Testing a suspected failed outboard engine thermostat.
When placed in hot water the thermostat should open fully which is measured by the lift as stated in the service manual.
When placed in cold water or left to cool, the thermostat should return to the closed position allowing the jacket water / engine to warm to the correct operating temperature.
If failed, the engine would overheat and cause damage.
The specs of your thermostat can be found in your service manual.
This is a part of your outboard motor which can be easily over looked but easily checked. Normally only needing 1 or 2 tools.
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How to remove a thermostat from a F25 Yamaha 25 HP outboard 2016 model
Is there a tool to test it without having to remove it from a F25 Yamaha outboard?
Mine is not doing anything on my 300r