Can Car Talk Diagnose Ed's GMC Sonoma Mystery?

Dear Car Talk

Dear Car Talk | Feb 18, 2016

Dear Car Talk:

I have a 2003 GMC Sonoma ZR2 with the 4.3 Vortec V-6 engine. I bought it used, with 145,000 miles on it, and it is a great truck. But it has one strange problem. I did a tune-up. I put in new plugs, new wires, a distributor cap, etc. Then about two weeks later, while driving, it started misfiring. It was dark, and when I opened the hood, the wires were arcing out -- I could see the sparks. OK, so I figured I got bad wires. I replaced them, and after a month, the same thing happened. Oh, and I purchased the wires and plugs that were original equipment. To make a long story short, I am now on my fifth set of wires. I can't figure out what's wrong. Any ideas would be greatly appreciated.

-- Ed

That is a weird one, Ed. Well, now you know why the previous owner sold it.

If the wires are original-equipment GM Delco wires, it's extremely unlikely that you'd get five bad sets in a row. So something is creating a huge amount of resistance in your secondary ignition system.

Like most husbands, electricity prefers to take the path of least resistance. So if electricity is jumping off all of your plug wires to get to ground, I'm guessing there's some enormous amount of resistance at your plugs.

One possibility is that you've gapped them all wrong. If there's supposed to be a .040-inch gap in your plugs, and you've gapped them all to .100 or more, the spark would not be able to jump across that gap to get to ground, and would seek another path.

And perhaps it takes a few weeks for the wire's insulation to break down enough to let the arcing begin. Or perhaps the wires aren't attached correctly to the plugs, and rather than try to jump that gap, the spark bleeds out through the wires. Or maybe, somehow, your coil is putting out way too much voltage.

But these are just guesses, Ed. I really don't know. Maybe some of our readers have had and solved this unusual problem and can tell us what they found. Email us from, and we'll pass along any credible-sounding answers.

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