You also could have a defective mass air flow sensor, which works with the computer and throttle actuator in controlling idle speed.
Typically, though, a failing mass air flow sensor would show itself with an oscillating idle going up and down.
A vacuum leak will also make the throttle actuator and / or mass air sensor work improperly.
If you have a vacuum leak, it could also be at the intake manifold seal. You can spray water at the gasket to see if air is leaking there.
My intake manifold gasket failed causing a vacuum leak that caused me to replace both the actuator and mass air sensor. The bad idle remained after two indie MB mechanics failed to find the vacuum leak.
I took it to the dealer, and it took them 30 minutes to find the leak at the intake manifold.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".