1984 Yamaha 40hp overheating alarm

[psteurer]

My brother-in-law has this motor and the overheating alarm occasionally goes off. The motor is peeing strong but very little water comes out exhaust relief ports when idling around in the canal and it looks like steam is coming out. The cylinders heads are too hot to touch when idling. But you can put your hand on the side of the motor where the water jacket is located and the temperature is warm. Seems like the water pump is working, but the water is not getting into the passages around cylinders. Any advice?

 

[99yam40]

Re: 1984 Yamaha 40hp overheating alarm

Change the thermostat and water pump. If it still is overheating then it time to pull the head and look at the cooling passages

 

[JUSTINTIME]

Re: 1984 Yamaha 40hp overheating alarm

how warm is warm
alarms go off around 173 F

sounds like bad sensor

 

[psteurer]

Re: 1984 Yamaha 40hp overheating alarm

The cylinder head where the thermostat is located is way too hot to touch. You put your hand there but only for less than a second. You can easily put your hand on the jacket on the side of the motor where water is circulating. The water pump was replaced last year so I am thinking as 99yam40 suggests that it is either the thermostat or some blockage. My brother-in-law is going to remove the thermostat and then give it a run to see it the water starts to circulate. Are those two bolts that hold the themostat cover tricky to remove? The motor is run predominately in a fresh water river that has a constant low 70 degree temperature year round.

 

[99yam40]

Re: 1984 Yamaha 40hp overheating alarm

With it being a fresh water motor, the bolts should come out without a problem, But being as old as it is you never know.
Pull the thermostat and test it in a pot of water as you heat it up to see what temp it opens, how far it opens, and what temp it closes.while it is out look to see what the passages look like(how much build up is in them)
Thermostats need to be replaced from time to time.
I would not recommend running without a thermostat because without it in place the motor will not circulate water properly to all parts of the motor as it was designed, If it is bad buy a new one.

I have had a water pump impeller loose a fin or two in less than a year, so it does not hurt to have a look at it when there is a over heating problem

 

[kaferhaus]

Re: 1984 Yamaha 40hp overheating alarm

And if the previous impeller had shredded there may be pieces of it blocking the water passages.

Most T-stats "stick" in the open position, but I've seen them stick closed, just not as often.

Do not just remove it if it's defective... the engine will not reach normal operation temperature if you do and that can cause piston/cyl wall scuffing. the pistons are designed to expand as the engine reaches normal operating temp. A good bit of the "normal wear and tear" on a engine happens while it's warming up.... so one that's not reaching normal temp.... you get the idea.

 

[psteurer]

Re: 1984 Yamaha 40hp overheating alarm

Thanks for all the advice. I believe what my brother-in-law plans, is to remove the thermostat and then give the motor a test run to see if that solves the overheating problem. If it does, then he will replace the thermostat. If it does not, then he will look for a blockage somewhere. It appeared to me that the impeller was okay since it had a very strong pee at start-up and at idle.