Thermostat 42ºC Yanmar 105582-49200

Function and technical context

This 42°C thermostat is part of the marine diesel engine's cooling circuit, regulating the coolant temperature so that the propeller reaches and maintains its optimal operating range. Acting as a thermostatic valve, it remains closed until the refrigerant liquid exceeds 42 °C, at which time it opens to allow circulation to the exchanger or the seawater circuit. In the Yanmar 3HM35 engine, two units are installed per engine—position 7 of part 39—which makes it essential to replace them in pairs to guarantee uniform thermal behavior. A thermostat that is stuck in the closed position causes immediate overheating of the block, while one that is stuck in the open position prevents the engine from reaching working temperature, increasing internal wear and fuel consumption.

When to replace this part?

The most common symptoms of a faulty thermostat in Yanmar marine engines are: excessively high engine temperature or, conversely, a temperature needle that never rises to the normal speed value; sharp fluctuations in the temperature indicator; and starts with persistent white smoke due to irregular heating of the block. It is recommended to check the thermostat during each seasonal maintenance and replace it preventively every 2-3 years or when intervening on the cooling system, given that the marine environment—with salt water and constant thermal variations—accelerates the fatigue of the wax element. Replacing it before it fails prevents serious breakdowns at sea and protects the cylinder head from heat stroke.