idle surge
 

I have an 87 S4 which intermittently will idle up & down and die....it will idle perfectly at times and the suddenly act up...then idle perfectly...nothing consistent ...hot, or cold...warm weather or cold makes no difference...sometimes act up on start-up & then when I get to a stop, start suging up & down will almost always die when this happens...I can hold the throttle down enuf to get 1100-1200 rpm & it is fine but let off the gas & will surge.....anyone have an idea???
Tom Taylor

MAF,
Then O2 sensor.

tbt
I have read this below which has led me to do an intake refresh to fix it, which by the way I am still doing (taking my time between vacation, work and some trips). Take a look at this link and it will give you some ideas....http://members.rennlist.com/fabric/i...eshooting.html

On the other hand, while doing some research I came across a ni9ce definition of the Idle Valve below that seems to be a possibility for your description....http://members.rennlist.org/pirtle/keepers.html
There are a few threads about using WD40 to improve it.....http://www.nichols.nu/tip443.htm\

Let us know what you discover. I will be interested in at least replicating the diagnosis on mine.

Good luck