I did the touch test, but don't think I learned much. The large coolant line that comes out of the firewall below the monovalve is hot at all times. The smaller coolant line that goes into the firewall above the monovalve is very warm, but 10-20 degrees cooler than the bottom line, and it's consistent in all conditions.

I did notice that following freeway driving, the heat very quickly comes on to full heat (in about 5 seconds) at 40 mph. If air flow becomes very quickly hot, doesn't it HAVE to be the monovalve opening?

Interesting point about the impeller, I'll have to figure out how to test it, hopefully without taking the whole area apart.

By new monovalve, I just mean a new valve insert that goes in the coil at the top of the monovalve body. Didn't do anything to the rest of the monovalve body.

I also know that my ACC pushbutton unit is probably bad, but by cutting power to the monovalve, aren't I taking the pushbuttons out of the loop?