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Mercruiser 5.0 water temp

Wayne asked on 7/2/2019 6:44:32 PM
I have a 5.0 mercruiser, water temp runs 160 to 180 degrees at idle all day long, under load 3000 rpms runs normal for 3 to 5 minutes then drops to zero, when backing off to idle comes back to normal. This happened before on this motor and it blew up ! 20 hrs on fresh rebuild w/ all new impeller and water pump, new cylinder heads risers and manifolds,thermostat, and all passages in out drive clear, I dont want to blow up another motor ! Any advise would help, please !!