More important even than the traditional tune-up is cleaning the air induction and fuel systems. Oil and other particulates, from the PCV system as well as the surrounding air, are drawn into the intake manifold with the incoming air. These impurities are then deposited on the walls of the intake runners and on the backs of the intake valves, impeding the smooth flow of air & fuel into the engine. This "soft carbon" build-up can cause driveability, emission and fuel economy problems if not cleaned regularly.
The method and product used in cleaning is as important as the cleaning itself, as an ineffective job will not produce the desired result. A thorough job will include some method of cleaning throughout the intake system, as well as running cleaner directly through the fuel injectors. Unlike tune-up and spark plug replacement intervals, which vary greatly from vehicle to vehicle, a thorough fuel system service should be performed every 2 years or 40 -50,000 kilometres, regardless of age or make, with few exceptions. Some engines may be more susceptible to the formation of carbon deposits than others and will require more frequent servicing.