As far as I know the 16V has the same setup as other contemporaneous Merc engines. There is a switch on the end of the throttle shaft on the throttle body under the air flow meter. It should open/close as the throttle just moves off the idle stop.
The switch itself is difficult to reach, but it should have a pigtail that you can disconnect from the harness to check it for function.
The throttle shaft switch works in conjunction with the microswitch on the linkage to tell the ECU when the throttle is in the idle position.
I had an idle problem on my 190E 2.6 and it was a sticky throttle position switch. I was able to clean it off with mineral spirits from a spray bottle to where it functioned properly, then gave it a shot of WD-40. That was about 18 months ago. No problem since.