a method and apparatus are provided for reliably detecting misfires in an internal combustion engine during actual driving conditions by monitoring fluctuations in engine speed between consecutive firing intervals . an average acceleration over a series of firing intervals is removed from an individual acceleration measurement corresponding to a particular firing interval to give an acceleration deviation which is then normalized to an expected torque to provide a power loss measurement . the power loss measurement indicates the occurrence of a misfire with a high signal - to - noise ratio . the information derived from misfire detection can be used to prevent destruction of a catalytic converter by unburned fuel mixtures and can be used to improve vehicle driveability by correcting for or indicating needed service for the vehicle . the misfire detection is implemented using existing sensors and processors and is adapted to be done on - board a vehicle in real time .