Originally Posted by Frank Reiner

Many of the idle speed controllers recognize the P & N positions, and are programed to provide an increased idle speed in those positions. The increase, however, is on the order of 50-100 RPM, depending on coolant temperature. When warm, the controller may be cutting off power to the idle air valve (IAV), rather than slightly reducing it, which will result in a wide open IAV, and the attendant high idle speed.