A microprocessor engine control system has been developed for a Caterpillar 3208 naturally aspirated engine to allow it to operate on natural gas using diesel pilot injection. The design criteria used for the control system included maintaining comparable peak pressure and rate of pressure rise when operating on dual-fuel. The inputs used for the control system include engine speed, load and inlet air temperature. The maximum diesel substitution level attained was approximately 85%. The conversion system consists of three main component groups, a positioning device to control the amount of diesel fuel, a natural gas control valve and a computer control module. Following completion of the system, the engine was put through a series of performance, transient and stability tests and the dual-fuel strategy was adjusted to optimize performance.