If you’re driving a late model vehicle, it's likely equipped with a direct fuel injection system.  Older vehicles with conventional fuel injection systems deliver fuel to the engine through injectors located in the intake manifold.  Newer vehicles with direct injection systems locate the fuel injectors directly in the combustion chamber of the engine; allowing the fuel to be atomized more effectively, resulting in increased fuel economy, power, and response.  The downside to this revelation in fuel injection technology is the potential for carbon-buildup to occur.  Because fuel no longer passes over and "washes" the back of the intake valves, carbon-deposits commonly form on the valve surfaces.