There are only a couple of things that will kill a 300E. I would start by looking at the wire that connects to the EZL...If a mercedes shop looked at it, they may have taken off this wire to prime the oil. If not, I would look at the fuel pump relay. You can pry the bottom open and look into it. If there are burned traces or spots, you have found the problem.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS