The time-history of engine speed fluctuations over one complete engine cycle, under steady-state or transient conditions, may be analysed to detect and identify faulty cylinders in a multi-cylinder engine. Diagnostic tests are usually performed using microprocessor based hardware systems. To validate software it is necessary to obtain representative data for various engine configurations with accurately controlled fault conditions. It is not easy to do this on a test bed and an alternative source of test data is required. This paper briefly describes a digital simulation program that can be used to generate repeatable test data for a variety of engines with differing degrees of cylinder power faults. The diagnostic software tested uses a simple dynamic model of an engine to estimate the gas pressure torque waveform from a measurement of engine speed fluctuation.