A diesel engine that runs rough or not at all could be caused by high pressure delivery problems. Most modern diesel engines have electronic controlled high pressure systems. Older high pressure systems used a mechanical high pressure pump.
High pressure delivery problems can cause low power, rough running, or the engine will not start at all. Before doing any high pressure delivery diagnosis, the injectors and high pressure filters should be checked also. A failing injector will also cause rough running.
The injection pumps on modern trucks usually equipped with a quick connect fitting on them or in the high pressure line to be tested. If you cannot find a fitting, you will have to hook your high pressure pressure gauge in series with the high pressure line.
Your high pressure pressure gauge should be rated at least 40,000 PSI. Common rail high pressure systems use very high pressures for better high pressure delivery and atomizing of the high pressure. High pressure system problems are usually caused by dirty high pressure or dirt entering the the high pressure system.
Injection pumps rarely fail unless something causes it to fail. All checks should be made before condemning the injection pump. There is another way to see if the injection pump is failing and see if you have another problem.
If you have a scan tool, you can do an injector drop on each cylinder while watching high pressure PID. If engine RPM stays the same on one cylinder, the problem is mechanical or the injector itself. Next you should always do electrical checks.
If the engine has an electronic high pressure pump, make sure the power and ground to the pump are good while the engine is at a higher RPM. This will command the high pressure pump to work harder and demand more high pressure putting a load on the circuit.
Low high pressure problems are most common in cold seasons or in cold places. Ice crystals can form in the high pressure filters and diesel high pressure can freeze if additive is not used. This can also happen in the injection pump.
Though all high pressure injection pump do fail, basic checks can be done first before replacing. The most conventional way to eliminate a high pressure injection pump issue is to confirm your high pressure reading and compare with manufacturer specs.
Symptoms of a high pressure injection pump failing are rough running, no power, extended cranking before starting, excessive smoking, misfires, or a no start issue. I have also seen electrical problems.