Many E30 M3 owners have experienced the high idle that is symptomatic of the throttle cables coming undone and sticking outside their "adjusting barrels" on top of the plenum. What this effectively does it make your throttle cable "shorter". So as far as your throttles know you are stepping on the gas pedal, even though you may be idling at a stop light.

There is an easy fix for this problem which was first presented to the E30 M3 SIG by Don Fields. It constitutes a pair of "clips" from another BMW application which can be used to secure the cables into their guides on top of the plenum. However, without photos it is difficult to know exactly how the clips should work. This page is an attempt to rectify that situation.

The part number for the throttle cable clips is:
p/n   35 40 1 165 393
They can be obtained from your BMW dealer for about $1.50 a piece. If you still have cruise control on your car then you will need two of them.

Thanks to Don Fields for sharing this handy little trick.