[capnward]

Okay, I will try to make this brief. MY A4 had been running well all spring until I replaced the freshwater impeller in the newer MMI pump. The old impeller was fine, only 2 seasons old, but the arms on it had developed a permanent bend, so i replaced it because I like to be proactive about these things. What's that saying, if it ain't broke..
15 minutes after restarting, the temperature alarm went off. Salt water flow was normal. I noticed water dribbling from the back of the freshwater pump. The water seal on the back had been pushed away from the pump, I assume from the added pressure of the new impeller, and was spinning on the shaft. Probably it had been leaking slowly there for a while, and finally got completely loose. Luckily I had the Moyer rebuild kit, so I replaced shaft, seals, and bearings. The water seal was shot and had scored the shaft. I thought that the overheating was because of the bad pump. But with the rebuilt pump, the overheating continued, after running for a few minutes.
I flushed out the manifold, heat exchanger, and block, until the water was clear. At first, the water was brown with tiny flecks of rust. I made a mental note to start using antifreeze in the coolant, to reduce rust.
I also changed the plumbing to the thermostatic mixing valve that Indigo used to sell, that had seemed to work for 15 years. When I had the engine rebuilt in late 90's, the shop left the thermostat out, and recommended I not use one. But I wanted to run hotter, so I went to the mixing valve, which worked like a bypass valve that automatically adjusts, to maintain a certain water temperature while maintaining full flow around the cylinders. Its the same Honeywell valve you see under bathroom sinks. I did not install the bypass with hand valve from the water jacket side plate to the thermostat housing.
Anyway, looking at the promotional literature for the valve, I noticed that I may have had it hooked up wrong all these years. I couldn't find the Indigo installation instructions after all this time, so don't know if I followed those right. The valve is designed to have cold water go in one side, hot in another, and mixed water out a third. But I had it installed so that hot water from the engine went in the mix end of the valve. Water to the upper, hot part of the heat exchanger went out the 'cold' side of the valve. Water to the pump, via a tee at the bottom of the HX, came out the 'hot' side. So I changed the plumbing so that water from the engine went in the 'hot' side, water to the upper HX out the 'cold' side as before, and water to the lower HX and to the pump went out the 'Mix' end.
I thought these changes had fixed the problem, because it seemed to be okay after a half an hour, so I took the boat out to try it at higher rpm under load. That was a Mistake; the alarm went off after 15 minutes, and after shutting down and listening to the engine pop and creak and squeak alarmingly, I was surprised the engine still started, I feared it would seize up. I should have shut down, and waited for it to cool, but I stupidly crept back to the dock in idle as temperature gauge was pegged at 240 and head temperatures next to spark plugs above 400. The engine actually ran fine, just hot as hell. The creaking noises went away once it got really hot. By the way, those laser temperature scanners are very educational.
The next thing to try, which should have been the first thing, was to eliminate the mixing valve completely, and go straight to and from the heat exchanger. This of course was the solution. Then I found the old Indigo temperature control system description, and saw that these valves become inoperative after an overheating episode, internal parts melting, parts to rebuild valve costing $35. Time for a new thermostat that won't melt. I will order one from Indigo when they reopen July 1. (Why are they shut down?) I like the external thermostat idea.
So I guess the bad seal on the water pump diminished the water flow through the engine to the point that it overheated the first time, and the fried mixing valve kept it overheating. Or maybe years of accumulated rust buildup going into the wrong end of the valve eventually clogged it up, forcing the pump to push out the water seal, frying the valve insides.
In 20 years of A4 ownership this was by far the worst overheating episode. I thought I had ruined the engine, but after changing out the very black oil, she still started and ran. Compression does not seem to be affected, all readings well above 100. No leaks in the head, no cracks. No water in the oil. What durable little engines we have! No doubt some damage was done, though, I guess where the piston rings touch the cylinders. There does seem to be some speed loss.
Now my problem is running too cool. With no thermostat, and no bypass, I can barely get above 100*. So I rigged up a bypass, with a hand valve at the hot entrance to the HX, to divert some of the water back to a tee at the lower HX exit, then to the pump. With this setup, and the valve at the HX closed halfway, I can get 150* on the gauge at 1500 RPM, vacuum around 12.5, knots around 4.5. This will, I think, keep me going until I can install a thermostat. At 1950 Rpm, vacuum is 5, knots approaching 6, gauge temperature is 160*, but that seems to be pushing engine limits. I have a new Indigo prop. I should mention that while the gauge temperature is 160*, laser scanner temperatures on the top of the head between the spark plugs and the nuts on the manifold side are near 200*, so a 160* thermostat may be plenty hot enough.