I may have misunderstood so here come the questions:
- When you said you pressure tested the water jacket in post #5 I thought you meant the engine water jacket. Now I understand the test was the exhaust pipe water jacket, correct?
- Have you pressure tested either of the exhaust manifolds you have?
- Have you pressure tested the block?
- Have you pressure tested the heat exchanger (HX)?
- In post #5 you mentioned checking for kinks in the exhaust system. I'm at a loss to understand how a rigid water jacketed exhaust system gets kinked. Please explain further.
- Is there a high loop right before the exhaust thru-hull? Can you provide pictures or a sketch of your exhaust system?
The water jacketed exhaust pipe is typically fed with raw water so it is not involved in your coolant loss. The presence or absence of a thermostat will not cause a coolant loss either but if there is no thermostat you certainly need a way to restrict the bypass (the coolant path from the alternator side of the block to the thermostat housing).