Pickup’s engine skips a beat now and then

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The engine in my ‘84 GMC S-15 pickup has begun to skip a beat at times, usually when I’m accelerating at mid-speed. I replaced the spark plugs, and fuel and air filters, but it didn’t help. Where should I look for the cause of this problem? I don’t mind taking it into the shop, but it would be great if I could fix the problem myself.

– Stan Lanos, Augusta, S.C.

You made a nice dent in the diagnostic process by renewing the spark plugs and filters. Assuming the engine runs smoothly most of the time, we can assume it’s OK mechanically. A burned valve, intake manifold leak or other mechanical fault generally causes a continuous, rather than intermittent, symptom.

If the engine idles and drives smoothly most of the time, this also rules out the carburetor or a vacuum leak as possible faults.

A more gradual power loss or stumble could be caused by the carburetor, but I’m keying in on your statement that “the engine skips a beat.”

What’s left? It’s either the ignition system or an intermittent fault in the engine management system. (The latter will be visited next week.) The ignition system works harder under part-throttle acceleration than any other time. A borderline fault in an ignition component or connection will usually rear its head first, intermittently causing a misfire, as you pass through a certain part-throttle driving condition.

If the symptom had surfaced after you had renewed the spark plugs, I’d be concerned that one or more plugs might have become cracked during installation. This occasionally happens due to an unfortunate angle or twist of the socket extension, especially when the plug is difficult to access.

My hunch is the Jimmy is suffering from a faulty spark plug wire or distributor cap. Excessive electrical resistance, due to a corroded terminal, makes it difficult for spark to reach the plugs. A pinhole or split in the wire’s insulation or boot might allow spark to escape to a nearby metal object rather than making it to the plug. A close visual inspection of the wires is a good start. The terminals at the cap and plug end of each wire should be shiny metal, not black or powdery white. The boots should appear flawless when bent and twisted, and don’t forget to check the wire between the ignition coil and distributor cap.

A resistance check also can be made, if you have a simple volt-ohm meter. Good wires should have less than 10,000 ohms per foot of length. Higher than this means trouble. Follow up with inspection and/or replacement of the distributor cap and rotor and an available voltage check. This requires the use of an inexpensive tool resembling a spark plug with a huge electrode gap. The spark-tester tool is temporarily connected to the end of one of the spark plug wires and clipped to a grounded metal object. If cranking or running the engine results in a sharp, snappy blue spark jumping the tool’s air gap, the ignition system is sound. If not, an ignition coil or module fault might be the cause.

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

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