Q: I have an excellent 1993 Ford Exploder with a 4.0-liter engine and electronic fuel injection. It's a daily driver and everything works.
But there's one problem: It idles around 500 rpm and, until it gets real warmed up, it stalls at all stops. I have learned to keep my foot on the accelerator and the brake at the same time, but this seems idiotic. Surely, there must be a way to make a minor adjustment so it idles at 600-650 rpm, which would alleviate this problem. The one mechanic who looked at it said I would have to replace the engine control unit for $500. No way that will happen. Any suggestions?
_Ross
A: I didn't edit your spelling of your SUV's name because "Exploder" is indeed a popular nickname for the Explorer. But don't worry: An explosion isn't likely!
Your engine's idle speed is controlled by a gadget called an idle air control solenoid, which is commonly called an IAC. This part is basically a variable position plunger that meters air around a closed throttle blade. When energized, a coil of wire surrounding the plunger creates a magnetic field, moving the plunger. The powertrain control module, or PCM, meanwhile, has programmed idle speed values for varying operating conditions and sends electrical pulses to the IAC to try to make this happen.
There are three possible causes for your Exploder to idle too slowly. The IAC might be gunked up with carbon or electrically faulty, there could be a wire fault or circuit fault, or the PCM may have a failed driver (control transistor).
Let's test the IAC by doing the following: Obtain a T-pin and 2-3 feet of insulated wire. Bare both ends of the wire half an inch and twist one end beneath the head of the T-pin. Locate the IAC with the engine not running. It's a cylindrical metal part similar in shape and size to a pill container and is located near the throttle with a two-wire electrical connector. Gently slide the T-pin into the IAC connector alongside the white/light-blue wire so it touches the wire's terminal just inside the connector and then drape the length of wire and its bare end safely away from moving parts or metal. Start the engine with the transmission in park and the parking brake set. Standing beside the vehicle, touch the bare end of the wire to engine metal. If idle speed increases considerably, the IAC works. If it doesn't, either the IAC is faulty or the other wire (red) doesn't have power (unlikely).
If the idle speed increases when you control it, this means the PCM can't do it when it wants to. If the white/light blue wire (circuit 21) is properly connected back to the PCM, the PCM may be faulty. When a PCM driver (transistor) fails, the failure's usually caused by a shorted actuator (the IAC solenoid's coil should be checked for possible insufficient resistance) or the part could wipe out a replacement PCM's transistor as well. Due to the age of the Exploder you should be able to get away with a recycled (wrecking yard) PCM, as long as it came from an identical vehicle (emissions, same engine, transmission, axle ratio, air conditioning, et cetera). PCM swapping doesn't work for some other and all more recent vehicles. The IAC test procedure I mentioned is unique to most Ford vehicles of this vintage. Other vehicles require differing procedures.
