MISFIRE DIAGNOSTIC DEVICE FOR ENGINE

PROBLEM TO BE SOLVED: To respectively properly diagnose a serious misfire trouble having high possibility for an adverse influence to be exercised on an exhaust gas purifying catalyst and a slight misfire trouble having low possibility for an adverse influence to be exercised thereon. SOLUTION: This misfire diagnostic device for an engine comprises a diagnosing period calculating means 41 to measure a diagnosis period for diagnosing a misfire trouble: a number of misfire occurring times measuring means 42 to measure the number of misfire occurring times; a threshold setting means 43 to set a threshold for deciding a misfire trouble; and a misfire trouble deciding means 44 to decide the misfire trouble of an engine by comparing the number of misfire occurring times, measured during a time in which a diagnosis period forms a preset reference time, with a threshold. The reference number of misfire occurring times for deciding vapor is set according to the number of misfire occurring times measured in a preset first reference time. A vapor deciding means 45 is provided to decide it based on the reference number of misfire occurring times and measurements of the number of misfire occurring times measured in a second reference time longer than the first reference time whether vapor of fuel is generated.