I fixed mine on a 1996 502 MPI by simply turning the barrel on the throttle cable. One nut, two minute job. The throttle cable wasn't going all the way back to idle. It did after a turn or two of the barrel.

A great thing the scan tool can also check is if the throttle is opening all the way when the control is wide-open. Last one I checked was only opening the throttle 88%!

(Rookies: Do this check with key-on engine off.)!