My mom’s ’92 Mustang 2.3L occasionally stumbles badly when taking off from a stop. It’s like someone momentarily shuts off the key, then turns it back on. I’ve done all the usual maintenance including plugs, plug wires, cap, rotor, filters, etc., and had a buddy scan the computer for codes. There were none. What gives? Shouldn’t there be a code for something like this?

It always seems to do the skip at about the same point during a take-off – just after you step on the gas. It starts and idles well, and once you get it rolling, it’s fine at speed. Any ideas? Or am I headed for a big-bucks repair bill?

-Johnny Chamberlain, San Jose, Calif.

You made a great start on this problem by getting the maintenance items out of the way first. Many of the parts you replaced were possible offenders and it’s nice to be able to rule them out. Since the engine idles well and runs OK at higher speed, I’ll roll the dice and assume the fuel pressure and volume are OK and the fuel injectors are clean.

Also important is the ignition system. To check it you’ll need a spark tester, available at a pro-level auto parts store or tool truck. This very cool $10 tool resembles a spark plug with a huge electrode gap and has an alligator clip attached to one side. The tool is attached to the end of one of the spark plug wires and clipped to a bare metal engine part. Should spark jump across the tool as the engine is cranked or running, you’ve got enough spark for two cars! If not, the ignition coil or control module might be faulty.

Based on your excellent description of the symptom and conditions under which it occurs, my hunch is the Mustang might be suffering from a faulty throttle position sensor. This sensor differs from most under the hood as it has a mechanical component within, which means trouble in the long run. The sensor contains a potentiometer – a variable resistor – and is connected to three wires. One wire supplies a reference voltage to the sensor, another is a ground and the third is a signal leading back to the engine computer.

As the throttle opens and closes, the potentiometer’s wiper sweeps across a resistor, like a windshield wiper across the windshield, and returns a portion of the reference voltage to the computer. After countless sweeps, perhaps more than 75,000 to 100,000 miles, the signal voltage might develop a momentary dropout in the throttle’s range of motion, kind of like your wipers skipping over a bug. This momentary loss of signal simulates a rapid close of the throttle, and the engine control computer chops fuel delivery, causing the stumble.

Were the problem more severe, a trouble code would set, but momentary glitches can escape detection by the Mustang’s rather primitive On-Board-Diagnostics-1 system.

Checking the throttle position sensor involves sneaking a T-pin into the back-side of the sensor connector, touching the gray/white wire’s terminal, and observing the voltage as the throttle is slowly swept open and closed. (Do this with the key on and the engine off.) A digital voltmeter is needed, preferably one with a rapid-display bar graph. Brief dropouts can be hard to see, so sweep the sensor hot and cold, many times. Anything but a smooth change in voltage indicates a faulty sensor.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.