hello my name is doug if i was looking at your engine i would disconnect your fuel return. That is the line coming from the front or your head, the water tank end.it will take 2 wrenches to hold the line and fitting.get a clear water bottle hold on fitting,crank engine (be carefull the engine will start so be prepared to shut it off)see if air bubbles goes in bottle if it does you have to have someone to replace the injector cups. and i would bet this would be your problem we build approx 120 engines(3116/3126/c7)cats per year i would say iam a pro with these engines doug