surging - air conditioner causing
On my E420 with surging at idle, we traced it to the air conditioner compressor coming on and off. Apparently, unlike other cars, the Mercedes compressor continually goes on and off (like very few seconds). Each time it kicks in the idle drops, sometimes so much that the car hickups like someone touched your back bumper. Only way we could cure was to move the idle speed up. Turning off the air conditioner solves problem, but the dealer says there is nothing else we can do, other than turn up the idle. In winter, when I can remember to do it, I run the air in economy mode and it does not surge.