Thermostat 71º 121750-49800

Function and technical context

The thermostat is the regulating element of the marine diesel engine's cooling circuit: it remains closed when the engine is cold to accelerate warm-up to working temperature, and opens at 71°C to allow the coolant to circulate through the exchanger or water jacket. Without precise thermal regulation, the engine can systematically operate below its optimal temperature – with increased fuel consumption and cylinder wear – or, if the valve is stuck in the closed position, shoot towards overheating in a matter of minutes, with the risk of serious damage to the cylinder head and gaskets. In Yanmar engines of the GM and HM series, this component is integrated in section 40 of the breakdown (callout position 3) and works directly on the thermal balance of the block.

When to replace this thermostat?

The most common symptoms of a faulty thermostat are: abnormally low engine temperature or the engine taking a long time to reach it (valve stuck open), or sudden increases in temperature with the overtemperature alarm activated (valve stuck closed). High fuel consumption or reduced performance may also appear without apparent cause in the injection system. It is recommended to inspect the thermostat during each seasonal maintenance and replace it preventively every two or three years, regardless of the visual appearance, since the wax element that operates the valve can lose calibration without showing external deterioration. Anticipating the failure prevents an engine stop while sailing and the costly damage associated with an episode of overtemperature.