George Murphy, the tech person for MBCA, has "fixed" a number of fuel pumps like ours by removing it and reversing the electrical hook ups on the bench, thus running the pump backwards and flushing it all out. I've never tried it myself, but it sounds like it's worth a try if you're planning to replace it anyway.

__________________
Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles