This disclosure provides system and method that can determine hydraulic start of injection (SOI) in engines using an in-cylinder pressure sensor. The system and method iteratively perform a wavelet transform on a signal based on the pressure information provided by the in-cylinder pressure sensors to determine a detail coefficient corresponding to a frequency band containing frequencies of pressure pulsations excited from a fuel jet injected into the cylinder. Hydraulic SOI is determined at the timing when the amplitude of the determined detail coefficient satisfies a predetermined threshold. The system and method provide diagnostic, control, and/or compensation opportunities for fuel injector operation in high pressure fuel rail engine systems without use of expensive or complex fuel injector components.