A method of testdiagnosing the operation of each of a plurality of fuel injectors of an internal combustion engine system includes inhibiting ignition of the engine, monitoring a fuel pressure within a fuel rail of the engine and pulsing a fuel injector of the plurality of fuel injectors of the engine. Whether the fuel pressure has stabilized is determined and a pressure differential is calculated based on a pre-pulse fuel pressure and a post-pulse fuel pressure when the fuel pressure has stabilized. A technician determines whether the fuel injector is operating properly based on the pressure differential.