Except for RV conversions which could lag by a model year, all NAFTA 2001 - 2003 OM612 diesel engines have a low pressure mechanical fuel pump mounted on the front of the engine. All NAFTA model year 2004 - 2006 OM647 diesel engines have an in tank electric fuel pump. All NAFTA V6 OM642 diesel engines (2007 and up) also have an in tank electric fuel pump.
The 2001 - 2003 fuel system "pulls" fuel out of the tank which means that for practical purposes the fuel filter and all fuel lines before the inlet of the low pressure mechanical pump are under negative pressure while the engine is operating. That design fuel system is normally very reliable. That said, if (when?) a leak develops in the normally negative pressure side of the system (a leaking fitting, leaking o-rings, cracked fuel line, loose clamps, etc.) then the fuel pump is unable to overcome the air intrusion to provide enough suck and pull fuel from the tank to supply the high pressure fuel pump. That often results in hard starting or no start conditions. In that situation (air leaks) there have been people who report that installing an in-line electric fuel pump between the tank and the fuel filter will often cure the no-start situation by pulling fuel from the tank and supplying fuel to the mechanical low pressure pump..If the leaks are small enough once the engine starts and RPM increases the added fuel pump isn't needed for the engine to run properly.