Mercruiser overheating

[rloucks3]

I have a 1973 mercruiser 120hp. Motor is overheating. I put new water impeller, thermostat. I took off the hose going to motor from impeller really good flow, I took the hose off after water pump good flow, took water off before thermostat good flow then took off hose after thermostat no water.

 

[Chris1956]

So your new thermostat is stuck, or is it just still closed 'cause the motor is cold?

Clogged exhaust manifolds and risers can cause overheat. Were those ever replaced?

Also, a head gasket leak can cause the same.

 

[Scott Danforth]

Did you find the bits missing from the old impeller?

 

[Scott06]

I have a 1973 mercruiser 120hp. Motor is overheating. I put new water impeller, thermostat. I took off the hose going to motor from impeller really good flow, I took the hose off after water pump good flow, took water off before thermostat good flow then took off hose after thermostat no water.

I would bet the passages in your thermostat housing are plugged. The one "after the thermostat" -I assume you mean the hose that feeds water to manifold? that should be flowing some water at all times to cool the exhaust even when the thermostat is not open. Or do you have a separate feed to the elbow?

 

[rloucks3]

So your new thermostat is stuck, or is it just still closed 'cause the motor is cold?

Clogged exhaust manifolds and risers can cause overheat. Were those ever replaced?

Also, a head gasket leak can cause the same.

 

[rloucks3]

No I didn't replace the risers nor the exhaust manifold has been replaced. Can they be cleaned out?

 

[Buster53]

No I didn't replace the risers nor the exhaust manifold has been replaced. Can they be cleaned out?

How old are they? Manifolds and risers need to be replaced after a certain number of years. In salt water, maybe 5 years. Fresh water, maybe 10

 

[rloucks3]

How old are they? Manifolds and risers need to be replaced after a certain number of years. In salt water, maybe 5 years. Fresh water, maybe 10

The boat is a 1973 don't know if ever replaced? I brought the boat as a fixer upper.

 

[Buster53]

Based on info given, most likely it is your risers/manifolds. Not an expert on this, but just for the heck of it, remove the T-stat, run the boat and see what happens

 

[Scott06]

How old are they? Manifolds and risers need to be replaced after a certain number of years. In salt water, maybe 5 years. Fresh water, maybe 10

In freshwater they outlast the boat. Have had boats 40+ years old with original exhaust manfiolds in perfect working condition

 

[Scott06]

The boat is a 1973 don't know if ever replaced? I brought the boat as a fixer upper.

Take the elbow off and see if the passages are clear. I think you may also be able to take a cap off each end of yours. As along as water can flow it is ok

 

[rloucks3]

Based on info given, most likely it is your risers/manifolds. Not an expert on this, but just for the heck of it, remove the T-stat, run the boat and see what happens

Does the boat have to be in water or can I use earmuffs? I am getting water to thermostat but not after. I brought thermostat from Amazon so it very well could be the wrong one??

 

[Chris1956]

You can use a flusher device at idle speed to test. A barrel of water is a bit better. Check the hoses out of the thermostat for hot water. If the hoses get hot, the thermostat is open and the water is being circulated.

 

[rloucks3]

You can use a flusher device at idle speed to test. A barrel of water is a bit better. Check the hoses out of the thermostat for hot water. If the hoses get hot, the thermostat is open and the water is being circulated.

Water is not coming out of the thermostat.

 

[Scott Danforth]

Water is not coming out of the thermostat.

you have a blockage or lack of flow from the pump

 

[Chris1956]

Gee, test the thermostat in a pan of water on the stove. Use a candy thermometer to see that it opens at the correct temp.

 

[Buster53]

Just a thought…
go to YouTube and watch a video on replacing the T-Stat to make sure you are doing it correctly. It’s easy to put it in backwards. Don’t ask me how I know.
But, as I mentioned earlier, see what happens if you run the engine without the T-Stat.