Various embodiments include a method for operating an internal combustion engine with a fuel injection system including a piezo actuator comprising: stopping the discharge phase of the actuator during an injection cycle; measuring a voltage profile at the actuator; comparing a feedback signal at the actuator with a setpoint value; varying the discharge time of the actuator in successive injection cycles until the feedback signal corresponds to the setpoint value; defining a servo valve closing time as a defined offset with respect to the optimized discharge time; and using the defined servo valve closing time to set an injection quantity of fuel for future injection cycles.