Abstract: The engine diagnosis problem lies in solution of the engine fault equations which express the relationships between the measurements and a large set of possible but improbable mulfunctions.The engine fault equations are generally underdetermined and can be solved by the aid of Main Characteristic Parameter Model(MCPM)suggested by the authors.The engine fault equations are based on engine fault samples and can be called empirical fault equations.The empirical fault equations are similar to fault equations obtained by single factor experiments,but the engine fault samples are used instead of the results of special experiments.Differing from the small change equation,the empirical fault equations are easily obtained and are of great use for diagnosis of such faults which are difficult to be diagnosed by employing small change equations.Some practical examples of engine fault diagnosis are also given.The fault diagnosis results obtained from are very satisfactory.