A fuel injection system for an internal combustion engine includes a fuel injector and an engine control system. The engine control system is configured to send electrical fuel injection signals to the fuel injector to open the fuel injector, monitor a current amplitude of electrical fuel injection signals, detect the mechanical openings of the fuel injector based on the monitored current amplitudes of electrical fuel injection signals according to inflection points between a decreasing slope and an increasing slope in the monitored current amplitudes of electrical fuel injection signals, calculate time delays between the sent electrical fuel injection signals and the mechanical openings of the fuel injector, and set durations of electrical fuel injection signals based on the calculated time delays.