1998 Ford Ranger Surging

Try cleaning your IAC valve.

Here's the standard procedure:

Disconnect air intake ductwork from throttle body.

Start the engine, then increase and hold the idle speed to 1,000 to 1,500 rpm.

Spray throttle cleaner or engine cleaner into the throat of the throttle body, aiming for the idle air bypass port (usually located on the side or top of the throttle body opening). Give this area a good dose of cleaner (about 10 second's worth).

Turn the engine off to allow the cleaner to soak into the IAC passageway.

Wait about three minutes.

Restart the engine, rev and hold at 1,000 to 1,500 rpm, and repeat the cleaning process again.

Turn the engine off again, and reattach the air intake ductwork to the throttle body.

Start the engine and rev and hold to 1,500 to 2,000 rpm until no white smoke is coming out of the exhaust pipe.

If it doesn't fix your problem, the IAC valve could be defective.