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Old 19-03-2020, 15:54   #1
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Yanmar Thermostat Overheat

Yanmar 3ym30

I had an overheat problem today. After checking a few things I ended up pulling out the Thermostat, I ran it without the thermostat in it and it was ok. Well, I figured the thermostat must have been stuck closed.

I decided to test it in a pot of boiling water and it opened and closed as it's supposed to. Well now I'm really confused

I'm assuming it would overheat if the thermostat was stuck closed?
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Old 19-03-2020, 16:20   #2
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Re: Yanmar Thermostat Overheat

I suggest you investigate the raw water cooling circuit i.e. seacock, water pump">raw water pump, heat exchanger and mixing elbow.

The thermostat can't keep the engine at the correct temperature if the heat exchanger isn't receiving a sufficient flow of raw water (sea water).

The removal of the thermostat perhaps allowed enough cooling with a partially compromised raw water circuit.
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Old 19-03-2020, 17:17   #3
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Re: Yanmar Thermostat Overheat

Did you have a thermometer in the pot of boiling water, or did you just boil water?
I ask as a thermostat in a boat should open about 165 degrees or so, and can be bad, but still operate at 212f.
Either way, get a new one and while your at it a new gasket too.
Checking / cleaning the heat exchanger can never hurt either, and flush and change the coolant. Since you have to drain it and could well get salt water in the fresh water side when you pull the heat ex tubes out, I flush and change the coolant any time the heat exchanger is opened.
It only takes a tiny bit of salt water to contaminate coolant, or so I have been told.
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Old 19-03-2020, 17:36   #4
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Re: Yanmar Thermostat Overheat

If you are away from port, ideas to read the water temp - use a thermal imaging gun, meat cooking sensor, oven thermometer, If you do not have any thermometer, try the thermostat in a pot filled with water from the hot tap. If not open under hot tap, then put on stove and raise temp, it should open before boiling.
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Old 19-03-2020, 17:43   #5
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Re: Yanmar Thermostat Overheat

Have you confirmed the engine was actually overheating? You might have an intermittent gauge or over temp. switch.
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Old 21-03-2020, 19:34   #6
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Re: Yanmar Thermostat Overheat

Once you have solved the problem, invest in one of these. Put one sensor just below the water injection point on the exhaust elbow & the other under a bolt that houses the thermostat.
https://engineguard.com.au/?gclid=Cj...4aAuj0EALw_wcBClick image for larger version

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