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Home›Repair guides›Thermostat Testing

Compatible — Thermostat Testing

For these models: MDJA, MDJB, MDJC, MDJE, MDJF.

Before you start

  • A defective thermostat may cause the engine to run too hot or too cold and must be replaced if faulty

Procedure

  1. Remove thermostat from the cylinder head
  2. Heat a pan of water to the opening temperature specified in the thermostat specifications table
  3. Check temperature using a thermometer immersed in water
  4. With the thermostat suspended in water at the specified opening temperature, check to see that the thermostat begins to open
  5. It should be possible to slip a 0.003 inch (0.08 mm) feeler gauge between the valve and thermostat body
  6. Continue heating the water until the temperature specified for full opening is reached
  7. Check to see that the thermostat is open by the amount specified in the specifications table
  8. Remove the thermostat from the hot water and allow it to cool in the surrounding air
  9. The thermostat should close within a short time
  10. If the thermostat sticks or does not operate properly, replace it with a new one
  11. Always install a new gasket when replacing the thermostat

Tools required

  • pan
  • thermometer
  • feeler gauge

Specs & torque values

  • 0.003 inch (0.08 mm) feeler gauge

Source: manual:974-0750_Onan_MDJA_to_MDJF_Marine_Diesel_Engine_Service_Manual_12-1985.pdf. Figures and steps are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.