This mod patches the fuel flow bug in the Orbx PAC-750 XSTOL so that fuel now draws from all four tanks. 

All credit goes to TheReproCase over on the MSFS forums (@Repro here) who created the original code; I've just packaged it into a drag-n-drop mod.

Technical Discussion:

(quoted directly from TheReproCase's posts on the MSFS Forums):

The Problem:

Currrently, when fuel flow from multiple tanks into a single tank is calculated, the flow from the first tank is calculated and that chunk of fuel fills the tank, then the rest of the tanks in the loop are like ‘nah, tanks full’. It goes back to the beginning of the loop and keeps doing this. In the time it takes to get back to the beginning, it calculates the engine fuel burn and voila there’s space in the tank again.

The Solution:

To avoid this problem, using triggers and valves the fuel can be set to only flow into the collector when it has enough room to receive fuel from all four tanks.

Basically, it takes the four lines that collect each tank to the collector, splits them in half, installs valves in them, and wires those valves to a 'collector tank level threshold' trigger to open and close them. They only open when the collector has at least 0.1 gal space available).

So the behavior you’ll see if you watch the fuel window is when the collector is below 6.7 gal, some flow starts from all four until it’s just over 6.7, then stops, then drops below and starts again. It won’t ever fill to 100%, because if it does it won’t draw equally from all four tanks on that last iteration.