Re: Yanmar coolant light on
Raw water cooled Yanmars generally have alarm temps of about 120F. This is below the temp to damage parts if action is taken to shut the engine down. Long term operation above about 140F will cause plating of salts on heat transfer surfaces. Action is needed to prevent further increase in temp as engine damage will occur if action is not taken.
I am not familiar with the construction and specific instrumentation of the YSE8 but I am familiar with other small raw water cooled Yanmars and offer that I would investigate three things in the first troubleshooting -
First I would remove the thermostat and test it on the stove. Put it in a pan of cold water with a cooking thermometer and turn the stove on and check to see that the thermostat opens when the temp of the water approaches 110-120F and is fully open by about 130F. If you have or can get the expected exact values for your engine use those temps instead. Then take the thermostat out of the pan and run cool water over it and check to see that it closes. Drop the thermostat back into the pan and check that it opens again...repeat as needed to convince yourself it's operating normally.
Check the thermostat when you remove it to make sure that it is not fouled preventing flow.
Second I would check the water pump impeller - inspect and replace it if needed. Check for dry rotting of the rubber/plastic vanes and for missing vanes that could be blocking passages. Even if you are seeing flow out the exhaust with the engine running the flow rate could be too low due to damage to the impeller or blocked passages.
Third I would remove the temp sender for the alarm and heat it with a hot air gun (being careful not to overheat the plastic parts) while checking the switch contacts to make sure they are opening and closing normally. If the contacts seem to operating normally you can heat a pan of water to measured temps to check the opening and closing points of the sender.
If this is found to be normal I would pull the exhaust manifold and clean it.
Hope this helps. Good luck.
"It's not what you know that's important...it's what you do with what you know." -H.G. Rickover