Finally! After replacing nearly everything else in the cooling circuit, I decided the problem must be the heat exchanger, even though we already cleaned it out with muriatic acid. I bought a new one from Transatlantic Diesel (they know their stuff over there) although I asked for a heat exchanger for a Perkins 4-108 instead of a Westerbeke and so they sent me the right one for the wrong engine (our Westerbeke engine uses a Perkins 4-108 block and so for most purposes it’s really a perkins 4-108). I called them and they sent me the right one no problem.
It took only a few hours to put it in. Afterwards I ran the engine at an idle at the dock, then put it in gear and let it strain against the docklines a bit. It never got above 185, whereas before it would overheat while sitting at the dock in idle.
The new heat exchanger is a better design than the old one, as well: the new one has a bolt with metal end caps and a gasket underneath, so that it can be fully dismantled. The old one had a single rubber endcap, allowing access only to the center of the tubes (one half of the circuit).
Granted, we won’t know for SURE until we take her out and run it hard, but I’m optimistic that we finally fixed the problem.
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