Thanks for all the responses, guys.

lawnmowerman: The IAC was my thought, too, as if it were sticking or failing to close a bit more, but then I found this funny behavior of the throttle body arm assembly, which I don't see how can be affected by the IAC.

garym: That's a great site you linked to -- thanks! Unfortunately, Toyota doesn't go into enough detail about the whole TB system. I'm familiar with a simpler TB, TPS, and IAC on my car, but this one has a dashpot and a "throttle opener" under vacuum, and from my test above the presence of vacuum clearly affects the TB's mechanical behavior and I don't understand how or why.

ErikB: Ha ha ... sorry, "MTX" is a term we use in the SHO community meaning Manual Transmission; our trans was labeled by Ford the MTXIV (instead of say R150F). ATX's are slower

Funny business with the IDL signal would make sense, except it looks like in 1997 Toyota changed the TPS from 4-terminal to 3-terminal, and there's no more IDL line!

Thanks for explaining the dashpot.

foxtrapper: A peculiar mechanical bind seems possible (it isn't the cables -- they have slack). I'll try more poking at the rotating assembly, and I guess I'll end up pulling it and going after it with a toothbrush and cleaner like you suggest. At this point that's just a blind guess, because the rotating assembly seems to rotate fully and completely closed with the engine off and vacuum applied to the throttle opener (per the manual), but *doesn't* with the engine on.

It's just weird. Need to smog the vehicle for registration right away, and this isn't going to help pass California emissions <img src="/forums/images/graemlins/confused.gif" alt="" />

-Alex
'97 T100 XC 4x4 MTX
'95 SHO MTX