ENGINE MISFIRE DIAGNOSING/DETECTING METHOD

Provided is an accurate engine misfire diagnosing/detecting method that solves a problem of erroneously determining normal combustion as misfire due to disturbance such as a gap input from a tire in rough road driving. The accurate engine misfire diagnosing/detecting method includes: defining in advance two crank angles A, B at mutually different points in an engine combustion cycle; calculating, as an index value of a combustion state, an engine angular acceleration (XTM) near an expansion stroke based on a difference in elapsed times of a crank angular speed C1 at the crank angles A, B; preliminarily determining that the angular acceleration (XTM) has been lowered by engine torque becoming negative due to a misfire phenomenon and that misfire is occurring when the calculated index value exceeds a predetermined threshold value; and determining the presence of a failure due to the misfire when the temperature of an exhaust gas temperature sensor provided downstream of a catalyst disposed in an exhaust system of the engine exceeds a specified value.