1971 Johnson 125 cooling problem

Hello, I am quite stumped here and looking for some help. I have a 1971 Johnson 125hp. I have replaced the impeller, housing and key after noticing the engine was getting hot. After replacement, I was getting good flow (as seen exiting at the water exhaust ports), however, only the port side cylinders were cooling. With a thermal gun, the block stayed around 147 degrees. We killed the motor after the starboard side would get to 170. We have removed the lower unit to make sure there was no connection issue between the pump and water tubes.

A marine mechanic suggested checking inside the thermostat. But when you look at the manual, the water flows from the pump directly to the cylinders before returning to thermostat which decides whether to return water to the pump or eject it. We did remove the hoses to the bottom of each cylinder and noticed water flowing freely from the port side but no water from the starboard side.

what is the next best step? If there is a blockage in the starboard side, where do I begin to clear it?

thank you for your help.

J

Jkrens said:

A marine mechanic suggested checking inside the thermostat. But when you look at the manual, the water flows from the pump directly to the cylinders before returning to thermostat which decides whether to return water to the pump or eject it. We did remove the hoses to the bottom of each cylinder and noticed water flowing freely from the port side but no water from the starboard side.

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Yes, the pump moves water TO the head and if the thermostat is closed it dumps to exhaust. Only when the thermostat opens do you get cool water FLOWING THROUGH the head and cooling the cylinder. Your issue really sounds to me like you have a stuck/plugged thermostat and it's not allowing cool water to flow through. It's worth the price of a gasket to pull the thermostat to clean everything, and reinstall. More than once I've seen an engine with 'good cooling' and when you pull the thermostat you find bits and pieces of old impeller clogging it.

Where are you running this motor, in a bucket, or??

kevinj said:

Yes, the pump moves water TO the head and if the thermostat is closed it dumps to exhaust. Only when the thermostat opens do you get cool water FLOWING THROUGH the head and cooling the cylinder. Your issue really sounds to me like you have a stuck/plugged thermostat and it's not allowing cool water to flow through. It's worth the price of a gasket to pull the thermostat to clean everything, and reinstall. More than once I've seen an engine with 'good cooling' and when you pull the thermostat you find bits and pieces of old impeller clogging it.

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ok. The image attached is from the service manual and I think this is what was confusing me. I will be doing the thermostat anyway and will be checking for any blockages in there.

fdrgator said:

Where are you running this motor, in a bucket, or??

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In the Lake.

Jkrens said:

In the Lake.

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Good. They weren't made for bucket cruising.
Make sure all the parts are in the thermostat. A bit of back pressure from the therm parts helps ensure complete filling of the block. Running it with the hoses disconnected can fool a fella. Air can enter the return pipe to the pump.

fdrgator said:

Good. They weren't made for bucket cruising.
Make sure all the parts are in the thermostat. A bit of back pressure from the therm parts helps ensure complete filling of the block. Running it with the hoses disconnected can fool a fella. Air can enter the return pipe to the pump.

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Pulled the Thermostat and all parts are there. Springs are good and valves open and close fine. Also tested the Vernatherm and it is in working order. So maybe a blockage in the cooling passage? Can this be cleared with air or water?

I've never seen one blocked, even here in salt water country. Of course it's had 50 years to crud up.

When you install the lower unit are you using the 2 plastic guide tubes for the water tubes, yes, no or no idea?

racerone said:

When you install the lower unit are you using the 2 plastic guide tubes for the water tubes, yes, no or no idea?

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yes. I installed the guides into the housing and then guided them over the tubes with a friend of mine.

kevinj said:

Yes, the pump moves water TO the head and if the thermostat is closed it dumps to exhaust. Only when the thermostat opens do you get cool water FLOWING THROUGH the head and cooling the cylinder. Your issue really sounds to me like you have a stuck/plugged thermostat and it's not allowing cool water to flow through. It's worth the price of a gasket to pull the thermostat to clean everything, and reinstall. More than once I've seen an engine with 'good cooling' and when you pull the thermostat you find bits and pieces of old impeller clogging it.

Click to expand...

With great respect, I submit that that is not quite correct. When thermostat is closed, water pressure pushes the check valve open and water returns to the pump for recirculation. When thermostat opens water is free to be discharged out through the exhaust. This is not an either-or situation. The 'stat is constantly sampling the water temp, and opens or closes as necessary to maintain desired temperature. Either way, water always flows through the powerhead.

I am starting to think that I may have installed the water pump incorrectly. The new pump kit came with some extra parts that weren't in the original system. For example, the round plate in the photo below was included in the kit but was not in the existing system so I assumed it may have been for some of the engines this kit was listed for. I couldn't even figure out how it would be installed so I left it out. Any insight on this and if I may have missed a step? Everything else went on like usual, the bottom plate was sealed down, key was installed correctly, pump fins were in the correct orientation, grommets in place and shaft seal and O-ring. Wondering if this round plate has a place somewhere and I missed it.

Well no wonder you are having trouble. The round plate goes in the black pump housing then the circular piece, then impeller. In other words, it is a wear plate above the impeller, just as the other flat plate is a wear plate below the impeller

fdrgator said:

Well no wonder you are having trouble. The round plate goes in the black pump housing then the circular piece, then impeller. In other words, it is a wear plate above the impeller, just as the other flat plate is a wear plate below the impeller

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Thank you so much. I think my confusion was because the housing already had the round cylinder piece already installed inside the box when I opened it. Didn't think I could remove that to install a different piece. Facepalm. Well that's why I came here. Thanks again.