Heat Exchanger Leaks: Symptoms, Causes, and What to Do

In a closed-cooled marine engine, the heat exchanger keeps engine coolant and raw water separated while allowing heat transfer between them. If that barrier fails, coolant leaks out, seawater leaks in, or both — and either situation puts your engine in danger. Heat exchanger leaks often start small and go unnoticed until symptoms appear. The sooner you recognize them, the better your chance of avoiding major repairs.

What Does a Heat Exchanger Do?

It acts like a car radiator but uses seawater instead of air to remove heat. Coolant flows through one set of internal tubes, and raw water flows over or around them in the opposite direction. The system keeps engine temps stable and prevents raw water from circulating inside the engine block.

Symptoms of a Leaking Heat Exchanger

• Unexplained coolant loss You top off the coolant but find no visible leaks — yet the level keeps dropping.
• Overheating after long runs As coolant leaks or becomes diluted, heat transfer efficiency drops and temperatures rise.
• White residue at hose fittings Dried coolant near the end caps or hose connections often indicates a slow external leak.
• Salt crystals or corrosion at seams Raw water leaks may leave salt deposits or signs of rust at the core or mounting flanges.
• Milky oil or steam in the exhaust A severe internal leak may allow raw water to cross into the coolant system or engine internals.

Common Causes of Heat Exchanger Leaks

1. Pencil anode failure If the internal anode is missing or corroded away, saltwater begins to eat the copper or brass core from the inside.
2. Cracked solder joints or seams Vibration and temperature cycling weaken soldered joints over time. These cracks may leak only under pressure or heat.
3. Electrolysis and stray current Improper bonding or marina current leaks can cause the exchanger to corrode even if anodes are in place.
4. Freeze damage If coolant isn’t drained or treated before winter storage, ice expansion can crack the core or end caps.

How We Diagnose a Heat Exchanger Leak

• Pressure test of the closed cooling side (up to cap rating)
• Inspection for external corrosion or salt staining
• UV dye or coolant tracer in the reservoir
• Exhaust gas detection in the coolant (to rule out head gasket)
• Removal and visual inspection of end caps, cores, and gaskets

What to Do if Yours Is Leaking

If the leak is external and minor, it may be repairable by re-soldering or replacing seals. If the internal core is compromised, replacement is usually required. Don’t delay — a leaking heat exchanger can allow raw water into the coolant system, or worse, into the combustion chamber.

Preventing Heat Exchanger Failure

• Inspect pencil anodes every 6 months and replace as needed
• Flush raw water passages regularly, especially after saltwater use
• Use only marine-rated coolant, not automotive antifreeze
• Inspect hose connections, clamps, and caps at every oil change
• Pressure test during annual service to catch early signs

Conclusion

Heat exchanger leaks are more than a nuisance — they’re a serious warning sign. Loss of coolant, contamination of the cooling system, or saltwater intrusion can lead to catastrophic engine failure. If you suspect a leak, don’t guess. Test it, confirm it, and repair it properly. Replacing a heat exchanger is far cheaper than rebuilding an engine ruined by corrosion or heat.