Since you see bubbles into
raw water I'm voting for a pinhole leak in the heat exchanger
core. The cores are supposed to be sealed and not allow exchange to the
raw water. I inherited an old
boat one time that had a known leak in the heat exchanger. Symptoms were: would run fine for hours after an initial fillup with fresh water. but when the
engine stopped the
coolant would escape through the leak and would overheat if not refilled upon next startup. As long as the engine ran any loss of water in the heat exchanger was replaced with saltwater (essentially making the engine
salt water cooled). But the heat build up and resulting pressure from a shutdown engine's heat sink would empty the water out of the heat exchanger down to the level of the pinhole leak.
The end cap for the heat exchanger or a leaking
gasket for the end cap can also cause
overheating but you would see
salt or water all over the engine. Once my genset overheated from a pinhole leak when the cast iron end cap rusted out from the inside. Outside on the painted surface almost nothing was visible except a tiny rust mark with a thin tail-tale rust stain going down from it. Thankfully my genset is painted a light tan and it showed up. Now I do a freshwater flush of the unit when it sits at the
dock for anytime without use. When cruising and using it regularly I don't bother but if I'm letting it set for a couple of weeks or more I flush it with Salt-X and leave it with fresh water inside. Just
buying the
parts and doing the labor myself it costs about $2K for the
repairs each time.Two end caps, gasket set, mixer
elbow, special long bolts, etc - and not including a cupernickel (sp)
core which has always been fine. Before I started doing the freshwater rinse/flush i would get about 4 years out of a set of heat exchanger
parts - now it is over 5 years and all seems well.