There is a switch at the back side of the throttle valve shaft below the flow meter that indicates when the throttle valve is in the idle position. It sends a signal to both the EZL and ECU that the engine is at idle, which fixes the timing at 9 deg., eliminates vacuum advance, and tells the ECU to maintain a fixed idle speed (700 on my 190E 2.6 with manual transmission.)

The switch itself is difficult to access (and change I expect), but it's wired to a connector located on the top of the inlet manifold, ahead of the fuel distributor. You can use this connection to test if the switch is working. (You'll have to remove the air cleaner to access it.) IIRC it has three leads - ground and one each for the ECU and EZL.

When I had the problem at least ten years ago it was not functioning properly, and my first step was to douse the switch with electrical cleaner, and that ended up doing the job and normal idle control resumed, but I recall it took multiple applications.

Also check that the micro switch on the throttle linkage is functioning, though this switch had a different function. It controls fuel shut off and cut-in. Notice that as you move the throttle linkage the switch changes before the throttle begins to move. This ensures smooth fuel cut in without a jerk.

When you lift off the throttle the switch changes position to signal fuel cut-off, but there appears to be a one to two second delay, and this keeps revs from falling fast enough to make a quick shift with smooth clutch engagement, particularly between 1-2 and 2-3 that are quite widely spaced.

I recall I adjusting it differently than recommended in an attempt to get revs to fall off faster on upshifts. I think that helped a bit, but I still have to granny shift between 1-2 and 2-3 to prevent a noticeable driveline jerk.

Of course, the above would not be an issue with the auto trans.

Duke