Why is rpm fluctuating on 94 Geo prizm?

Answer 1

If you have a "surging idle" (idles speed goes up and down repeatedly cycling every few seconds) this is most commonly caused by a lean condition and lean conditions are usually the result of a vacuum leak.

If the idle speed is significantly greater when the engine is cold than when it is fully warmed up, this is normal.

If the idle speed is greater when warmed up than when cold, this indicates a malfunctioning idle speed control (IAC). You can get a replacement unit from your parts supplier or Toyota dealership parts department.

If the idle switches between two speeds at regular intervals this may indicate that you have the air conditioning (or defroster) engaged.

Check throttle positioner adjustment.