It is ALL controlled electronically by input signals to ECU from various sensors fitted to engine.
The ECU then sends signals to the stepper motor to open or close the throttle as appropriate. All this happens in the closed throttle position ie. when you do not have your foot on the accelerator.
300-24, saloon and coupe, E300 24 valve diesel estate
As Dieselman but if the problem persists then disconnect the battery overnight and try again in the morning. After that get the throttle body bootstrapped again (electronically teaching it where to sit )
I am currently having the same issue, took it to a local garage and they adjusted the idle using the cable because my car would just cutt out completely. The mechanic told me that the 'idle valve' would need replacing