Im not an expert on the 280E but this is my educated guess. You may want to check the 123 forum in BenzWorld.

The top picture in post #7 is the pressure regulator. It controls the system pressure. It is adjustable by shims.

In the top picture in post #8, the small cylinder with the electrical connection, is a Frequency Valve. It is controlled by a ECU that reads the voltage of a O2 sensor.

It works similar to a EHA valve in KE Jetronic in that it controls fueling to the injectors by leaking fuel back to the fuel tank.

The ECU is controlled by a relay. If the relay is not working, the ECU can't control the FV and you will have all sorts of running problems.

So the first thing you check is if the FV is working. It should vibrated when the car is running.

The injector buzzes with the key on and during start-up attempts. Guessing it is the pressure regulator valve which makes sense if there isn't enough built-up pressure to push fuel through the injectors. The question is do you add shims (washers) to increase it and how many. I don't think I have a fuel pressure tester that fits the metric threads to try and hook a meter up. Ugh.

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1984 280E