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Correct Operating Temperature - 1974 Johnson 6hp

F

FishyBC

Contributing Member
So I have my Johnson 6hp almost ready to head out on the water trout fishing and I decided to run it in a 55 gallon drum to make sure it's all good to go. It's running well. I replaced the water pump and thermostat as the old thermostat was plugged up and broken.
After about 5 mins in the tank, it seems to be running too hot (> 180 F) so I shut it off.
When I removed the thermostat out and it runs more like 110 or 120 F which from what I'm reading is too cold.
I have been researching around here and see that a properly functioning thermostat is supposed to open at somewhere between 145 F and 165 F. I did the "test it in a pot" test and the thermostat does open up at around 165 F.

What is considered too hot for the motor to run at and would the thermostat restrict flow enough to cause not enough flow if it's opening as it should ? As mentioned it's brand new with a new impeller and lots of water is pumping out the exhaust. It's warm water coming out the exhaust when the engine is running hot which tells me it is opening.

I read that sometimes they run hotter in a drum than they would out on the water but I want to make sure this is dealt with so it's just a good running little motor for my trip. I searched for this but didn't find it and suspect others are curious as well.

Thanks for any thoughts about this.
 
Rule of thumb is the head should get hot enough you want to pull your hand away but do not have to for a slow count to six. The thermostat should open up around 145 degrees. Did you use the string test? Put a string in the thermostat valve and hold it center of a pot of water with the meat thermometer right next to it. What temp does it fall off the string? Is the water level in the barrel at least five inches above the water pump?
 
Was there much corrosion in the head when you replaced the thermostat? If so get a jug of saltaway and pour it in the barrel and let it idle for at least 15 minutes. Shift in gear a few times to stir it up good.
 
I really like the idea of pouring salt away down there. I don't have any here so I tried the Zep Rust, Calcium and Lime cleaner. I ran right out the exhaust port and out the leg. Should it run out or should it stay in ? Maybe a catch basin at the bottom so that it can be repoured through if it runs through would do it.
I'll give it a try.

Also, when you say let it idle, are you doing this with the thermostat housing cap off and with the motor running ?
 
30 minutes or more should do it,recycle the water and saltaway thru engine, thermostat housing installed, Divert exhausted water back into drum with engine running at low rpm
 
Thank you both for your suggestions. I intend to get some Saltaway and try this. It is probably a good idea to do to any outboard at least once or twice a year. Kinda like a seafoam treatment for the cooling system.
So I did find the following when I took it back out to my tank. I fired it up again and watched it jump up to 165F fairly quickly.
I had my Infrared Thermometer pointed at the power head and my other hand on the exhaust water. It started to cycle. It would get up to 165 or 167 and the water coming out of the exhaust would go from being cold to hot. as the hot water came out the temp would drop into the 140's. This continued on over and over. Looks to me like the thermostat is working and that I'm going to guess that anything under 170 deg F is considered OK.

One last question. So running the motor with saltaway in a tank is safe and would work for any outboard so long as it continuously has the solution available in the tank ?
 
Just a FWIW, there are two water flow paths through that powerhead. One path can be open and the other clogged. Try your Saltaway if you want, but I'm not a believer in it.

EDIT: Remove the thermostat cover and thermostat and study what you will see. One flow path comes out above the 'stat and then out the back of the head (bypassing the thermostat). The other flow path comes out below the 'stat, through the 'stat (if open), then out the back of the head. Only that flow path is thermostat-controlled. The other one flows all the time, open or closed.

Now that you know, start the motor with the cover and 'stat removed and see if water comes out both flow paths. If it does flow freely from both, it isn't plugged up.
 
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