Short answer
Most marine diesels use raw water (sea or lake water) to cool the engine coolant through a heat exchanger. When raw water flow drops, the engine heats quickly. The classic cause is a worn or failed pump impeller; its broken vanes often lodge in the heat exchanger. Check the seacock, strainer and raw water flow at the exhaust too.
Safety first. Diesel injection systems run at extremely high pressure; a pinhole leak can inject fuel through skin. Exhaust and turbo parts stay very hot long after shutdown. Never open a hot cooling system. If you are unsure, stop and see a qualified diesel technician.
Start with the easy checks
- Confirm the seacock is open and the strainer is clean.
- Check for normal water flow at the exhaust outlet.
- Inspect the raw water pump impeller on the maker's schedule; replace it if worn or cracked.
- After any impeller failure, find all the missing vane pieces, often in the heat exchanger.
- Check zinc anodes on the maker's schedule.
Likely causes, most common first
- Worn or failed impeller
- Most common.
- Blocked intake or strainer
- Weed, debris.
- Fouled heat exchanger
- Debris and scale.
- Belt, pump or closed-system problems
- Also check coolant level.
These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.
When to stop and see a diesel tech
- Overheating continues with good raw water flow.
- Exhaust system damage from overheating.
Questions
How often should I change a marine diesel impeller?
Follow the engine maker's schedule. Many operators replace it on a regular schedule rather than waiting for failure.
Related symptoms
Cooling
Why does my diesel run hot when towing?
Troubleshoot →CoolingWhat is cavitation and liner pitting in a diesel cooling system?
Troubleshoot →Power & drivabilityWhat is wet stacking on a diesel generator or idling truck?
Troubleshoot →Where this comes from
- Trans Atlantic Diesels: Marine engine overheating (PDF)
- Cruising World: Replacing impellers and raw-water pumps
- Northern Lights: Marine generator operator manual (PDF)
Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.