GM Users’ Group: Thermostat replacement on the 3GM30 (Raw Water Cooled Engines)

Referring to the service manual for this one, Yanmar’s advice is (somewhat surprisingly) to change the thermostat every year. Many owners test annually and carry a spare part but don’t go as far as changing it every year. BUKH, for comparison, recommends changing their (much more expensive) thermostats on the DV24 every 5 years. Engine usage and dedication to maintenance will dictate how often you swap these out for replacements. Always remain vigilant for any sign of engine over-heating though.

The location of the thermostat is found at the end of the cylinder head. It is mounted on the thermostat housing which is combined with the generator mounting base on the cylinder head at the gear case end.

Thermostat Construction and operation

The thermostat remains closed until the cooling water temperature reaches a fixed temperature. Until the cooling water reaches this fixed temperature, it collects at the cylinder head and the water flowing from the water pump is discharged through the bypass circuit. When the cooling water temperature exceeds the fixed temperature, the thermostat opens and the cooling water flows through the main circuit of the cylinder and cylinder head. The thermostat serves to prevent over-cooling and improve combustion performance by maintaining the cooling water temperature at the specified level.

Inspection of the thermostat

1. Remove the water outlet coupling at the top of cylinder body to remove and inspect the thermostat, Remove any dirt or foreign matter that has built up in the thermostat, and check the spring, etc. for damage and corrosion.
2. Testing the thermostat – Place the thermostat in a container filled with water. Heat the container with an electric heater. If the thermostat valve begins to open when the water temperature reaches about 42°C and becomes fully open at 52°C, the thermostat may be considered operational. If its behaviour differs much from the above, or if it is found to be broken, you must immediately replace it.

3. In general, inspect the thermostat after every 300 hours of operation. However, always inspect it when the cooling water temperature rises abnormally and when white smoke is emitted over a long period of time after the engine starting.
4. Replace the thermostat when it has been in use for a year, or after every 2000 hours of operation.

When attaching the thermostat to the cooling water system, before attaching the thermostat to the system, be sure to check its packing and make sure there are no leaks.

Care must be taken when assembling the thermostat – The thermostat cover must be assembled with the arrow mark kept upward.

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