Expert: Everett Peacock - 4/23/2010

Question
I have a 5.7 L mercruiser(1996) with carb.  The water temperature is fluctuating and acting strange.  It will go along at 170 or so at 3800 RPM for a long while then suddenly jump to 250 causing the alarm to go off.  It will drop again suddenly to 170 even if I keep going.  I replaced the thermostat and the gauge.  The sending unit could be the problem I guess but I just don't know what to check after that.  The out drive was rebuild just a few months ago and bellows changed water pump replaced.  Any thoughts about what could cause this strange jump in temp?

Answer
make sure the elbows are not cloged up....  I had a 89 454 that could not keep cool....even replaced the engine...still run hot....I installed a thru hull fitting for the water intake and the problem went away.. the water passage that gos thru the transom assy built up enough corrosion to restrict water flow enough to overheat...