Misfire Diagnostics

The ECM receives 58 A/C voltage pulses every crankshaft revolution from the crankshaft position sensor to determine the position of the engine at various degrees of rotation. The ECM bases misfire diagnostics on the principle that the crankshaft velocity will increase as each cylinder fires. To detect misfire, the ECM monitors the time between the cylinders 120( firing interval, which equates to 20 crankshaft reluctor wheel notches. A double space between notches is used to identify cylinder #1. A misfire on a cylinder occurs when the time to pass 20 notches takes too long. The time is based on what the maximum time between combusting cylinders should be at a given engine speed and load.

The ECM begins misfire detection as soon as the engine is started. The ECM will simultaneously perform 200 and 1000 crankshaft revolution tests using two different internal counters. The 200 revolution tests (600 combustion events) are used to determine if a catalyst damaging misfire (based on speed and load) has occurred. The 1000 revolution tests are used to determine if an emission related misfire (1.5 times the emission level standard) has occurred. During this time, if any misfires are occurring, the Scan tool will display the number of misfires under parameter MISFIRE CURRENT CYLs. #1-6. The Scan tool will display whatever value the ECM has stored for the 1000 revolution counter. NOTE: The maximum amount of misfires the Scan tool can display for the current counter is 255.

Once every second, the current misfire counters will be moved to the MISFIRE HISTORY CYLs. #1-6. The current counters will be reset to zero once the 1000 revolution test is completed. The history counters will only be reset when a clear DTC command has been issued to the ECM.

If an emission related misfire occurs, the ECM will command the MIL On steady and DTC P0300 will set. This will usually take 30 misfires during a 1000 revolution test OR a sum of 45 or greater (not exceeding 30 per 1000 revolutions) during four consecutive 1000 revolution tests. If a catalyst damaging misfire occurs; the ECM will command the MIL to flash.

The ECM controls the MIL by sending a message over the controller area network (CAN) to the body control module (BCM), which in turn sends a message over the Class II link to the I/P cluster.

NOTE:
^ The higher the speed and load, the quicker the ECM will command the MIL to flash when a catalyst damaging misfire is occurring.
^ If a catalyst damaging misfire occurs on a specific cylinder for a certain length of time, the ECM will command that cylinder fuel injector off for the remainder of the ignition cycle regardless if the misfire condition no longer exists.

CRANKSHAFT RELEARN - In order to correctly detect misfires, the ECM must learn the variation between the 58 crankshaft notches. This notch learn variation is done automatically by the ECM for 24 ranges determined by engine speed and load.

The ECM continuously updates the notch variation whenever misfire is enabled and no misfires exist in the particular range. The ECM will take about 20-30 seconds to learn the notch variation for each speed/load range. If a speed/load range has not been learned, misfire diagnostics will still be enabled, but misfire criteria will be relaxed.