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Normally coolant overflowing is an early indication of a head gasket failing.

Does the coolant level drop after the engine cools? When a head gasket starts leaking into the cooling passages, the leaking combustion gases push out the coolant and the gas isn't a good heat exchange mechanism.

If the coolant is being contaminated with sea water , that's an indication of a failed heat exchanger, but normally the fresh water cooling loop runs at higher pressure than the raw water side and you'll see cooant loss, not overflowing.

Probably not the news you wanted, but I think it's time for a good diesel mechanic. A compression check will help diagnose the problem.