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i meant if your plugs are already kinda old. I usually do mine right before i do a plug changeThe plugs look fine, tint darker than when I put them in but they look good.
you were lucky. i knew a guy (mechanic) who was in a hurry and doing a JB emission service for a customer (similar to seafoam). he stuck the brake booster line in the can and let it eat. it hydrolocked the engine and his shop had to repair it and eat the bill.I might have been lucky
A Cadillac mechanic actually destroyed a Northstar by doing this (broke a rod).