You are right, it would open up to whatever idle speed is set to in your chip, but the point is, it wouldn't go all the way back up to 1000.
If it does go back to 1000, then it's something in the computer. If it starts to idle normally however, your scan tool reading a requested idle of 1000 rpm is not true and you know that the scanner is not giving you the right information. If the computer is truly requesting a 1k idle, that is where it will idle if it can. The only real way it has to achieve that is by allowing more air through the throttle. The only way it can do that is through the IAC. If with the IAC all the way closed and disconnected you are still idling at 1000 rpm, there are really only two possibilities. You have a vacuum leak, or the throttle plates are open too far.
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