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Message: <blockquote data-quote="MrGiggles" data-source="post: 499782" data-attributes="member: 22613">A 20+ year old water pump seems far more likely than a somewhat rare head gasket failure, Occam's razor and all. Some of those Mercs especially have a tiny water pump that is hardly enough when it's in good repair.Try just seeing if the powerhead is actually hot when it's alarming, it should be hot to where it hurts to hold your hand on it for more than a few seconds, but not hot enough to burn you instantly. Can also shoot it with an IR temp gun if you have one.While you have the lower unit off to service the water pump, remove the thermostat and flush the powerhead from the top down with a garden hose. Also remove the grates from the lower unit and make sure that passage is clear.</blockquote>