What's the history of the injection pump you're using? Is it a known-good pump? If the fuel rack is full and the orifice is installed to build pressure, the pumping elements should be doing their thing.

If the IP sat for a long time, or it was from a WVO vehicle, the internal parts of the fuel rack could be gummed up or seized.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]