Does the "wire" part of spark plug wires ever really wear out. In doing a tune-up the other day, a bunch of black dust fell out of the boot when I removed coil wire from the dist cap. That center terminal on the cap was all burnt up and black. So was the connector on the cable end. I think it was loose and arcing. Anyway, I can buy just the single cable and save money over buying a set. I'd rather not buy the entire set if its unecessary.
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The wire itself doesn't wear out specifically, but the insulation around it deteriorates and will cause a voltage leak. This can lead to stalling and stumbling, especially in wet weather, and can also get you zapped pretty good if you touch a wire with the engine running.