Check the idle air valve is plugged in, and the connections aren't loose or corroded. Its also a good idea to give it a clean through with carb cleaner.
Try unplugging it to see if the idle increases further. If so, then you must have a vacuum leak on the engine side of the throttle body ie. the intake manifold.

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Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i