88 3.0 overheats when throttling up.

[stang32]

we have an 88 omc with overheat condition , at idle it creaps up but when throttling up it goes sky high. iwe replaced the water pump assy but still the same issue. i pulled the therm housing while it was still hot and the thermostat was wide open, when i pulled the intake hose there was a lot of pressure that shot out of the engine leading me to think there is a blockage so i pulled the manifold and elbow, there is no exhaust flapper in the exhaust pipe and i checked down the pipe and did not find it, i also ran the hose down the pipe with no restriction at all. . i ran water through the manifold and could not confirm any restrictons in the manifold or elbow.
now, i already pulled the manifold so checking the flow of water from the waterpump was not possible. i did pull the waterpump and checked for any restrictions but blowing thrpugh the hose was nice and clear. no restriction at all. i then ran the muffs and water flowed freely up to the waterpump housing,
any thoughts?

 

[Scott Danforth]

when you replaced the impeller, did you find all the old pieces of the old impeller?

over heating is from lack of water flow. the fact that you pulled the line from the raw water pump and there was a lot of pressure was the warning that there is no water flow.

most likely what ever is blocking the flow is in the thermostat housing area.

 

[kenny nunez]

Add a length of clear hose between the thermostat housing inlet and the hose coming from the drive pump. With the boat in the water watch which way the air bubbles travel. If the bubbles travel toward the thermostat housing then there is an air leak from the drive. If the bubbles start from the thermostat and go toward the drive pump then the head gasket or a combustion leak is entering the cooling system.