1980 Mariner Overheat at high RPM ok at Idle.

xzttpw

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Jun 5, 2012
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Overheat -I have a 1980 Mariner 30 hp. bought it new. It overheats at higher RPM but is ok at low RPM. I have replaced the water pump & housing and replaced the Head Gasket with no change in the symptoms. I have checked the thermostat with boiling water; it opens, and even removed the thermostat with no change in symptoms. The water pump telltale stream seams strong at idle RPM it never overheats, when the thermostat opens you can see increased water out of exhaust. As the engine RPM is increased aproximately 1/2 throtle the water pump telltale starts to spew steam. It is HOT. If you return to idle about 1 minute the water pump teltale returns to a water stream. It seems like exhaust pressure may be overcomming water pump pressure at high RPM. I am not sure what is next? I did not see any cracks when the head was off. Does any one know what is behind the side plate on the cylinders? Is this the reed valves? Could there be a gasket failure at this plate? Is there water circulated around the exhaust on the plates on the side of the engine block? I hate to continue to spend money on gaskets that don't seem to acomplish a repair. I know part of any repair is to eliminate possibilities but don?t want to go off in the wrong direction? Suggestions appreciated.
 
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