Sometimes they "leak" voltage right out the side, through the insulation. It's especially prevalent where there is metal to wire contact or near metal to wire contact.
Sometimes you can develop a carbon track down the side of the spark plug insulator. There will be a matching carbon track down the inside of the spark plug wire boot, even if you can't see it. I've tried just replacing the plug for this, and I've tried cleaning the carbon from the plug and replacing the wire, but the carbon track always come back. The only true cure is to replace both.
Buy only high quality plug wires (I use ACDelco). The best one's are from the factory. Cheap parts don't fix problems, they cause more.
-------------------- 1990 Montero RS (In pieces... for now)
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