Abstract: Engine misfire failure when acceleration is a frequent failure. Misfire failure was diagnosed from engine principal shaft instantaneous speed. The instantaneous speeds in rapidly accelerating process from idle speed to high speed, at which the engine runs under the four formal and fault working conditions, were measured with magnetoelectric sensor. After decomposing and reconstructing the speed signals with db3 wavelet, and then comparing the information in all the four conditions so as to judge whether the engine was misfiring or not and what types of the misfire would be. The results show that, the wave amplitude, the stable idle speed, the stable high speed and accelerating time of misfire condition are distinctly different from those in normal condition. Therefore this kind of misfire diagnosis method is feasible and effective.