Good afternoon and welcome to JustAnswer!!!
Do you know what cranking pressure is on the rail? Cummins needs 6,000 to bust off and if it's low it's usually a leaking injector.. Have you performed a injector return test or a lift pump supply test? The lift pump should be producing 500 ml in 10 seconds, if good and your injector return test shows excessive then start blocking off injectors until the rail psi comes up and then replace that injector.. If you find that you need 3 or more I would replace all the injectors..
To test the Pressure Limiting valve you will need to connect special tool #9013 If the engine runs, start the engine and quickly / instantly depress the throttle to wide open, then release back to idle position (make sure the parking brake is set and that the transmission is in park or neutral). Do this a few times. If the engine will not start, disconnect the Fuel Control Actuator on the CP3 high pressure fuel injection pump, then crank the engine over for no more than 30 seconds. If there is no fuel from the PLV, the valve should be okay. If fuel does leak, the PLV should be replaced.
Using tool #9011, cap off each port of the high pressure manifold (rail) and try to start the engine. If the engine starts, remove and inspect the High Pressure Connector Tube and the injector from the capped off cylinder. Inspect for any loose or damaged connections. If necessary, replace the effected parts. If the total injector return is greater than 70 mL while cranking the engine (two 15 second cycles), it may be necessary to replace all six injectors.
I hope this helps and if you need anymore assistance feel free to reply..
And thank you for the positive rating,