The paper reports on both the processes of harmful deposits formation in spark ignition engines and their threats related to engine operation. The connection of a dynamic development of piston engines and fuel injection systems with increasing problems of deposits formation on various engine parts has been discussed at length. The effect of fuel composition, engine design and its operating parameters on the formation of a variety of engine deposits has been described. Possible causes of deposits formation have also been indicated. The paper confirms great importance of modern deposit control detergents and state-of-the-art technology of manufacturing of injection systems components in counteracting this detrimental phenomenon.