I tried a new air mass meter today and although the readings are now equal or above the desired g/s the fault code still appears, that is the P1260.
You had good evidence to condemn the MAF but I think you secretly knew it was unlikely to cure the P1260. How many times have we swerved a fault code thinking its unrelated to the fault and then it comes back and bites us.
Had a word with AD and they think it is either a software related issue or a fault in the ECM
Then they need shooting !! Right between the eyes !!
That ECM & its software has 3 concerns with the LP fuel Circuit
1. To Click the Relay in and switch the Pump on
2. To monitor fuel pressure and throw a DTC if the pressure is wrong.
3. Push accurate LP Pressure to your scan tool
Its doing all 3 jobs marvellously well from where I am sat ??
KOER terminal  = 1.64V @ idle
KOER terminal  = 1.54V @ 2500 RPM (Max rev's)
KOEO terminal  = 2.1V (with codes cleared). [pump is running here]
That looks wrong to me. Have you got wiring or Pin Grab issues in the connector. It is a long thread now but would I wrong in thinking you appear to be getting different readings every time you make a measurement ?
Working on the principle that your fuel pump is causing Fuel to Flow & Not creating Pressure.
From the Diagram above my understanding is that item 11 -"Pressure Relief Valve" is the calibrated "Restriction" responsible for creating then maintaining the correct pressure in the circuit between the outlet of the LP pump and the Inlet to the HP Pump. I'm thinking it "Reliefs" anything above 2.6 bar. If that is the correct Pressure Figure. I will try to verify that later from another source.
If that Fuel Pump is NOT demand controlled, the diagrams suggest it is not. Then If the pressure relief valve in the tank does not have wires, cables or Bluetooth then it should maintain a constant pressure unless its broken ? or controlled by something.
The figures above shouldn't move - Ever ?
I have also measured the low side fuel line pressure with KOEO pump not running and found the line pressure holds 1.4 bar stable
That proves you don't have a leak. You need to be testing that with the Engine & LP pump Running.
If not already done, Scope the Power and Earth at the pump, Amp Clamp on the 3rd Channel, Fuel Pressure sensor on the 4th channel to rule out any Relay or Control Issues responsible for the LP figures dancing around. Wire a 55w bulb in parallel to load it up (might blow the fuse) but it will confirm the circuit.
While you have the scope out, leave the Amp Clamp on the pump then poke 2 into the pressure sensor supply pins, the last into the Sensor signal ideally back probed at ECU
I have then connected the pressure gauge to the low side pressure line directly from the fuel pump and filter and completed a KOEO pressure test for the supply to the Common Rail, I recorded [2.6] bar with the pump running.
Now I am reading this wrong or Baffled !! That tells me that you have your pressure Gauge Teed in to the very same piece of fuel pipe that Item 16 - Pressure Sensor is fitted into and you reading 2.6 Bar - The Correct Pressure, In the correct part of the LP Circuit, with the pump running hence correct test conditions ????
Does it look like I have a problem with the HPP?
In theory, you could unbolt the HP Pump, cut it in half, throw it in the dustbin, securely block the inlet pipe (the one with the LP sensor in it) Then run the LP pump and maintain 2.6 bar all day if the LP side is functioning correctly. As long as the Relief valve in the tank is fixed and not variable by design.
Educated guess, but In short it is critical that you have a constant 2.6bar in 4 Places and P1260 will go away ?
1. At the measuring surface within the Pressure Sensor to keep the ECM happy
2. At the inlet of the HP Pump so the party can start, so to speak.
3. On the screen of your diagnostic tool.
4. On your Gauge which is T'eed in for independent verification.
I would advise that you limit your initial testing to those 4 places only. Measure where the ECU is measuring, and where it needs to be.
Bosch suggests that your system pressure should be 2.5bar. The pressure sensor at 2.5 bar should have 2.6 - 3.0v on the signal wire. I would trust that over the 4.3v AD are quoting, makes more sense.