When you were pushing the primer pump down the contents of the primer pump were looking for the easiest way out or path of least resistance. It was through the leaky relief valve.

When the pump was retracting it was sucking the air back into the injection pump through it . That had been driven out initially. The leakage was so bad that the pumping had a substantial back and forth component, Rather than directional to build pressure and expel the air in the injection pump enough.

What really got fuel up was when you were depleting the batteries. The lift pump has a check valve on the output side as well. It was working as you were cranking. It provided your real elimination of the air. As in operation it cannot support reverse suction.

Do not expect to find much literature on this. If the maintenance plan Mercedes wanted on these cars is followed. Issues like this are detected far earlier than when they become problems. There are many simple tests that can be conducted on this supply system to detect what the problem is.

I seriously toyed with the ideal of getting the system in shape to retain the fuel pressure at least until the next use of the car. As a subject on the site. First I would have to get members interested in buying a pressure gauge.

It was not happening so I put the concept on the back burner. After a lot of thought I suspect that rebuilt engine drop in the 240d may do this. I also suspect when brand new they all did it for a percentage of their early lives.

I see no reason that with a tight relief valve and tight lift pump valves. Primary fuel pressure cannot remain after shutdown. At those low pressures it probably cannot escape past the injection pump elements and even if it does. The older delivery valves may have leaked down enough fuel over time. It blocks the loss of pressure. It may even increase it a little and that would open the relief valve to restore the residual pressure.

All the clamping off of the return line really achieved was a positive identification the relief valve had far too much leakage.

We have over time seen some members examples start in temperatures most will not even consider doing. I assumed they just did not have internally leaky fuel system issues. Compression checks on these older engines remain generally very good for their age and miles. Some are substandard but the majority are still decent.

As I have mentioned many times doing a real maintenance run of these fuel supply systems is cheap . Plus it gives you a better understanding. So if some issue arises it is far easier for you to deal with. Most problems in that fuel system designed in the 1930s do give warnings well before the problem erupts.

I almost cringe when a new member comes on board. Quoting I just acquired a thousand dollar old diesel Mercedes. What should I check before leaving on a five thousand mile trip. It is amazing that so many make it in my mind.

A percentage do not and a proper maintenance run on the car plus a few things that have been learnt over time ..Could avoid many of the expensive road breakdowns that do occur. Generally it is pretty inexpensive to do this yourself