Well, I removed the throttle body, only mildly dirty, and cleaned it. Out of the car, with vacuum applied to the throttle opener, the mechanism rotates shut completely just as it should. Back in the car, same behavior: high idle, then I push it closed just that little bit more. It's not the throttle opener; I could feel the pin retracting under vacuum, so it's not the throttle opener screw out-of-adjustment.

This is all the obvious stuff, so I'm left confused. Somehow, in the car with the engine running, the TB behaves /differently/ than it does out of the car, even with vacuum applied to the throttle opener. What other things could influence the TB?

-Alex