The "salt" deposits in heat exchangers and outboard cooling passages are not salt, i.e. NaCl. Rather it is a combination of hardened mineral deposits such as calcium carbonate and calcium sulphate. These solids cannot be removed by fresh water flushing because they are minimally soluble in water. You can flush forever and they will not be washed away. That is why you need a solution such as barnacle buster to remove from surfaces. Richard P.