'91 190E 2.3 rough, low idle - fixed
This intermittent rough idle had become almost a constant occurance when starting from cold, and caused missing even when warm, though not severely. Off-idle the engine has always been smooth.
I noted when the idle was rough, I could sneak my toe behind the accelerator pedal and pull up a bit, and the idle speed would increase and smooth out. Checking the linkage, it was apparent there was a less-than-positive complete return to idle. I suspected the throttle was not closing completely, and I could tell the idle return microswitch was only occasionally closing.
I found the 'full-throttle stop adjuster' - which is plastic - had broken, and part of it was binding on the cable and would move around, changing the effective cable stop point. It seems to be part of the throttle cable, which is probably pricey, so I repaired it with a good epoxy - no longer adjustable, but everything slides smoothly, and the idle seems to be completely normal now.