To evaluate your problem you will need to know a number of things. They may start all the way back with how an internal combustion engine works. You might not know or understand that engine speed is controlled by controlling the air not the gas. The "Gas Pedal" is a misnomer. You don't step on the gas; you step on the air and the engine control system adds the appropriate amount of gas.
Once you feel comforatble with general engine performance then the individual MB systems involved are very well explained in these archives. THe system you should become familiar with is the idle air control system.
Understand the theory and the testing concept becomes natural. To understand why it doesn't work one must understand why it does work.
Bosch Master, ASE Master, L1
33 years MB technician