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I'd like to add a comment that hopefully won't be rubbing peoples noses in the dirt.
The term vacuum leaks is in some ways a bit misleading. The important thing to ensure is that there are no leaks in the rubber piping that goes to and from the air idle control valve, the crankcase and the air flow / mixture sensor (plate) assembly.
As explained above the air flow plate is just a mechanical balance that is connected to a plunger that allows more fuel into the fuel distributor. The amount of air drawn past the air flow plate is meant to be proportional to amount of air drawn into the cylinders...
...now the way Mercedes (well Bosch actually) have set up their air balance is that it is designed so that the engine will struggle to idle! To allow the engine to idle air is diverted behind the air sensing plate via the (diversionary) pipes that go to the air idle control valve...
...the important message concerning "vacuum leaks" is that if this pipework (that by-passes the air flow sensing plate) leaks then you get what is called "false air" in the system. This buggers everything up! Well it doesn't bugger up absolutely everything but it certainly messes up the fuel air mixture and confuses the ECU.
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