If you experience cold starting problems, stalling at idle, or hesitations during acceleration, it may be the idle air bypass valve. This valve helps the vehicle idle steadily and run when the accelerator pedal is not pressed.Google+
Car Problem Reports
Ford Ranger Hard to Start Cold, Stalling at Idle
Ford Ranger Problem
Engines Affected: 2.5L 4 Cylinder, 3.0L V6, 4.0L V6
Average Mileage: 177,000 mi (130,000 mi - 215,000 mi)
Visitor, 1998 Ford Ranger, 2.5L 4 Cylinder, 215,000 mi
doesn't like the cold weather. idles very low and won't stay started. when it warms up, it's fine
Visitor, 1998 Ford Ranger, 4.0L V6, 145,000 mi
Truck hard to start when cold. Replaced aic valve mass air flow sensor and battery. Truck still stalls and lacks power.
Visitor, 1991 Ford Ranger, 3.0L V6, 204,000 mi
Truck runs fine when warm, but has trouble starting when cold and holding an idle. Anything above 3000 RPMs and it stalls outright. Other random times during idle it will mysteriously sputter and cough, then die. Already spent $100 replacing distributor cap, coil, rotor, plugs, and wires (everything except spark plugs, because I was lacking the correct tool). After doing work, Ranger ran fine (because it was already warm). Next day went out to start it up and go to work, but it barely made it a block down the street, died, and wouldn't start back up.
Visitor, 1998 Ford Ranger, 2.5L 4 Cylinder, 130,000 mi
Engine begins to rough idle while stationary, at a light etc., but If the gas pedal is given a quick tap it will smooth out and continue to run, otherwise it will stall. Restart has not been a problem so far and the number of occasions has been minimal. Has not been in for repair yet.