Abstract: The vibrations of diesel engines high pressure lines during injection time contains much information of the fuel system condition,the research shows that the signal has fractal characteristics, i. e. self-similar stochastic structure within the data. It belongs to 1/f noise family. The information of the signal can be concentrated in one parameter, Hurst Exponent or Power Spectrum Density (PSD) exponent. By analyzing the sensitivity of the PSD exponent in variant conditions of the fuel system, a non-intrusive fault diagnosis method to diesel engines is supplied.