Specific/Random Cylinder Misfire Detection

The PCM receives A/C voltage pulses from the crankshaft position sensor to determine the position of the engine at various degrees of rotation. The PCM monitors the acceleration and deceleration rate of the crankshaft when each cylinder is in its firing event. The PCM bases misfire diagnostics on the principle that the crankshaft velocity will increase as each cylinder contributes its power input. A misfire on a cylinder occurs when the acceleration rate of the crankshaft falls below what the minimum acceleration rate should be on a normal firing cylinder.

The PCM begins monitoring the engine for misfires once misfire enable criteria, which can be found under DTC P0300, has been met. The PCM will perform 200 revolution segment tests. During this time, if any misfires are occurring, the Scan tool will display the number of misfires under parameter MISFIRE CURRENT CYLs. #1-4. Once a 200 revolution test is completed, the current misfire counters will be moved to the MISFIRE HISTORY CYLs. #1-4 and the current counters will be reset to 0. The history counters will be reset to 0 when the ignition is cycled.

There are two types of misfires: an emission related misfire and a catalyst damaging misfire. An emission related misfire indicates that within a 200 revolution test (400 combustion events), 2-19% (8-76 combustion events) of the combustion events failed. If the PCM fails 10 out of 16, 200 revolution tests, DTC P0300 will set. If after the second ignition cycle, the PCM fails another 10 out of 16, 200 revolution tests, it will send a message to the I/P cluster over the Class II link to turn on the Malfunction Indicator Lamp (MIL).

A catalyst damaging misfire indicates that within a 200 revolution test (400 combustion events), 20% or greater (77 combustion events or greater) of the combustion events failed. If the PCM fails four out of 16, 200 revolution tests, DTC P0300 will set. The PCM will send a message to the I/P cluster over the Class II link to flash the MIL until it no longer detects a catalyst damaging misfire.

The MIL will be turned Off after three passed trips under the same criteria that set DTC P0300 or 80 passed trips if the criteria could not be met. DTC P0300 will be the only DTC stored in memory for a misfire condition regardless if the misfire was specific or random. The misfire history counters under DTC P0300 Freeze/Frame Failure Record will have to be reviewed as to which cylinder(s) was misfiring.