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MD3B Volvo-Penta sea water to fresh water conversion
The salts inside your engine wont migrate to the cooling passages in the heat exchanger .... but they should be cleaned out!
For the ''salts'' (precipitated carbonates) that are inside your engine, simply get a commercial ''descaler'': Marsolve or RydLyme (available through most marine supply houses such as West Marine or Boat/US,
etc.) and recirculate (pump) this into a drained engine, let sit a few hours, flush (into a barrel, etc.) .... that''s it. Both of these products will dissolve the accumulated scale without dissolving the base metal of the engine. Some folks use muriatic or other acids which can dissolve the base metal as well as the scale - therefore harmful.
BTW - when you change to fresh water cooling, dont forget to change the thermostat to one that open at approx. 190 - 195 degrees. The higher operating temperature will be better for combustion efficiency, etc.
hope this helps ;-)