•  Individual fuel injectors are measured for flow characteristics and assigned a calibration code which is printed on the head of the fuel injector. The calibration code is used by the PCM to adjust the fuel injector injection signal to achieve optimum efficiency. The fuel injectors are designed to allow a quantity of fuel to return to the fuel system. The returned fuel is developed during the opening cycle of the fuel injector control valve. This returned fuel is used to lubricate the fuel injector moving components and is essential to the operation of the fuel injector. This lubricating fuel is returned to the fuel tank through the fuel injector fuel return line. Only by accurate measurement, at precise engine conditions, can the volumes of fuel from the fuel injector fuel return be used to diagnose concerns with the fuel charging control system.