Yamaha - Thermostat Engine Cooling Control Component 67F-12411-01-00. Overview. Keeping your Yamaha outboard running at the right temperature is critical for efficiency and engine longevity. This genuine Yamaha thermostat regulates coolant flow through the engine so it warms up properly and maintains a stable operating temperature on the water. When thermostats begin to stick or wear out, engines can run too hot or too cold, which affects performance and fuel efficiency. Replacing the thermostat