Misfire Monitor
BACKGROUNDMisfire is defined by the California Air Resources Board as the lack of combustion in a cylinder due to absence of spark, poor fueling, compression, or other cause. As a result, the air/fuel mixture will not burn, and during the exhaust stroke, it enters the exhaust system. The raw fuel and excess oxygen adversely affect the fuel system's feedback mechanism (the oxygen sensor) and can cause permanent damage to the catalytic converter.
OPERATION
The OBD II misfire monitor uses information provided by the crank position sensor to determine engine RPM and detect slight variations due to engine misfire. Crankshaft RPM is calculated between the 69° and 90° falling edges of the crankshaft position sensor signal.
The threshold for determining what amount of RPM change indicates misfire varies with engine speed and load. This is required because, as engine speed increases or load decreases, the overall effect of a single cylinder misfire diminishes due to the momentum of the crankshaft.
The misfire monitor contains an adaptive feature that can take into account component wear, sensor fatigue, and machining tolerances. The PCM notes RPM variance between cylinders during normal vehicle operation, and then uses these figures as a foundation for calculating the threshold at which RPM variance is considered to indicate misfire.
NOTE: The Powertrain Control Module (PCM) requires normal vehicle operation to update its memory. Because the PCM has no reference for "normal operation", the PCM considers a misfire condition to be normal of the vehicle was misfiring when the PCM was replaced. The same situation can occur any time battery power to a PCM is lost and a vehicle is misfiring when the controller is reconnected.
The misfire monitor triggers operation of the Malfunction Indicator Lamp (MIL) under two different operating situations:
1,000 Rev Misfire
Misfire is continuously monitored (in 200 revolution segments) once the enabling conditions have been met. If the monitor detects misfire in more than 2% (automatic transmission) or 1.6% (manual transmission) of the engine cycles in a 1,000 revolution period, a temporary fault is set. Freeze-frame data of the operating conditions during the last 200 revolutions of that 1,000 revolution period is stored. As with other monitors, failure of the misfire monitor on the next trip matures the code, and the MIL illuminates and a DTC is stored. It is important to note that two trips are required for the code to mature. Continued misfiring during the initial trip does not illuminate the MIL unless it exceeds misfire in more than 15% of cylinder firing opportunities.
200 Rev Misfire
If the monitor detects misfire in more than 15% of cylinder firing opportunities during any 200-revolution monitored segment, the MIL immediately begins flashing, a DTC is stored, and the freeze frame is stored with information regarding the conditions during which the misfire occurred. This indicates that engine misfire has reached the point at which damage to the catalytic converter is likely to occur. The vehicle defaults to open loop operation to prevent the adaptive fuel controls from dumping additional fuel into the cylinders and accelerating damage to the catalyst. Driving the vehicle while the MIL is flashing is not recommended.
Once out of the operating condition where the 15% misfire is occurring, the MIL will stop flashing but remain illuminated. The vehicle may be driven, but should be serviced immediately.
Because some misfire is a common occurrence in a number of driving conditions, MIL illumination due to misfire may occur when no component has failed. Try to identify conditions that could trigger the misfire monitor that are not related to component failure. Examples of these include:
^ Damp ignition system components (high humidity)
^ Low fuel/running out of gas (monitor triggers as engine sputters)
^ "Lugging" the engine (manual transmission)
^ Pulling heavy loads
^ Low quality gasoline
In addition, the following situations could mistakenly trigger the MIL:
^ Large potholes
^ Extended rough road operation
As advancements in computer technology occur, Chrysler's engineering efforts continually decrease the chances of setting a false misfire fault.
ENABLING CONDITIONS
The following conditions must be met before the misfire monitor will run:
^ MAP voltage is less than 1.60
^ RPM is between 2,200 and 2,800
^ Engine coolant temperature is greater than 176° F
^ The engine has made a start-to-run transfer
^ Vehicle speed less than 3 MPH
PENDING
The misfire monitor does not run if the MIL is illuminated due to one of the following:
^ The vehicle is in the limp-in mode due to Manifold Absolute Pressure (MAP), Throttle Position Sensor (TPS), camshaft or crankshaft position sensors, or engine temperature.
^ A speed sensor Diagnostic Trouble Code (DTC)
CONFLICT
The misfire monitor does not run if any of the following are present:
^ One trip fuel system rich maturing code
^ One trip fuel system lean maturing code
^ One trip purge monitor maturing code
^ One trip EGR monitor maturing code
SUSPEND
There are no suspend conditions for the misfire monitor.
Because false misfire readings could be a customer concern, there are several conditions under which the misfire monitor is disabled. These include:
^ Low fuel level
^ Periods during which MAP voltages change quickly
^ Severe engine deceleration
^ Throttle toggles between open and closed
^ Engine cranking (below 598 RPM)
^ RPM greater than 3,000 (automatic transmission), or greater than 3,500 RPM (manual transmission)
^ Full lean or deceleration fuel shut-off
^ Cold starts (engine coolant below 100° F)
If a misfire is detected and a DTC stored, the message on the diagnostic scan tool screen appears as follows:
Multiple Cylinder Misfire
Flash Code - 43 J 2012 Code - P 0300
Cylinder #1 Misfire
Flash Code - 43 J 2012 Code - P 0301
Cylinder #2 Misfire
Flash Code - 43 J 2012 Code - P 0302
Cylinder #3 Misfire
Flash Code - 43 J 2012 Code - P 0303
Cylinder #4 Misfire
Flash Code - 43 J 2012 Code - P 0304
The MIL goes out if the misfire condition does not recur during three consecutive trips where the vehicle is operated within 375 RPM and 10% of the load condition is stored in the freeze frame. The DTC is erased from memory following the successful completion of 40 warm-up cycles.
A misfire condition could possibly be caused by problems with any of the following components:
^ Spark plugs or wires
^ Coil
^ Crank position sensor
^ Timing belt
^ Cam or crank sprockets
^ Piston rings (worn)
^ Valves (worn)
^ Head gasket
^ Head (cracked)
^ Fuel lines and filter
^ Fuel rail
^ Fuel pump module
^ Fuel pressure regulator
^ Injectors
^ Exhaust pipes and muffler
^ Catalytic convertor
^ Fuel pump relay
^ Wiring harness and connectors
^ Crankshaft