New to me boat that's been sitting for two years. I recently got the engine started. Engine temperature was fine for the first hour of run time, then it began to get overly warm. The Sherwood impeller had come apart. Upon replacement, I found several impeller vane pieces in the water pump ports, both the inlet and outlet. I didn't find any pieces in the thermostat or thermostat housing. I back flushed the engine from the thermostat housing through the water pump (impeller cover removed), with the full flow of a water hose, no noticeable resistance.
Now my engine will eventually overheat unless I pinch closed the recirculation hose, this forces the hot water to exit through the muffler instead of heated water recirculating endlessly. I've blown through the thermostat housing to muffler hose by mouth, no noticeable resistance. The thermostat is new and I've verified that it opens when warm and closes when cool. With the recirculation hose pinched closed, the engine never gets up to operating temperature (as if the thermostat isn't closing).
To recap- With the recirculation line open, the engine overheats. With the recirculation line closed, the engine never gets warm.