Your diagnosis is probably correct based on my experience with my 250C and a 230SL that I worked on. You might try to force some carb cleaner through the fuel pump and replace it it that doesn't work. With pressure from the fuel pump, I would then see what the injection pump does.

You will probably need a good used unit or send it out to be rebuilt based on what I have read and experienced with the 230SL.

Good luck.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe