My insight and $.85 will get you a cup of coffee. But here goes.

You clearly have a fuel problem if you can pour gas in the system and start the car. This tells me that gas gets to the cylinders through the sensor plate, rather than being delivered by the injectors. Once, started it seems to run, which means that then it starts getting fuel through the injectors. Presumably you are not driving around with someone pouring gas!

Possible culprits -

Fuel filter - disconnect return line into container, run fuel pump and see if you get a liter or so in 30 seconds

Air flow sensor plate rest position incorrect - no air moving at start, therefore no fuel to injectors, once engine starts, enough flow generated to get system working

No air through idle valve - same as above - try disconnecting

Bad fuel pump relay. Runs in ON but not in start. Jump 30 to 87 and see if it starts.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe