TAMD41A Fuel Drain Back

I have issue with my fuel supply not staying primed or getting air, problem is when starting i have to crank & crank the starter multiple times before engine eventually starts, after starting engine runs normal. If i wait ten minuites & try to restart i have to crank again multiple times before starting. I have since replaced the lift pump, but no changes. I rigged a small fuel tank elevated about 4ft above the engine connected a line from it to the inlet of the lift pump, blead the air & ran the engine, it has started the first turn every time ever since. it seems I have a drain back issue or a air issue under normal connection from the tank in the boat, I have also bypassed the suction from the starboard tank (problem engine) & tried from the port tank, same problem, so I think my problem is between the injector pump & the injectors. Any other way of confirming injector pump is the problem prior to removing?
Any suggestions are appreciated.

Take return from both engines and let it run into a canister.
The amount should be the same and no air bubbles.
Even test under load.
If too little fuel, it may vaporize and cause a vapor lock in the system.
Report back after test.