1994 merc 5.7 overheating

[HT2 LAMB]

1994 Chaparral. Alpha1 Gen2 operated in fresh water. Pulled boat out of barn after winter and fired it up, slowly got hot on muffs. Ran great when I wintetized it. Replaced H2O pump (old impeller intact) and tstat, checked flow from tstat housing to outdrive. The risers stay cool with the upper hoses staying cool and the lower riser hoses getting hot. Can't get above 120 with tstat pulled. Anyone see a circ pump go bad that doesnt leak or make noise? Have not done output test yet as I have to drive to the lake for that.

 

[alldodge]

1994 Chaparral. Alpha1 Gen2 operated in fresh water. Pulled boat out of barn after winter and fired it up, slowly got hot on muffs. Ran great when I wintetized it. Replaced H2O pump (old impeller intact) and tstat, checked flow from tstat housing to outdrive. The risers stay cool with the upper hoses staying cool and the lower riser hoses getting hot. Can't get above 120 with tstat pulled. Anyone see a circ pump go bad that doesnt leak or make noise? Have not done output test yet as I have to drive to the lake for that.

:welcome: to iboats

You should not remove the Thermostat, this is what keeps the block full of water.

What's your engine serial number?

 

[achris]

Which thermostat housing do you have, balls or no balls?

 

[HT2 LAMB]

At a ball game now will have add the sn later but its 0F something. It's the 6 hose housing run without the check balls. Never had an issue, ive had the boat 4 years. Removal of tstat was diagnostic, I put it back after testing it. I also ran a hose with nipple to top of circ pump and muffs on lower unit and couldnt get temp over 100 but heats up if i turn off water to top of circ pump. I think Ive narrowed the problem between the circ pump and raw pump even though i just replaced it. Gonna put a temporary auto circ pump to see if that solves it. I have and use my manual. If it doesnt im gonnal pull PS cooler and also do outflow test. Again, anyone see bad circ pump that is quiet and doesnt leak? Looks original. Thanks

 

[HT2 LAMB]

Ty for the welcome, glad to be aboard. Haze gray and underway.

 

[Watermann]

Worn out or eroded impellers on the motor water pump can cause lack of proper circulation in the block.

 

[alldodge]

At a ball game now will have add the sn later but its 0F something. It's the 6 hose housing run without the check balls. Never had an issue, ive had the boat 4 years. Removal of tstat was diagnostic, I put it back after testing it. I also ran a hose with nipple to top of circ pump and muffs on lower unit and couldnt get temp over 100 but heats up if i turn off water to top of circ pump. I think Ive narrowed the problem between the circ pump and raw pump even though i just replaced it. Gonna put a temporary auto circ pump to see if that solves it. I have and use my manual. If it doesnt im gonnal pull PS cooler and also do outflow test. Again, anyone see bad circ pump that is quiet and doesnt leak? Looks original. Thanks

Wouldn't be the first time to have the circulation pump go out. You could just pull it and remove the back cover to check the impeller vanes

 

[HT2 LAMB]

Thanks alldodge, i need a core anyway so i'll pull it off and check it. My earlier post I indicated the lower riser hoses but I meant the manifold hoses get hot as well as the circ pump hose and I can feel water bubbling (not boiling) when I feel the hose, the riser hoses stay cool and the tstat supply hose stays cool and there is flow out the transom. Will post what I find out.

 

[achris]

Marine circ pump should have a cast brass impeller, not a automotive pressed tin one. Also the shaft seal is different (which is why you can't use an auto pump on a marine engine)....

Chris..

 

[HT2 LAMB]

Problem solved! Did I ask about a circ pump? Should have been jerk pump. Got the auto pump, the casting was too deep, no way it would work. Took boat to the lake to test the output. Never got above 150 degrees, perfect. Had a couple cracks in my muffs and not enough volume through the garden hose to cool the engine. Live and learn. Hope others will benefit from this. Thanks fellas. Go Navy!

 

[alldodge]

couple cracks in my muffs and not enough volume

Sometimes the simplest things bite us, good to hear your going and thanks for letting us know