Misfire Diagnostics

The PCM receives an AC voltage from the crankshaft position sensor, which relays the position of the engine at various degrees of crankshaft 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.

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 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 if vehicle speed is over 48 km/h (30 mph) with MAP greater than 70 kPa when in the catalyst damaging misfire event.

The PCM begins monitoring the engine in 200 revolution segments for misfires once misfire enable criteria, which can be found under DTC P0300, has been met. 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 cleared. Once a certain number of misfires based on engine speed and load have occurred, DTC P0300 will set. The higher the engine speed and load, the quicker a misfire DTC will set. Even if a specific cylinder was initially misfiring, DTC P0300 will always set first. If that specific cylinder continues to misfire, a specific cylinder misfire will also set. The number of misfires that must occur to set the specific cylinder misfire DTC is also based on engine speed and load.

The PCM only stores misfire history counts, which are displayed on the Scan tool as MISFIRE HISTORY CYLs. #1-4, when the MIL is commanded On. The Scan tool DATA DISPLAY is used to review the misfire history counters to determine which cylinder(s) were misfiring. The misfire history counters will NOT reset to zero until DTCs are cleared in the PCM.

If a misfire DTC is set and the MIL is On, the PCM will send a message to the I/P cluster over the Class II link to turn the MIL Off after three consecutive passed drive cycles under the same misfire criteria that the DTC set under. The PCM will also turn the MIL Off after 80 passed drive cycles if the criteria could not be met.

CRANKSHAFT RELEARN - In order to correctly detect misfires, the PCM must learn the variation between the 7 crankshaft notches. This procedure is performed only once unless a crankshaft or PCM is replaced. The following is a description of what occurs when this procedure is performed with the Scan tool.

When the procedure is initiated, the Scan tool will send a message to the PCM to reset the crankshaft learned adaptive to NO. When the engine coolant temperature is above 70°C (158°F), A/C request is Off and the transaxle in Park or Neutral, the PCM will be prepared to learn the crankshaft notch variation. The accelerator will now have to be manually depressed to fuel cut-off and released. The PCM will cut fuel off near 4000 RPM and will allow the engine to decelerate. The PCM will learn the crankshaft as the engine decelerates. An engine misfire will NOT affect the learning process since the PCM is turning Off the fuel injectors.