The ecu is targetting a certain idle value (example: 750rpm) and if the idle falls the ecu should adjust the idle air valve to compensate to try and maintain the target idle.
Maaaark, have you resolved the other idling issues that you mentioned in another thread?
you need to ensure that
- ignition distributor timing is set correctly
- throttle body plate and air screw are properly adjusted
- tps properly adjusted
- idle air valve and ports are clean and clear. Especially the port/passage way that runs through the throttle body.