Abstract: A misfire detection system and method for onboard diagnosis of engine performance detects an engine speed as a function of a rotational angle of the crankshaft, and determines crankshaft acceleration as a function of this angle based on the detected engine speed. In this manner, the power stroke acceleration of each cylinder is determined and ranked. After excluding the greatest acceleration, the accelerations of a group of consecutively ordered cylinders including the second greatest acceleration are averaged to produce an assessment level against which the remaining lower-ranked cylinders are evaluated to detect misfires.