On my Volvo MD17C, the raw water runs through the reverse gear box first and that is a fairly common failure point for obstructions. I would also pull the end caps off the heat exchanger and look for obstructions and I would check for obstructions in the very beginning of the circut, your thru-hull inlet. Over the years with differnet boats, I've experienced some heating problems from each one of those areas, especially in the fall when we get a lot of loose eel grass floating around.
1983 Fraser 41
La Conner, WA
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Boating for over 25 years, some of them successfully.