Breaking news: Gene Cooley's idea that the water pump may have been vapor-locked, might have been the answer.
After Carl left I thought about the situation some more and it seemed unlikely to me the radiator is bad, so maybe the water pump wasn't pumping? I disconnected the bypass hose from the top of the water pump and poured a little coolant into it to try to 'prime' the pump, then reconnected the hose and started the engine. As before it idled up to maybe 115C, but this time I revved the engine to about 2000 rpm and held it there. The coolant temp steadied, and all of a sudden it dropped down to maybe 90C and stayed there. At about the same time, a slight whine from near the water pump disappeared. I'm guessing that bit of activity broke the vapor lock and finally got the coolant flowing again.
Next step: Reinstall thermostat, and retest/drive. Will probably have to wait until next weekend for what I sincerely hope will be the final chapter in this embarrassing story.
After Carl left I thought about the situation some more and it seemed unlikely to me the radiator is bad, so maybe the water pump wasn't pumping? I disconnected the bypass hose from the top of the water pump and poured a little coolant into it to try to 'prime' the pump, then reconnected the hose and started the engine. As before it idled up to maybe 115C, but this time I revved the engine to about 2000 rpm and held it there. The coolant temp steadied, and all of a sudden it dropped down to maybe 90C and stayed there. At about the same time, a slight whine from near the water pump disappeared. I'm guessing that bit of activity broke the vapor lock and finally got the coolant flowing again.
Next step: Reinstall thermostat, and retest/drive. Will probably have to wait until next weekend for what I sincerely hope will be the final chapter in this embarrassing story.