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Electrical short in spark plug wires

My 1995 GMC (350 cu in 8 cyl) runs rough sometimes. At night (in the dark) I can see electricity jumping right through the spark plug wires to metal, near the plugs, and the engine lugs down when this happens. Not just in one wire - many of them. Any idea why? Bad ground somewhere?

that is the textbook illustration of what constitutes bad plug wires. also possibly, the wires have been pulled up from the plug tip, while the rubber boot stayed in place, then there is not much coverage over the back end of the plug contact. either way, excellent reason to replace the wires.

Time for a new set of wires. Advance Auto had an AC-Delco OEM wire set for my 93 Caprice for about $40.

Ed B.

The insulation on your plug wires is failing. They have to be replaced. It has nothing to do with ground connections. The shorts that you are seeing are due to cracks in the insulation or other insulation high voltage break down issues.

Tip for the job: Replace only one wire at a time so that you don’t end up plugging the wires into the wrong place.

I’m not sure what kind of ignition design your truck uses but if the high voltage coil has a ground then I would check it. It does sound like the system is looking for the least resistance to ground, I don’t think there would be multiple wire failures causing this so I think a ground problem is a good bet. Also check the battery to engine grounding. You may have to replace the wires now due to damage to them. Another possiblity is the plugs are faulty and are causing the trouble.

The only ground on the coil is going to be the one that completes the 12VDC primary circuit. If that one’s bad, there’ll be no (or poor) current through the windings and no (or poor) field collapse into the core, resulting in no (or poor) voltage to the plug wires. In short, it ain’t the coil ground. It’s the ignition wires themselves that are bad.

Seconded. OP’s description is a textbook description of bad wires. Or rather, failing insulation around the wires. Not terribly unusual that the rubber would begin to fail in multiple places at the same time, it’s all the same age and exposed to the same conditions…do you ever find your windshield wipers tend to wear out at the same time?

The wires are likely due for replacement but the root cause will probably be the spark plugs. Aged plugs with widened gaps and subtle misfires will cause this kind of problem.

A spark has a much harder time jumping a aged/widened plug gap when it’s under compression. The spark is going to go to ground somewhere. If it has too much trouble jumping the gap it’s going to take the next best option; going through the wire.

OK’s reply above deserves six stars.

I agree. His point is an excellent one. The plugs should be replaced as well as the wires. And, frankly, if this '95 is a disributor based ignition system, my bet is that the cap and rotor are overdue also. Although not relevant to the problem, new filters are a good idea also.

Thank you all for the responses. I replaced the spark plug wires and THE PROBLEM IS RESOLVED!

I had already replaced the distributor cap and rotor the day before, and the plugs are only about 10K miles old.

The thing that threw me off is that there was shorting on so many of the wires at the same time… but as you-all pointed out, I guess they were ALL old…

Thanks again.

Thanks for the follow up post. We often never hear the results, and it’s good to know when someone has been helped.

Happy motoring.

I think the point is still being missed here. You have replaced the wires and the problem has been solved; for now and apparently.
If there’s a chronic subtle misfire problem due to aged plugs, widened gaps, etc. then down the road the same thing is going to happen with the new wires.

Eventually those errant sparks, which could be compared to a rabid dog trying to get out of an enclosure, are going to keep gnawing at the wires, boots, distributor cap, coil terminal, or whatever until they find their way out.
The only thing you may have done at this point is stiffened the dam up a bit.

If you go back into the plugs I suggest you check the plug gaps before installing them. It’s always assumed the gap is preset and cannot be corrected if necessary. This is not true.

This is better designated as “electrical leakage from spark plug wires”.

Thanks for the update PS. I agree with OK4450 about the plugs and have to wonder if this trouble was really caused by replacing the plugs. I would first want to know if the replacements were OEM recommened ones and if they aren’t I would change them out. Since the plugs only have 10k miles on them my thought is the new plugs caused the real trouble. It may be the gap issue but I think there may be a internal resistance issue with the plugs. The plugs just may be faulty from the manufacturer. That is a pretty remote possibility but it does happen. Looking at the pulses with a scope would be a big help.

I missed the part about replacing the plugs about 10k miles back. My bad.
The plugs could still be behind this problem to some degree though.

My memory is hazy on this but it seems to me that each additional .001 in the gap translates to roughly 1000 volts extra from the coil. A plug, or plugs, that have .010 extra on the gap for example (translating to an additional 10,000 volts) could be what was tipping the spark into exiting somewhere else.

If the plug gap on this truck is around .035/.040 (guessing, don’t know off the top of my head) and the plugs out of the box are wide at .060 that could be a problem.
The effect of failing wires on the plugs should also be a consideration.

Lol just change the wires, check

He’s changed the wires. The purpose of the spark plug exercise is to try and assure the wire changing process is not repeated the same time next year.