A method for adjusting a fuel control system of an internal combustion engine to account for variations in the energy content of a fuel delivered to the engine involves monitoring the O.sub.2 level of the engine exhaust gases. The fuel may be a gaseous fuel for example. The fuel control system utilizes a stored gaseous fuel energy content value (E.sub.G) to determine the necessary duration of a gaseous fuel admission valve control signal. The method includes sensing an actual engine exhaust gas O.sub.2 level and comparing the actual exhaust gas O.sub.2 level with a desired exhaust gas O.sub.2 level. The stored gaseous fuel energy content value (E.sub.G) is adjusted based upon the comparison. The desired exhaust gas O.sub.2 level may be determined as a function of one or more engine operating parameters which indicate a desired or expected air/fuel ratio, the level of O.sub.2 in the exhaust gases being an indicator of the air/fuel ratio.