The fuel injector works like the spray nozzle of a pressure washer, this vital engine component is located at the opening of the cylinder or inside the inlet manifold. Liquid fuel is injected at high pressure via a small nozzle.
There is no cleaning agent, able to clean the valves from any debris. The problem with direct injection systems is the buildup of carbon and dirt behind the intake valves and ports .
Engines start losing performance after 45000 miles and the only remedy (on most models) is usually costly as it requires removal of the cylinder head to remove the carbon and dirt buildup.
A regular (6 monthly) carbon engine clean will prevent these build ups from occurring.