I had a head gasket starting leaking very slowly. The 12 months or so before I realized it, I noticed a white residue on many of the coolant hoses. The only thing I can think of is exhaust gases in the coolant somehow permeated the rubber hoses and evaporated on the exterior, leaving a white, dusty-looking residue that wiped right off. The residue ended exactly where the radiator necks began, so there's no question that it was directly related to the coolant inside the hose.

Strangest thing...

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Suppose you were an idiot, and suppose you were a member of Congress; but I repeat myself. - Twain