A precise determination of the crankshaft angular position, at which the self fuel ignition occurs in a diesel engine, enables a credible diagnosis of the technical condition of the engine working space as well as the fuel feed system. An observation of the Langmuir probe signal provides entirely new possibilities for engine diagnostics. The probe is introduced into the working space of a cylinder through its indicator valve. This paper presents preliminary results of diagnostic tests performed on a D10 type Farymann Diesel engine. The main aim of the investigations was to confirm the diesel engine control susceptibility to the applied, original measuring method that enables a precise determination of the crank-shaft angle, under which the fuel self-ignition occurs. In order to verify the diagnostic results, simultaneous measurements have been conducted of the cylinder pressure as well as vibrations (measured on the cylinder head cover or its mounting bolts) generated by the engine fuel injection system and the valve timing system. A satisfactory qualitative and quantitative agreement of the recorded control parameters has been obtained using a simplified comparative analysis. The results showed that further upgrade of the diagnostic method as well as the computer software is necessary in order to synchronize all the monitored variables and enable a comparative analysis in relation to the angular crankshaft position.