The lift pump can only supply ~15PSI of pressure. The injector requires >1500PSI of pressure to open. Whether or not the pumping element dribbles fuel with an open circuit is irrelevant, the injector is only going to pop when the pumping element closes the spill orifice and compresses the fuel in the hard line.

Rolling black smoke usually indicates an overly rich fuel condition. If you have EGR, have you pulled the intake to see if it's coked up?

If you have an ADA device on the IP, have you tried adjusting it leaner? When's the last time the injectors were cleaned and/or rebuilt and pop/spray tested? If you're dribbling or making a poor spray pattern, you can expect poor combustion as a result.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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