What determines Idle RPM?

my 02 540iT seems to have a low idle now. when i first got it the with AC idle was around 700 and non-AC 600 RPM. i had my accelerator pedal go bad causing Engine Failsafe Prog mode and after replacing the pedal and resetting the car the idle was around 800 for AC and 700 without AC and the car was quite a bit different in feel. i then started playing around with getting DIS setup on the car and nearly rand the battery dead playing with it and needed to reset the car again. i did not do any programming. i was only playing around in diagnostic modes. since then the idle is now 600 RPM with AC and 450-500 without AC. this is consistent with whether the car has warmed up or not. though i do think my MAF may be needing replacement since cold start is a bit rougher than fully warm.

this just seems like a lot of variability in rpm from what seems like base engine program. i am wondering if the MAF controls the rpm's or does the engine management program do this with regards to input from the various sensors and not just the tach?

i was planning on upping my rpm anyway in non-AC mode because of the hard downshift problem that occurs with auto's, but was wondering if i am missing something since i have what seems to have been three different engine programs running on the same car within the past month and a half.

G~

The DME controls engine RPM via the electrical, or "drive-by-wire," throttle, and a bunch of other sensor inputs.

My engine, when at operating temp, idles at 700 RPM with the A/C on, and 500 w/o. When warming up, the speed is 7-800. It has a conventional throttle cable.

Any stored codes?

How did you know your peddle was bad?

i checked the codes when the SES light finally showed up. i was getting the Engine Failsafe Prog several times before i got a SES. i checked the codes even during the EFP, and zero codes. anyway. i was getting P0120 and P0122. throttle position sensor error. this means either the throttle body or the pedal it seems. when i finally got the code i stomped on the pedal a few times when the car was off and each time i could go a bit further before the EFP would start up again. i would also start getting the EFP error when easing off the throttle going over the crest of the hill. it was a gamble, but from what i read the pedal was the more likely candidate than the throttle body. so far so good on that, i think.

as for pending codes. the last time i checked i was clean, but will check again tomorrow. the 02's are throttle by wire, so i figured it was determined by the DME, but it seemed weird that i would have three different idle speeds each time that i reset the DME by leaving the neg. terminal unplugged for an hour.

G~

It's possible the the potentiometers in the throttle bady are bad. If so, the entire assembly must be replaced; it is not serviceable.

you are right it could be in the throttle body, but since the problem would go away after several "poundings" of the throttle when the car was off. i determined that the throttle body was not the problem in this instance. if the problem persisted after the pounding of the throttle when the car was off i would suspect the throttle body. all of this assumes of course that with the car off there is not any communication going on between the two. though i was searching for sources of throttle bodies if the pedal was not the cure.

G~

I've had the Engine Failsafe Prog pop up a few times and never really found out what really caused it.
I've ohmed the pot and the throttle body pots. The last thing I did was to spray a control cleaner on the throttle body pot and wiper. It's the part under the spring.
BMW had a free clinic shortly after this happened the last time so I brought my car in for a DME scan, they couldn't find anything wrong with it.
The Service Mgr said that mashing the gas sometimes cause it, as the pedal can hit the floor before the throttle body can open up fully so the dual pots don't match, which causes the error.
Sound good anyways.

Your M62tu should idle at 500-580 when warmed up and 700 with the AC on when warmed up.

Ohhhh plastic gears!!! How lovely! (To save weight I presume )