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The ones on my car right now SUCK at keeping water/debris out of the plug wells. I've had quite a few misfire problems in the last few months because of them. I'll try to remember to check what brand they are after I get out of work.
lots of times this happens not because of the wires but because non-Motorcraft plugs are installed. the Motorcraft plugs are designed to a Japanese spec, not a SAE spec, so the non-Motorcraft plugs are too long. this makes the wire end not sit down against the valve cover, therefore it does not seal, therefore you get junk in your plug wells.
Two of my donuts worked, but one didn't.