The research proposes a method for detecting valve system faults in an engine. The methodology is based on the detection of impulsive sound embedded in the high level of background noise by using the sliced Wigner fourth order moment spectra (SWFOMS) which is inherently more robust in the presence of random (Gaussian) noise. However, in previous work, the SWFOMS for multiple signals had problems with its application which were due to the existence of non-oscillating cross-terms not smoothed by conventional methods. In this paper, the γ-method is developed to smooth non-oscillation cross-terms. Finally, the techniques developed are applied to the diagnosis of valve system faults in an engine.