Instantaneous angular speed of a diesel engine contains a lot of information about gas pressure in cylinders, which can be used to diagnose combustion-related faults and other faults affecting the gas pressure, such as fuel leakage in fuel system, valve leakage, etc. In this paper, a dynamic model for simulating the instantaneous angular speed is presented, and the instantaneous angular speed waveforms on a small four-cylinder diesel engine are simulated. The tangential forces induced by the gas pressure and the vertical imbalance inertial force are calculated and analysed based on the proposed dynamic model, respectively. It is concluded that the gas pressure and the vertical imbalance inertial force have a great influence on the instantaneous angular speed. The simulated results of the instantaneous angular speed fluctuation ratio based on the dynamic model are confirmed by the experimental results. The instantaneous angular speed waveforms both in the fuel leakage condition and in the normal condition are measured under various engine speeds and loads in laboratory condition. The characteristic parameters for detecting the faults relating to the gas pressure in the cylinder are obtained successfully. It is promising, therefore, to develop and to apply the diagnosis technique further using the instantaneous angular speeds to detect the faults relating to the gas pressure in the cylinder.