At our department we deal with continuous tuning of torsional oscillating mechanical systems during their operation using pneumatic tuners, mainly in terms of torsional vibrations magnitude, whereby we use the methods and means of technical diagnostics. One of the manners of continuous tuning realization is the application of extremal control – experimental optimization, which main advantage is that we don’t need to know the mathematical model of mechanical system. We must know only that the objective function of mechanical system has an extreme. For that reason it is very important to know this form of objective function not only at failure-free operation but at failure occurrence as well. The objective of this paper is therefore the investigation of the influence of piston compressor inner failure, namely cylinder(s) fall-out on mechanical system objective function form.