producing a first signal corresponding to rotational characteristics of an engine crankshaft taken at selected times, said first signal including a measure of variability corresponding to characteristics unique to the internal combustion engine system irrespective of misfire;

converting the first signal into a frequency-domain misfire detection metric having components selected from an engine cycle frequency and harmonic orders thereof;

normalizing said misfire detection metric using a non-misfire metric corresponding to a non-misfire operating condition of the internal combustion engine system to remove said measure of variability;

detecting a misfire condition when the normalized misfire detection metric satisfies a predetermined threshold;

identifying one of (i) an individual cylinder and (ii) multiple cylinders that misfired when said misfire condition is detected as a function of a calculated phase.