Abstract: Disclosed is a method and apparatus for controlling an internal combustion engine having two inputs and one output affected by both inputs. One input is perturbed and the effect on the output is compared with a desired effect on the output. The other input is varied until the actual affect on the output is the same as the desired effect on output. In one embodiment, ignition timing is periodically varied and the effect upon engine speed is detected. The detected output variations are compared with predetermined output variations for the existing engine speed and load demand. Any error is used to correct fueling to the engine. In a further embodiment the error corrects the ignition timing directly and corrects fueling indirectly at a slower rate. Further embodiments control air/fuel ratio, engine roughness, exhaust gas recirculation, for individual cylinders or groups of cylinders or the engine as a whole, depending upon the change in engine output in response to the variation in engine input.