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October 3, 2010

10 Common Causes Of Marine Diesel Engines Overheating

Filed under: Yachting — Tags: , , — Richard Jordan @ 12:33 pm

Perkins 4-108 is the most common diesel on sailboats and not immune to overheating.

Overheating is a common symptom of marine diesel engine blockage. Marine diesels should run at 180 degrees Fahrenheit. When the temperature goes above 200, start getting worried. Dialing down the RPM’s may allow the engine to recover and run well at a more leisurely pace. Overheating is difficult to diagnose, and the only real way to solve the problem is a systemic examination of every engine element, i.e. trial and error. Here is a starter list of parts to examine:

  1. Raw Water Impeller – the most common source. A spoke of the plastic impeller breaks off and clogs the raw water flow. Easy fix: buy a new impeller.
  2. Mixing Elbow – Corrosion here blocks the flow. Solution is a new or boiled out elbow.
  3. Heat Exchanger – Scale builds up in the heat exchanger. Solution is to snake with a wire and dip in acid.
  4. Thermostat Sensor/Receiver – Is the thermostat stuck? Or is the engine even overheating? Check the exhaust temperature. How hot is the water? Shoot a heat sensor on the water reservoir.
  5. Fresh Water Tank – Is it leaking?
  6. Hoses – Delamination inside engine hoses restricts the flow, causing the engine to overheat.
  7. Fresh Water Pump – An aluminum impeller inside the pump can corrode and limit flow.
  8. Oil Cooler – Similar to the heat exchanger. You cannot snake her out because the tubes narrow at the ends but dipping the cooler in an acid bath does the job.
  9. Over-propped – Too large a diameter or too much pitch can cause an engine to overheat prematurely. Correlates with not reaching full RPM’s. What does the tachometer say?
  10. Aeration – Air getting in from a leak in the line anywhere can lead to overheating.

Please suggest more options in the comments below. I have specifically not included catastrophic failures like water jacket blockage or cracked heads which are alarmist ideas and not the common answer though certainly a worse case scenario to keep in mind.

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  • Dan K.

    The use of a silicone based antifreeze can gum up the fresh water cooling passages through a process called silicone fallout. This happened to my yacht. I had to use Dynamic Defender to flush out the gunk. I will only use old style, non-silicone antifreeze in the system from now on.

  • david shepherd

     DK: Where is this product or service available? A quick search didn’t turn up a specific product. Please email me at Thanks!

  • Marzipand_666

    Engine overspeed can cause overheating??? Any idea…..

  • Scott Chalmers

    I Have a mitsubishi marineised diesel that is overheating and there are black specks coming out with the coolant through the overflow any ideas please ?

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