In this study, the performance and emissions of a 4 cylinder 2.5L light-duty diesel engine with methane fumigation in the intake air manifold is studied to simulate a dual fuel conversion kit. Because the engine control unit is optimized to work with only the diesel injection into the cylinder, the addition of methane to the intake disrupts this optimization. The energy from the diesel fuel is replaced with that from the methane by holding the engine load and speed constant as methane is added to the intake air. The pilot injection is fixed and the main injection is varied in increments over 12 crank angle degrees at these conditions to determine the timing that reduces each of the emissions while maintaining combustion performance as measured by the brake thermal efficiency. It is shown that with higher substitution the unburned hydrocarbon (UHC) emissions can increase by up to twenty times. The NOx emissions decrease for all engine conditions, up to 53%. The thermal efficiency of the engine is highest with the most advanced injection timing for high load conditions; however, the tradeoff is an increase in CO emissions by up to 23% and up to two and a half times the NOx emissions. As a result, it is observed that by altering the injection timing, optimum performance can be achieved taking into consideration all of the tradeoffs when methane is added to the intake air.