The PCV valve is a device which recycles moisture, unburned fuel vapors and combustion products back into the system so they can be reburned. It is difficult to get to it on a Mercedes, and it would be best to leave any work to a professional mechanic.
You need to take the valve cover off your engine. The valve cover is the top cover of the engine that houses your spark plugs. This is a very sensitive part of the engine; be sure to not let it get contaminated with debris when you do open it up.
The PCV valve will be a small black rubber or metal valve. It will be between two hoses: one leading to the intake manifold, and the other hose will lead to the crankcase.
When looking for the PCV valve, be sure to let the engine cool first, as the engine will be very hot for hours after extended operation. Not only will the valve be hot, but the valve itself will be as well. If you need to detach the valve, be sure to disconnect it and step away for a minute to let the airborne toxins disperse before working further.
