Function and importance of the thermostat
The thermostat is a central component in the cooling circuit of a marine diesel engine. It regulates the flow of cooling water depending on the temperature:
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Closed at cold start: engine reaches operating temperature more quickly
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Partially or fully open at operating temperature: cooling water circulates to dissipate heat
A correctly functioning thermostat stabilizes the engine temperature, reduces wear and optimizes consumption and performance.
Structure & mode of operation of a thermostat
A thermostat consists of a temperature-controlled valve that is located between:
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Internal engine coolant circuit
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Coolant circuit to the outside / heat exchanger
can switch. Metallic or wax-filled elements are used to adjust the degree of opening according to temperature.
Thermostat gaskets: Function & selection
Thermostat seals ensure:
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Tight connection surfaces between the thermostat housing and cooling water ducts
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Prevention of leaks and pressure losses in the cooling circuit
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Safe medium routing without mixing coolant and engine oil
The right gasket is matched to the thermostat design in terms of dimensions and material and prevents subsequent leaks.
Selection criteria: Yanmar engine version & OEM reference
The following applies for a perfect fit:
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Yanmar engine model & power class
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Year of manufacture / serial number of the engine
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OEM reference number of the thermostat or gasket
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Cooling circuit configuration: internal or external cooling
Structured matching reduces incorrect orders and ensures a perfect fit.
Assembly & installation instructions
The following applies when installing the thermostat and gasket:
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Remove old gasket residue completely
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Clean & check contact surfaces
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Position the new gasket correctly (do not kink)
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Insert the thermostat in the correct position
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Tighten the fastening screws evenly to the specified torques
After installation, bleed the cooling circuit and check the temperature behavior.
Maintenance & inspection of the cooling circuit
Recommended maintenance measures:
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Visual inspection for loss of coolant / dripping
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Function test of the thermostat at starting and operating temperature
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Temperature monitoring during longer journeys
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Check adjacent hoses, sensors and connections
Regular checks minimize the risk of failure and thermally induced damage.
Typical fault patterns and symptoms
Signs of problems in the cooling circuit:
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Engine runs cold for a long time - thermostat remains closed
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Overheating despite cooling - thermostat stuck open
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Leaks in the housing - seal worn or incorrectly fitted
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Temperature fluctuations under load
Targeted diagnosis prevents engine damage and downtime.
