Well, I couldn't deny it any longer. My port exhaust has been consistently showing more steam than the stb. My temp gauges consistently show the port engine about 170 degrees while the stb is 155. Using my IR temp gun on the tops of the risers while cruising, both port risers were 175 while the stb engine's risers were both around 150. With my thru hull exhaust ports I can also see a definite reduction in water flow at both idle and 2000rpm with the port engine vs stb. I assumed the problem was the fresh water pump. My engine room actually has room and the engines are facing forward with direct shaft drives so it only takes about 1/2hr to remove the pump. Once removed I cracked it open and... the impeller looks relatively new... Not looking like the problem. Took another 1/2 hr to reinstall the pump. Next, I looked at the strainer and its hose and didn't see anything out of the ordinary there. Next, I vacuumed out hose going from the pump to the engine - nothing. Next, I decided to take off the main heat exchanger end caps. Removing the hot side first, I was shocked to see that the rubber gasket between the cap and exchanger end was horribly deformed. It was swollen up and was almost gel like and some of it had broken off and lodged into the hose end heading for base of the stb side exhaust manifold. The "cool" side end cap was similar, but not as badly swollen however some of the rubber had become lodged in a few cooling tubes. They started out as flat disc like gaskets with a hole in the centre. I've cut two new gaskets from sheet gasket material, purchased two O-Rings and plan to reinstall tomorrow and take her for a run to see if this is the cause of my overheating issue. I really wouldn't be surprised after seeing how much the deformed/swollen rubber gasket was intruding to the working room within the exchanger ends and even blocking some core tubes as well as restricting the output hoses. I plan on running a wire thru the cooling tubes before installing the caps though since a few of them have rubber stuck in them. I'll update after completing this job. Sidenote - my stb engine had an issue with it's exchanger in 2009 with an internal leak and resulting loss of coolant. I tried unsuccessfully to repair it and ended up buying a new one just under one boat buck so I'm sure it's end cap gaskets are still ok.