctuate unless the computer is telling it to.

Ditto.

just look like the same problem with my honda crv 97
finally i took out the IAC valve clean with carb cleaner and the problem solve.(usually there are dust block the air flow )

good luck