Method and apparatus for retrofitting a low impedance fuel injection system to a high impedance fuel injection system internal combustion engine is disclosed. The original high impedance electronic control system may be retained, while system modification circuitry is added along the fuel injector control path. In one aspect, an original fuel injector control signal is intercepted along the fuel injector control wire. The intercepted signal is then modified from a simple on-off signal to a signal which varies the fuel injector current as a function of time, such that the on-state from the original high impedance system is converted to a current controlled signal. Moreover, using a plurality of parameters, the fuel injector pulsewidth may be modified, as well as the peak and hold current levels provided to the fuel injectors.