When I worked on boats, I fixed many overheat problems others couldn't solve by doing a proper flush of the fresh water side. Often the people before me had installed new pumps, heat exchangers and exhaust elbows and never solved the problem.
Also the anti-rust additives in coolant solutions only lasts so long. Eventually you get a flak rust buildup on the water jackets and in the head water passages. The rust doesn't transfer heat well and you go from normal operation temps to too hot quickly.
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