Mercedes: 2002 Mercedes C220 automatic desil, fuel water separator, desil

Question
The car will not accelerate at speeds above 80kph but will build up speed slowly.

Answer
Peter,
As an air pump the engine must have balance between incoming air and exiting air.
How many miles of this engine? HOw long since you drained the fuel/water separator and installed a new fuel filter? If this vehicle has a catalytic converter it may be clogged and restricting the outflow exhaust.
Is the vehicle smoking during acceleration?
Has the air cleaner filter been changed lately.