I tried everything to figure out why my Yanmar 2GM20 was overheating quickly, only to discover that the thermostat was stuck. It looked fine, but the little washer at the base got cocked a millimeter and simply locked up. Try it yourself with a spare. Several people told me to remove the thing altogther, and now the engine works fine. Whew.
Overheating
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I would have no problems doing it as an emergency measure but would try to get a replacement as soon as possible. There are documented occurrences of engines running at higher operating temperatures when the thermostat has been removed. It has something to do with the rate of flow of the coolant, its compression, the raising of the boiling point and the ability to remove heat. Even when fully open a thermostat does act as a partial blockage which slows down the coolant's flow to the manufacturer's desired level.
Ditto on re-installing a thermostat. Before putting yours back in, I'd suggest checking it for proper operation. Simple to do. Put it in a pot with water over the stove. Check water temp with a cooking thermometer. Yanmars are quite often set-up with 71 deg C thermostat (160 F). Heat the water until it reaches 160 and if the thermostat is good you'll see it open. Cool the water down and it should close again. A couple of times should re-assure you. If not, get a new one.
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