I just helped a friend who had the same problem with a Fiero he just bought.
Try this:
Get engine fully warmed up. Shut off engine.
Jumper the ALDL connector.
https://www.fiero.nl/cgi-bin/main.cgi?ECMCodesTurn key ON, but do not start engine.
After 30 seconds, unplug wiring from IAC motor. (#8 in diagram)
Start engine now (ignore flashing Check Engine light).
Turn adjustment screw until engine idles at 800rpm.
Shut off engine, remove ALDL jumper and reconnect IAC motor.
The ECM should now idle engine at proper RPM unless there is something else wrong.
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[This message has been edited by fierosound (edited 10-22-2018).]