Methods and systems are described for monitoring air/fuel imbalance in cylinders of an engine. In one example method, air/fuel ratio of a cylinder is modulated to produce a series of rich, lean, and stoichiometric conditions in the cylinder and corresponding crank accelerations are measured to calculate a peak function indicating an air/fuel ratio in the cylinder. The peak function is calculated over a plurality of modulations to provide a more reliable computation of the air/fuel ratio of the cylinder and its deviation from a pre-determined air/fuel ratio.