A comprehensive program encompassing laboratory engine, laboratory vehicle, and taxi fleet tests was undertaken to evaluate fuel variables and induction system deposits in modern spark-ignition engines. The widespread fuel injector deposit problem was the primary focus.Injector and vehicle design, driving cycle, and fuel composition were found to be major contributing factors to injector deposition. Commercial gasoline additives capable of preventing and removing injector deposits were also identified. Some of these additives control intake valve and port deposits as well, while others actually contribute to valve and port deposits.Like injector deposits, intake valve and port deposits are shown to have adverse effects on vehicle performance. Associated functional problems may be even more apparent in high-swirl, fast burn engines.