This morning I cleaned the IAC and re-installed it. I guess I didn't have the electrical connector pushed on all the way, as when I started it, it idled low and I had to hold the gas pedal down a bit to keep from stalling. It was -8 degC outside.

Pulled the connector off and put it back on again and re-started the engine. The idle did go up over 3000 rpm, but didn't hang there like before. It kept coming down in 500 rpm decrements every 1 to 2 seconds and eventually stopped at a stable idle.

However the stable idle is now about 1500 rpm. Yet the engine sounds like it's actually about 750 rpm. It's almost like the tach is reading double.

Next I'll check the tach against the tach in my timing light.