The method of modelling the signal of vibrations of the internal combustion engine based on the discrete wavelet transform (DWT), is presented in the paper. This method is suitable for the representation of transient processes, which occur in the vibroacoustic signal (VA) generated by the engine. The model is identified on the basis of the vibration signal recorded during the car driving at a constant speed. The base model and its measures are created for the new engine and the control measurements, performed either at the determined time periods or car mileage, are compared to them. This base model must be actualised after each engine overhaul and as its wear and tear progresses, due to changes of the vibration characteristics. On account of a random character of vibration responses the envelope of the modelled signal is utilised for diagnostic purposes. The model based diagnostic method was verified in the paper on the example of the engine exhaust valve defect.