1987 OMC Cobra 3.0l Over-Heating Issue

[Blue Fin Tuna]

What am I missing? My 1987 OMC Cobra 3.0 is still over heating. I have good raw water flow from the drive and a new impeller and housing, replaced thermostat, replace fan belt, replaced riser, hoses are good, no signs of issues at the motor circulation pump, exhaust flapper is good. The manifold had only a little visible crud at the riser. Nothing alarming with manifold. I have front and back flushed the drive with impeller housing off and have good flow from lower unit to the impeller. What the heck am I missing here??? I can only think to throw a new manifold at this thing for good measure!? Any ideas you guy may have would be appreciated.

 

[Scott Danforth]

did you back-flush your system?

how are you measuring temp?

what is your timing set to?

 

[kenny nunez]

The only other thing to check after doing Scott’s suggestions and if not finding any blockages will be to put a section of clear hose in the line that goes from the thermostat housing to the manifold. If bubbles are observed when the overheating happens then the head gasket is leaking. It is also possible that a head gasket leak can have enough pressure that overcomes the water pump impeller output. Has the engine ever overheated from the water pump impeller failing?

 

[Blue Fin Tuna]

I did also replace temp sending unit but checking temp with IR gun. I have back flushed from raw water hose to outdrive and the reverse by disconnecting raw water hose at thermostat. Get good water flow on test while in the water when disconnecting raw water hose. Water flows 4-5 inches high when hose held level with the block. Haven’t checked the timing and no signs of leaking head gasket. And I have checked the impeller primer hose and there is a steady stream of water so its not plugged. Re-tested new thermostat in boiling water and it opens at 160 degrees as it should. It must be somewhere after the thermostat housing. It got up to 215 degrees on IR gun and pressurized hot water shot backwards and began causing normally cold raw water hose to become hot.

 

[kpg7121]

Did you put a clear hose on it to check for air bubbles?

 

[Blue Fin Tuna]

Not yet but I will check that too! Thanks.