Abstract: A supplemental fuel regulator for diesel engines, and a system for adding supplemental fuel to diesel engines is provided. The fuel regulator has two chambers, one which receives fuel from the fuel tank through a fuel inlet valve, which also reduces the pressure of the fuel. The fuel at much reduced pressure is passed to a second chamber through a fuel control valve. Fuel is drawn from the second chamber into an outlet duct, directly into the airflow duct of the diesel engine. A vacuum line from the engine pulls fuel from the second chamber. As engine demand increases, the engine vacuum increases, and additional fuel is supplied by the regulator. A solenoid switch provides control over the system, which can be activated by a user when the supplemental fuel system is deemed desirable. The solenoid switch can be configured to shut off the supplemental fuel system whenever the diesel engine is shut off.