Having read all the posts on rough idling engines of the 300E, I read nowhere the item that effects idle speed and smoothness. When my '87 with the 103.98 engine idles rough I've been able to smooth the idle by adjusting the Lambda Control of the output voltage from the O2 sensor. I use a dwell meter to read the output from the sensor and using that long allen wrench, down through the hole in the airfilter, adjust the idle mixture until I get the right dwell at 2000 rpm. The smart box controls the speed, you adjust the mixture. Check it out. Happy Trails Beep Beep from Houston.
and all little Benzes is smuv.