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What I'm thinking is that the insulator cracked, but the plug is still firing. The heat and combustion is shooting out of the plug and killing the coil boot.
With mine I think that the gap was way too far and the cylinder wouldn't fire. It was missing the whole way home and had been ticking for months. (I thought it was an exhaust leak.)
Luckily the fragments from mine didn't break anything else. This 5.4 of mine has had a rough life.
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