The article presents problems occurring during the signals processing of the parameters of working processes of internal combustion engines in tests, while powering the measuring apparatus from the electric network of the marine power plant laboratory. Various experimental methods of separation were used to identify and then eliminate interference. On the basis of analyses carried out in the domain of time and frequency of the in-cylinders pressure signals of internal combustion engines and their wavelet decomposition, attempts were made to determine and eliminate the sources of interference. It has been shown that interference was penetrating the measuring equipment through the supply network. The developed methods of reducing interference in the laboratory conditions can be applied on the marine vessels. On the basis of the conducted tests, it can be concluded that the reduction of interference of pressure measurement signals in the engine cylinder is most effective when the computer is powered only from the battery while the measuring circuit is powered directly from the power network.