The normal water flow in the raw water
cooled MD17 goes from the sea water pump
to a "T" fitting on the underside of the exhaust manifold. At the T fitting the water can go two directions:
#1 it can continue on past the T and into the thermostat housing, flowing alongside the closed thermostat and out of the housing to the exhaust mixing elbow
. This is the normal flow when the engine is cold and the thermostat is closed. Or,
#2 it can flow into the T and into the exhaust manifold water passages, from there into the cylinder heads, down into the cylinder liner passages, back up into the head
and out of the head into additional passages in the exhaust manifold. There it contacts the thermostat and when it gets hot enough it causes the thermostat to open and it flows thru the thermostat and into the thermostat housing and then to the mixing elbow
When the thermostat gets hot enough to open, it's motion blocks or partially blocks the flow in path #1 and causes flow to begin in path #2. So the
thermostat regulates the engine temp by causing water to flow in one path or the other, or perhaps a bit in both.
When you start the engine from cold, there is plenty of water flow out the exhaust because the thermostat is closed, causing all of the pumped water to bypass the engine and go out the exhaust. As the engine starts to warm up, the thermostat begins to open, which closes off the the bypass water path in the thermostat housing and that should force water thru the engine path. Evidently this engine flow isn't happening, so the water that's in the engine overheats and steams, probably until it's mostly gone and then nothing comes out the exhaust.
When there is no more water in the engine, the thermostat, (no longer touching water) closes, again allowing water to flow in the bypass and cooling
When you run the engine without the thermostat in place, bypass water flows into the thermostat housing and also into the front end of the exhaust manifold, because there is no longer a thermostat there to keep it out. This cools the temp sender which resides in the front of the manifold and that gives you a cool reading on the gauge. But there is really little or no flow in the heads and liners, and the engine is running hot.
So the question is, why is there inadequate flow in the engine when the thermostat opens? It could be, as mentioned earlier, that the pump is worn out and there isn't enough pressure to overcome the normal flow restriction in the engine. Or it could be that there is a flow restriction in the T fitting underneath the manifold, something like an impeller blade perhaps. Or maybe after 35 or 40 years of life there is a lot of corrosion
inside of all the little passages in the engine and it just needs a good cleaning
All something to think about this winter!