overheat mystery

[slowboatman]

had overheat horn go off at 5000 rpm, back off to 4700 and could run all day. boat is a 2004 mercury 50 hp big foot 4stroke. took boat to dealer, he did a 100 hr service. took boat out could not go over idling speed without horn going off and motor shuuting down to idle. took boat back to dealer, they had boat for 3 months couldn't find problem told me they would have to charge me for any more work. took boat to another boat shop they did everything possible, now motor will get up to 3500 rpm and horn goes off motor shuts down. have spent a fortune i cant afford. any help on this problem would be greatly appreciated

 

[CharlieB]

Re: overheat mystery

Curious to know the water level on the gearcase at those engine speeds.

Not always, but sometimes,

As the water level begins to drop to the level of the water pump, or below, air can be sucked into the pump via a warped lower water pump housing or an ill fitting water inlet tube (if used) under the water pump.

All's well at lower engine speeds as the 'leak' remains submerged, until you reach enough speed that the boat (and water pump) plane out and rise out of the water.

 

[slowboatman]

Re: overheat mystery

thank you will relay information you gave me to mechanic. they have been running motor in large steel tank. He removed head today and checked head gasket, it was good had machine shop look at head for any warpage there was none. this mechanic is at end of his rope. I will take boat back to original dealer hoping they can find something.