This is a hose that goes from the diaphragm of the fuel pressure regulator to the crankcase. The reason for it is IF the diaphragm fails, the fuel will be pushed into the crankcase instead of spilling inside the engine compartment and causing a fire.
PS...when you order parts from us, be sure to get a new oil cap. That one is ugly! The OE one is black.
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MERCEDES SPECIALIST 11 YRS