My SHO surges at steady throttle; why?

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Thanks to John Holowczak: A SHO surging under steady throttle is most frequently caused by a dirty/clogged idle air bypass (IAB, or IAC for idle air control) solenoid. This is usually accompanied by longer crank times/difficult starting. A vacuum leak might do similar things, but assuming its the IAB (IAC) your options are:

  1. Remove and clean it, instructions on the FAQ, then be SURE to clear your EEC-IV Keep Alive Memory (my dealer didn't do this after replacing my IAB under warranty, so I did it myself to stop the surging at freeway speeds - see #2 below).
  2. Clear the Keep Alive memory on EEC-IV - if its not completely clogged this will stave off having to clean or replace it in 10 degree weather. Per Doug Lewis, disconnect the negative cable, turn key to engine run, press brake pedal and hold for 30 seconds or so. Turn key to off and reconnect battery cable. The current drawn by the brake lights gently discharges any saved up electricity in the EEC-IV computer, and the computer then knows to relearn all the cars sensors, including either a partially cloged, cleaned, or replaced IAB (IAC).
  3. Replace your IAB (IAC) with the 1996 design part Prefix is F96x-xxxxxx-xxx - there is a TSB on this. Be sure to clear the EEC-IV afterwards per #2 above. Ford changed the IAB to a non-cleanable one sometime in '93 or 94 I believe and screwed up. So they did another redesign in 96. My 1995 MTX now has the '96 IAB and its been almost trouble free except for some hard starting recently as the weather dipped below 20 degrees.

Idle Reset/Relearn Procedure

Always make sure to reset your idle after disconnecting the battery, as otherwise stalling and surging can occur.