I need to double check everything when I am back on my boat next week, but I believe this is how it works. Here is a picture I should have included before,
https://photos.app.goo.gl/VRaNcUXGusATyTUJ7
I believe the raw water enters the engine at the diverter. Unfortunately, we can not actually see that because it is hidden behind the alternator. But it *is* clear that I do *not* have a t-fitting with a bypass hose on that side.
On top of the manifold there is a t-fitting. On one side of the t-fitting is the sender. On the other side there is a right angle hose connector fitting. That hose runs from the manifold to the T-fitting above the red muffler. The output of that T-fitting then runs uninterrupted to the stern where it goes overboard. Only water exits that hole.
That is the only 'hose' that leaves the manifold. The manifold also has the insulation wrapped standpipe (?) that goes to the muffler. There is a large diameter hose that leaves the muffler and goes to the stern. That spits out a mixture of exhaust and water.
I believe the ball valve on top of the muffler is what controls the amount of outgoing raw water that gets mixed into the muffler to cool the exhaust. As you note -- too much water could cause issues like too much back pressure.
I do not know why the engine was rebuilt -- aside from the fact that it was at least 30 years old at the time.
I am thinking that I don't have a thermostat.
So my system would be most similar to the early model diagram in this thread,
http://www.moyermarineforum.com/foru...ad.php?t=10248
Except with the thermostat and return line missing.
So -- I am curious how the early model works. Here is my guess,
When the engine is cool, the water is recirculated. This means the warm water leaving the manifold is mixed back into the input of the water pump. As a result the 'cooling water' is now not so cold and doesn't cool as much.
As that water gets warmer, the thermostat sends less warm water into the recirculation loop and so the water entering the cooling system is colder.
I've heard rumors of systems that had a ball valve instead of a thermostat. I am guessing that to add that to my system, I would need to restore the recirculation loop and add the ball valve in the recirculating portion?
The other question is -- how much does this matter? I've been running my engine for 8+ hours and it is still basically room temperature. It could be that my sender and gauge are working fine and the temp really is so low it is off the scale.
Is this going to cause issues long term? I typically boat it water that is 50-60°F, so it has quite a bit of cooling power. I think it leads to less efficient and incomplete combustion? Are there other side effects?