On a similar note, I bought my 560sel a couple years ago with 135k on the clock. It needed one fuel pump, and although everyone suggested that I replace both (to be safe), I replaced only the bad one...

2500 miles later, in the middle of nowhere, my car felt like it ran out of gas. Coasted to a stop, waited a minute, and then tried it - started and ran fine, for about 20 seconds, and then "ran out of gas". We scratched our heads for quite awhile (most of a day), and then finally removed both fuel pumps and bench tested them, which showed the fault.

Our problem with diagnosis is that since the pumps are mounted together, when one is buzzing it feels like they are both running...

The clincher in the diagnosis process was when we hooked up a fuel pressure gauge - this showed "normal" when we started it, and then slowly went down to (below 40psi where it quits...)

-David