Abstract: The present invention is a method and apparatus to clean the intake system of an internal combustion engine including the intake valves. An air metering block is connected to the intake manifold of the engine by means of an adaptor tube. The only air that can get into the intake manifold of the engine is through inlet ports in the air metering block. An air adjusting screw selectively uncovers the inlet ports to vary the amount of air which can be introduced into the intake manifold. An electrically operated injector is fitted in the air metering block to inject solvent directly into the intake manifold. An adjustable injector driver varies the duty cycle or the on time of the injector to control the amount of flow of solvent into the intake manifold. A pumping unit is provided to pump solvent from a holding tank, through the injector, into the intake manifold.