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:
- 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.
- Mixing Elbow – Corrosion here blocks the flow. Solution is a new or boiled out elbow.
- Heat Exchanger – Scale builds up in the heat exchanger. Solution is to snake with a wire and dip in acid.
- 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.
- Fresh Water Tank – Is it leaking?
- Hoses – Delamination inside engine hoses restricts the flow, causing the engine to overheat.
- Fresh Water Pump – An aluminum impeller inside the pump can corrode and limit flow.
- 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.
- 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?
- 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.