Abstract: An Electronic Engine Controller (EEC) is calibratable to control the rate at which fuel is delivered to an intake of an internal combustion engine in order to achieve a bias in the air/fuel ratio combusted by the engine. The EEC operates under closed-loop control to alter the fuel delivery rate in response to an output of an oxygen sensor which detects the concentration in exhaust gas produced by the engine. The EEC achieves a predetermined bias, in the air/fuel ratio, which is in a rich or lean direction from stoichiometry, by maintaining the fuel delivery rate at a jump value upon the indication by the oxygen sensor of an air/fuel ratio in the same direction from stoichiometry as the desired bias. The fuel delivery rate is maintained at the jump value for a variable period of time, which preferably depends upon the engine speed and load. Upon expiration of the variable period of time, the fuel delivery rate is abruptly altered to a fuel delivery rate corresponding to a predetermined air/fuel ratio.