An injection device for an internal combustion engine includes a piezo actuator for moving a valve piston, and a control unit for actuating the piezo actuator. The control unit supplies electrical pulses having different pulse energy to the piezo actuator and determines a return stroke between the piezo actuator and valve piston or a time delay caused by the return stroke. The control unit selects the pulse energies such that a maximum excursion of a movement of the piezo actuator caused by at least one pulse is smaller than the return stroke, whereas a maximum excursion of a piezo actuator movement caused by another pulse(s) is greater than the return stroke, after each of the pulses detects a frequency spectrum of a voltage signal at the piezo actuator during the movement caused by the respective pulse, and determines the return stroke or time delay based on these frequency spectra.