How to Tell If Your Heat Exchanger Is Internally Leaking

The heat exchanger in a closed-loop boat cooling system keeps engine coolant and raw water separated. But when the internal barrier fails — usually due to corrosion, cracking, or poor maintenance — those two systems mix. Coolant disappears. Water gets into places it shouldn’t. And if left alone, engine damage follows. Internal leaks are hard to see from the outside, but there are clear signs you can watch for.

How the Heat Exchanger Works

Inside the heat exchanger, coolant flows through a series of tubes. Raw water flows around those tubes, absorbing heat. The two fluids should never mix. The core acts as a barrier between the two systems. When that barrier fails, coolant can leak into the raw water — or raw water can leak into the closed loop.

Warning Signs of an Internal Leak

  • Coolant level drops with no visible leaks: If the reservoir is always low, and there’s no sign of leaking hoses, suspect the exchanger.
  • White or milky residue at the exhaust outlet: Coolant leaking into the raw water side may exit with the exhaust.
  • Coolant in the bilge without an external source: Internal leaks can weep from the exchanger end caps or tube bundles.
  • Oil looks milky or frothy: If the leak crosses into the oil cooler as well, contamination can occur.
  • Steam from the coolant reservoir: A leaking core can allow exhaust pressure or heat to enter the coolant loop.

How to Test for an Internal Leak

  1. Pressure test the closed cooling system: Attach a pressure tester to the coolant cap and pump to rated pressure. A slow drop with no external leak may indicate internal loss.
  2. Inspect raw water discharge: Look for green or pink fluid (depending on your coolant) in the raw water stream while running.
  3. Remove the end caps: If you see coolant where only raw water should be, the internal barrier has failed.
  4. Dye test: Add UV coolant dye to the closed loop. If it appears in the raw water or exhaust, you have a breach.

Can It Be Repaired?

If the core itself is leaking, the exchanger usually needs to be replaced. Some high-end units can be re-cored, but in most cases, replacement is faster and more reliable. End cap gaskets and seals can be changed if the leak is limited to the perimeter.

Prevention Tips

  • Flush saltwater from the raw water side after every use
  • Replace pencil anodes regularly to protect the internal core
  • Use the correct coolant with corrosion inhibitors
  • Never mix coolants or top off with water only
  • Inspect the heat exchanger every season during layup or service

Conclusion

Internal heat exchanger leaks are sneaky. They don’t drip. They don’t make noise. But they can destroy your engine if ignored. If your coolant keeps vanishing, or your exhaust looks strange, or oil starts turning milky, check the exchanger. A $500 part is easier to replace than a full rebuild. Catch it early, and you keep your boat running — not stuck in the yard with the valve covers off.

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