1977 Mercruiser 165hp overheating

Engine runs great but overheats after idling for just a few minutes. The intake hose has good water flow, like a garden hose, from the lower unit. There is no thermostat in, so it's like the thermostat is always open. Have tested both the exhaust manifold and engine block for obstructions with a garden hose, and there is good flow with no back pressure. The temperature gauge has indicated as high as 210. Checking the water hoses with my hand reveals that they are very hot, too hot to touch, so the temperature sensor is correct in my opinion.

The water drains out though a flexible boot at the lower unit pivot point rather than through the center of the prop. I think the boot is torn, but the water flows freely out of it, without any pressure, so I don't think this would be affecting the engine's temperature.

I'm not sure how to proceed with troubleshooting. I am considering tearing into the engine's water pump, but I am not hopeful that I will find a solution here. There are no unusual noises, leaks, etc. Like I said, it runs great. So why is it overheating? The last time we ran this boat, it ran great. I would really appreciate any help or guidance.

David

Re: 1977 Mercruiser 165hp overheating

Shameless bump.

Re: 1977 Mercruiser 165hp overheating

Not knowing what engine it is makes taking a guess useless.serial number helps

Re: 1977 Mercruiser 165hp overheating

Well then put in a thermostat and see what happens, Might just work! The design calls for it. I don't know how the cooling water is routed in that motor but there are flows and direction of cooling water affected by the stat in mine.

Re: 1977 Mercruiser 165hp overheating

Its a GMC 165 hp, 250ci (i think) straight 6 engine.

My boat mechanic took the thermostat out and advised that we don't need it. Only need for cold starts per his recommendation. When the thermostat is out, it runs as if it is always open. I can see and agree with his rec that it isn't necessary in warm weather. The boat has functioned well many times in the past without overheating, also without a thermostat in place.

Today I took the front and rear plates off the exhaust manifold and cleaned them out as best I could. Also the riser. I did notice that the water holes in the riser were constricted from scale and some rust, and I opened them back up. It seemed to idle ok with muffs and a garden hose. Going to put it in the water and give it a try. Keep your fingers crossed.