My 190E will not start. Starter and battery seem fine. When I spray ether in the air intake the car will start indicating to me that the problem is in the fuel system. Recently, the car suddenly died while running, but restarted immediately. I have replaced the overvoltage protection relay also recently because the previous unit was blowing fuses on cold start. I still have the same problem with the new relay before the current situation (i.e., not starting).
Any suggestions as to what I should check now?

Sounds as if the fuel pump relay isn't powering up the fuel pump, very common problem & easy to repair(replace). Turn key on, the pump should run for at least 1 sec.

Where is the fuel pump relay located?

Could my problem also be due to the cold start valve?

I don't think it is the cold start relay.

I'd bet on DOC. There are two locations for fuel pump relays on your car depending on year. One is in front of the fuse box and the other is behind the black plastic panel behind the battery. It should say "4zyl kickdown" on top of it. We sell them in partsshop if you need one.

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

Both Doc and Benzmac were right. The problem was the fuel pump relay and is now solved. Thank you very much for your help and excellent information.