Art's description is correct, and the ICU will servo the valve for a larger or smaller opening depending on the rpm sensor feedback. If the feedback adjusted amount is sent to the valve as a voltage, the older valve with larger opening will cause a high idle and/or surge.
Usually replacing the valve fixes it, but an old valve and new valve can servo the same amount, will measure the same dqwell as if the ICU is working correctly, but from a different reference, which is the static opening size. If you have a Bently's or Greeenbook there's a good test and spec for the gap.