Abstract: The method of cleaning a fuel injection type internal combustion engine wherein the engine has a block, a combustion chamber, fuel injectors operatively connected to a fuel tank and a fuel pump, an engine starting mechanism and a fuel supply line connecting the fuel tank and the fuel injectors with the fuel pump imposed in the fuel supply line, and a fuel return line connecting the fuel injectors and the fuel tank, wherein the fuel pump is rendered inoperative and the fuel return line is blocked. A combination of fuel and cleaning fluids is imposed under pressure into the fuel supply line. The engine is then started and operated on the supply of fuel and cleaning fluid which have passed through the fuel injectors. After the engine has been operated for a period of time, perhaps 15 minutes or so, the engine is shut off, the fuel return line is unblocked, and the fuel pump is restored to its original condition.