For larger 2- and 4-stroke diesel engines it is necessary to test the fuel injectors after a specified amount of running time, to determine the performance of the injector. This includes tests like: 

• Verification of Opening Pressure - determine the pressure needed to initiate injection through injector nozzle
• Calibration of Opening Pressure - calibrate/adjust the opening pressure while the injector is under pressure (4-stroke engine injectors only) 
• Nozzle Leakage test - make sure nozzle is tight during idle time
• Injector Spray Pattern - inspect direction of jets as well as quality of atomising
• Non-return Valve Functionality
• Tightness of o-rings and seals
• Correct operation of safety valves
  

And other pressure-related tests for diesel engine fuel injectors and safety valves