The forced level of permissible vehicle exhaust emissions requires accurate control of the air-fuel ratio. In other words, the amount of air flowing into the cylinder and the fuel quantity must be known accurately. The wall mass of deposited fuel in the manifold of an injection system has a significant effect on fuel maldistribution and the air-fuel ratio excursion. This paper outlines an identification process of a mixture preparation model of the fuel injection in a spark ignition engine.A type of a prototype combustion probe integrated into a spark plug has been developed. This in-cylinder sensor which measures the combustion luminosity is characterized by the good correlation with traditional combustion parameters, including the air-fuel ratio. This paper describes the application of in-cylinder probe in the mixture strength examination. The proposed measure method estimates the fuel mass in a cylinder (where air mass has been constant) as well as the fuel film model parameters. Basic elements of the instrumentation and data processing methods as well as general ideas of the experiment were presented.An experimental test bench has been established. This paper describes procedures for estimating the parameters of a mixture formation model. The comparison of the model approximation and experimental results remain in agreement. The paper also comprises the regression equations and summary conclusions.