danalinscott, it depends on the age/generation.

The old stuff is gravity fed injection pump, meaning a low pressure fuel pump whose function is only to lift the fuel out of the tank and into the secondary filter (cartridge style). If the secondary filter-to-injection pump line comes out of the bottom of the secondary filter housing then it's absolutely a gravity fed injection pump.

The early spin-on secondary filter engines also had a gravity fed injection pump (Mercedes liked to do things incrementally - the glow system of the 1970s decade is nightmare to diagnose). Didn't work too well, so they jacked up the fuel pump pressure (then changed fuel pump?) so the fuel can go uphill through the secondary filter.

Then the injection pumps became pressurized on the inlet side via a relief valve on the injection pump. Different fuel pump from the very early example.

So you have to look at the injection pump/secondary filter to figure out the answer for a specific engine generation.

Biodieselers like you would be better off with a gravity system, in my opinion. The injection pump relief valve can cause problems unless flushed out well before shut down.

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daBenz - 1970 220D