Not sure if this is the problem you are having, but I know of a number of ’99s, including mine, that has developed a coolant leak at the copper elbow on the line from the engine coolant pump to the Aquahot.
I “fixed” mine by removing the engine to/from hoses from the Aquahot and connecting them together with a 3/4″ plastic barb fitting. Now I have no engine preheat. I also removed the engine coolant pump fuse to keep from inadvertently turning it on and running it dry.
Fixing it requires a small contortionist. I would guess that you need to remove the burner head, make sure the engine preheat loop is dry and then sweat on a new elbow. Roger Berke has seen (and fixed a few) these and sent me the attached picture. The elbow in question is circled and is behind the burner. I needed a mirror to confirm that was where my leak was.
Bob McGee 1999 U320
