Water flow on the MD11D (raw water cooled) is as follows:
From the saildrive
valve to the sea water pump, then to a "T" fitting on the bottom of the exhaust manifold. At the T fitting the flow can split, depending on the status of the thermostat. One way is into the exhaust manifold, the other is to continue to the thermostat housing, then to the exhaust elbow
When the engine
is cold and the thermostat is closed,the water flows from the sea water pump, past the "T" fitting and into the thermostat housing. From there it flows out the top of the thermostat housing and to the exhaust elbow
. Very little water flow restriction in this path.
As the engine warms up, the thermostat starts to open. The movement of the thermostat partially blocks water flow thru the thermostat housing, and this causes some of the water to flow from the T fitting into the exhaust manifold water jacket. From the exhaust manifold the water flows into both cylinder heads, down into the cylinders, around and back into the heads and back into the exhaust manifold. Then it flows past the partially open thermostat and out to the exhaust elbow.
As the engine gets hotter the thermostat will completely close off the bypass opening in the thermostat housing forcing all the water flow thru the manifold, heads and cylinders.
Over the years the water passages in the exhaust manifold and heads will get plugged with corrosion
, rust, etc., and this restricts the amount of water that can flow and will cause the engine to run hot. The passages need to be cleaned, either by using a cleaning
solution or by disassembly and mechanical cleaning
As an aside, the MD11D engine went out of production in 1982/3, so it's a little older than you think.