An electronic engine control system for a lean burn engine includes a unit for detecting an amount Qa of intake air fed into a cylinder of the engine, a unit for detecting an engine speed Ne, a unit for calculating a basic fuel injection pulse width TPbas on the basis of the intake air amount Qa and the engine speed Ne, and a unit for determining control parameters containing any of at least an air-fuel ratio, an ignition timing, a fuel injection timing, a throttle opening and an EGR rate in accordance with an operating state of the engine in optimum. A reference TPref having the same dimension as the basic fuel injection pulse width TPbas and which is a function of an accelerator operating amount determined on the basis of the accelerator operating amount is determined as a load parameter used upon determination of the control parameters of the engine in operation with a lean air-fuel ratio. Because the accelerator opening amount is inputted to calculate the reference .sub.Tpref so that the throttle opening is controlled, the measurement accuracy of the accelerator opening and throttle opening amounts can be ensured, particularly in a vicinity of the completely closed state of the throttle where the increasing rate of intake air to throttle opening exists.