Abstract: The diagnostic of the class described has a tank for holding an auxiliary fuel supply. A fuel pump is mounted in the tank. The fuel pump is provided with a fuel inlet and a fuel outlet. A conduit leads from the fuel outlet of the fuel pump to the motor fuel lines. In this way operation of the fuel pump forces fuel into the motor fuel lines through the conduit. An adjustable valve is connected to the conduit for adjusting the fuel flow into the motor. A fuel flow meter is connected to the adjustable valve so that the valve can be adjusted until the fuel flow meter indicates that the fuel flow into the motor using the diagnostic tool is the same as the fuel flow when the motor is operating without the diagnostic tool.