In order to optimize operating costs and more efficient maintenance and repair of automotive engines it is necessary to accurately forecast the technical operation parameters, which is achieved by selection of the correct method of diagnosis. The development of any method of diagnosis involves determining the most informative diagnostic parameters and ranges of their change. To compile the algorithm of in-place diagnosis according to Bayes, it is necessary to identify patterns of change in compression, cyclic flow and air flow, depending on the operating time of the tractor, which conducted statistical studies of tractors MTZ-80/82 fitted with engines D-243 in the agricultural enterprises of the Republic of Tatarstan. The research is to measure compression, power systems, air supply of tractors with different operating time. The dependence of data according to the parameters of the developments was composed, which are presented in a graph. As the intensity of the changes in diagnostic parameters of the engine speed have been chosen, the intensity of the cyclic changes in supply and changes in the intensity of the air flow. The results of development tests of D-243 engine with various kinds of faults were made up. The compression of 2.7 MPa was set for serviceable engine, 2.4 MPa for engine in good condition and 2.1 MPa for a faulty engine, the fuel injection pressure was 18.5 MPa, 16.5 MPa, 15.5 MPa, the discharging in intake manifold at a nominal speed was 2 kPa, 4 kPa and 7 kPa for the respective states of the engine. The carried experimental and theoretical studies allow us to determine not only the dependence of diagnostic parameters from compression, cyclic flow and air flow, but also to establish acceptable ranges of diagnostic parameters, which is the basis of the proposed method of in-place engine diagnostics and serves as a basis in the preparation of diagnostic matrix and establishes rules for the application of solutions.