The push the pedal before you crank method will not help a Mercedes because most are fuel injected...This only helps on a carb equiped engine!! IT sounds to me as if you are experiencing fuel pressure leak down. Where from? I don't know. Also if the battery has been down and jump started lately, the overvoltage relay can be faulty or the fuse inside of it. It is located behind the battery and is small and silver with a red top on it...Goood luck

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM