1 of the 2 fuel pumps works intermittingly. Is it possible to use an aftermarket or even a MB single fuel pump to replace both of the old pumps? And if so how much psi should the pump have?

1. Not unless you want to reengineer the entire system.

2. I know the MB or Bosch pumps are expensive, but I replaced my 380's pump with a no-name pump. It lasted a grand total of 3,000 miles. A waste of time and money.

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm

See the post here by George Murphy, technical director for MBCA - http://mbca.cartama.net/showthread.php?t=19298. He describes how to run the fuel pump in reverse to clean it out. If you were going to replace it anyway, seems like you've got nothing to lose trying this.

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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