We have 1700+ hours on our 4JH3TBE and have yet to clean the heat exchanger. Temperature holds steady at 180ºF even at flank speed. True, we rarely run it all out. We do change the antifreeze once in a while. Saw no tendency to overheat while in warm Mexican waters.

I assume you have checked for any flow restrictions in the design and layout. Sounds like yes, that you were on top of it from the get-go.