Misfire Detection Monitor

DESCRIPTION
The Misfire Detection Monitor detects fuel, ignition or mechanically induced misfires. Misfire is defined as the lack of proper combustion in the cylinder due to the absence of spark, poor fuel metering, or poor compression. Any combustion that does not occur within the cylinder(s) at the proper time is also a misfire. The intent is to protect the catalyst from permanent damage and to alert the customer of an emission failure or an inspection maintenance failure by illuminating the Malfunction Indicator Lamp (MIL). When a misfire is detected, special software called "Freeze Frame" data is enabled. The Freeze Frame data captures the operational state of the vehicle when a malfunction is detected from Misfire Monitor strategy.

Hardware
Misfire is not a system of components, but rather a software strategy that utilizes a variable reluctance sensor (Crankshaft Position Sensor) to provide information to the Electronic Ignition (EI) system, as well as the Misfire Detection Monitor. The Misfire strategy could also be affected by the Fuel Delivery System, Comprehensive Components, the Vacuum System, Evaporative/Purge System and base engine valve train.

Operation
The Misfire Detection Monitor uses the Crankshaft Position (CKP) signal to determine whether a specific cylinder has misfired. The Powertrain Control Module (PCM) timestamps each PIP signal from the Ignition Control Module (ICM) and calculates the time between their edges. Acceleration due to each cylinder firing can be calculated yielding an indication of power for each cylinder event. The acceleration values are compared, subtracting the median value from each acceleration value. Power loss greater than a given threshold is labeled a misfire per cylinder.

DTCs and MIL Operation
When a misfire that would cause catalyst damage is detected, the MIL will turn On or flash and a Diagnostic Trouble Code (DTC) is stored. If the misfire is the type that will cause an emission failure or an inspection maintenance failure, the MIL will remain On continuously and a DTC will be stored after two malfunctions on separate drive cycles. DTCs will be erased after 40 warm-up cycles if the fault does not recur and the MIL is turned Off. The MIL is turned Off if the fault does not recur during three sequential drive cycles with conditions similar to those when the malfunction was first detected.