Sorry for being out of the loop for a while.

I think we are troubleshooting this in the right direction.

The symptoms needs to be looked at separately.


1. The loss of prime when fuel level is below the fuel tank saddle
2. The x-mas tree error messages.

Loss of prime:
Reading up on how the fuel system is made, makes it easier(read: NOT easy) not troubleshoot why the system is loosing prime.
The workshop books can be found here:
https://drive.google.com/drive/u/0/f...G9xSTVEdVFPQlU

https://1drv.ms/f/s!Auket738SUJ3igJkke7T1hPSiRBm

RIGHT CLICK ON the ? IN THE IMAGE AND DOWNLOAD TO SEE IT. Image hosting is such a pain.

Im still struggling with deciding which check valve that must be stuck open in the fuel tank, preventing proper prime. Im leaning towards one of the valves(1,10,11) within the right fuel pump.
However, it could also be the check valve(7) located in the top of the left pump(jet)

Instead of throwing parts at this problem, I think I will remove all parts from within the tank and have a look on the bench. Since the problem only exists when the tank level is below the saddle (Thats how I explain why this issue is intermittent), and the fuel pump seems to deliver enough pressure when air has been purged, I think some crude is blocking a valve.

My understanding is that this priming issue, introducing air into the system makes the starting so difficult. Thats the only way I can explain this, since I was able to start it without issues when purging the new fuel filter, then connecting it.
Doing the ignition on-fuel pump on like 5 times in a row sometimes also gives a successful start as the air is then purged.

The price of the in tank parts are not trivial. I can get parts (like from the X6 diesel) for cheap on Ebay, but I would like to verify that the p/n is interchangeable. Unsure how to do that, as I can't figure it our just using realOEM.com

2. Error messages
I believe this is connected to low battery voltage while cranking.
Even though I changed the battery about 3 years ago, I must admit that the car mostly do short trips and stand for a month at the time while Im at work. Since the error messages disappear as soon as the car eventually start, I have put this away to low voltage, as I saw this when the old battery was done too.


BWTHDIK

Just to let everybody know the progress on my E70 3.0d

I took the fuel pump out of the tank.
Took the whole pump/reservoir apart. Didn't look to bad, but there was some gunk in the bottom of the reservoir. Especially at the bottom flap/valve.

Clean everything out and blew any valve through with compressed air. Put it back together and reinstalled. No more gurgling voices from the LPFP and starts are immediate now.

Cant tell you guys exactly what the issue was, but my guess is that a return valve was stuck in the open position.

Much more satisfying than just throwing new parts at the problem.