Over Heating of 30 Liter OMC IO

" I recently rebuilt a 181/3.0

" I recently rebuilt a 181/3.0 Liter (in the past they were called "Iron Dukes")

I put a car water pump on the engine, (just plugged of the heater port). I made sure the impeller on the pump extended out of the face of the housing the same amount as the marine one I replaced.

At idle, or low speed, it operates at its designed temp of 175 degrees, but, I'm having problems with its overheating at high rpm's.

List of things I've done:
1. Installed new thermostat
2. Checked impeller pump
3. Back flushed intake hose and lower unit.
4. Inserted a clear line section into the intake line to watch for bubbles (air leaks in system) However, I haven't ran the boat since I installed the clear hose.

The engine head is getting HOT. You can't put your hand on the valve cover or oil filter. I'm guessing that I'm note getting water to the upper part of the engine.

Any suggestions,
Bert "

" The first question is "w

" The first question is "why did you replace the water pump in the first place?" You may have had a partial blockage, and the pump was fine. Also, the impeller pump can look OK, but may have lost its resiliency, and loses efficiency as the RPMs increase. Before you blow a head, I'd replace the rubber impeller. "

ok at low rpm and too hot at

ok at low rpm and too hot at high rpm is often restricted water flow in exhaust manifold/riser/elbow.

" On a boat 3.0 there is a by

" On a boat 3.0 there is a by pass hole, built into the housing, by the water pump. This allows water to by pass the thermostat, and keeps the engine running cooler. Your car water pump and thermostat probably doesn't have this by pass hole. Also, a man told me that if you have a Cobra outdrive, then you have two different water pumps. Another thing is that, maybe, your new thermostat isn't opening or maybe it's to hot of number. You can check the thermostat by taking it out and dropping it into a hot cup of boiling water, and watch to see if it opens up. "