Yamaha Thermostat Assembly YF1-E3997-30-10. Overview. The Yamaha YF1-E3997-30-10 Thermostat Assembly is a genuine original equipment manufacturer component designed to regulate engine temperature and maintain optimal cooling system performance. The thermostat controls the flow of coolant within the engine, opening and closing at specific temperatures to ensure the engine warms up properly and operates within its ideal temperature range.. A properly functioning thermostat assembly helps prevent b