Pull the vacuum line off of this, http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1QF0V26SB1UX0O8OB1&year=1994&make=MB&model=C-220-001&category=All&part=Fuel+Pressure+Regulator

It should be easily visible near the front of the engine. If it's moist with gas, the regulator is bad.
Good Luck, Dan

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2012 E350 Bluetech