I finally received new multimeter leads so I'll be able to measure if the sensor and regulator get correct voltages - Chroma has more information in ePer than Stilo.
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With the engine idling and warm, with all the consumers off and the accelerator pedal not operated, the pressure regulator opening (MPROP) must be less than 45%.
Engine at idle, temperature = 70 degree, outside temperature 2 degree. Then push accelerator pedal to 2500 rpm for 20 seconds and idle again. You see also the graph of high pressure sensor voltage. MRPOP = 34.5 at idle and 32.3 at 2500 rpm.
Try to measure again the fuel pressure sensor. As you see in the graph, when the output of sensor = 2 volts, the real pressure = 663.7 bar and this is happen at 2600 rpm not at idle.
Today I swapped in a used original in-tank pump and the pressure is back to normal. The Delphi FG2435-12B1 pump I had installed before is listed as compatible with my car and engine in their own catalogue...