After messing with a surging idle on my 91 740 for a week or two I finally found the throttle body (TB) switch was not seeing the closed position. Previously I had removed and cleaned the TB and checked that the switch was clicking when the TB was opening and it was. But when I reattached the short linkage arm it was a tad short and must have held the switch open and the result was a surging idle. So happy story I fixed it. However in checking different things I removed and tested the idle air control (IAC) valve. I put 12 volts to it and it popped open and took the 12 volts off and it closed. Repeated several times and it worked great. So my question is how does the IAC valve achieve different idle settings if it has an open and closed position only? While I had the TB switch in the not closing mode the IAC valve held different idle RPMs at different times. I could unplug the IAC valve electrical connector and the engine would idle right down. So how's it work?