Does anyone have any evidence on this one?
Perhaps an exploded parts list view?
Perhaps the fuel pump tests from the online manual
http://skinnerbox.steaky.org/Service/W123/Main.html(Section 07-145) gives the answer.
The Fuel Delivery Pressure (with the return line open) specifies a pressure of 0.6 - 0.8 Bar at idle and 0.8 Bar @ 3K RPM. This is adjusted by lengthening the pressure relief valve spring in the IP return line banjo bolt.
The Fuel pressure End Test requires that the return line is pinched (closed off), then the maximum pressure attainable should be 1.1 Bar at idle or 1.3 Bar at 3K RPM.
This indicates that, with an unloaded OM616/617 the fuel consumption would be minimal, the pressure is limited by the fuel pump, not the return line pressure relief valve (which is blocked off, along with the air bleed from the filter banjo bolt.
I would post the text, but it is a pdf file scanned from the paper document.