The computer knows it'* not under load from the gear selector. In P or N, there can be no load.
There is no such thing as too much air unless you have a code set for an idle condition or lean condition.
It sounds to me from your symptoms that the IAC is not controlling the idle air quantity correctly. I'll move your topic to the 92-99 section so the Gearheads can find it and take a crack at the problem. My personal experience and advice at this point would be to remove your throttle body and MAF sensor, and clean them per the instructions in Techinfo. It'll cost you a couple hours and a throttle body gasket. After that, if the problem persists, replacing the IAC with a known good unit may help.
Your solution may have actually hurt performance if you prevented any amount of airflow. You want as much air as you can possible get, but protected from the heat of the engine bay.