General Misfire Algorithm Processing
The acceleration that a piston undergoes during a normal firing event is directly related to the amount of torque that cylinder produces. The calculated piston/cylinder acceleration value(s) are compared to a misfire threshold that is continuously adjusted based on inferred engine torque. Deviant accelerations exceeding the threshold are conditionally labeled as misfires. A threshold multiplier is used during startup CSER to compensate the thresholds for the reduction in signal amplitude during spark retard conditions. Threshold adjustments may also be applied to compensate for torque reduction during gear shift events, and to compensate for changes in driveline coupling with torque convertor lock status.
The calculated deviant acceleration value(s) are also evaluated for noise. Normally, misfire results in a nonsymmetrical loss of cylinder acceleration. Mechanical noise, such as rough roads or crankshaft oscillations at low rpm/high load ("lugging") conditions, will produce symmetrical, positive acceleration variations. Noise limits are calculated by applying a negative multiplier to the misfire threshold. If the noise limits are exceeded, a noisy signal condition is inferred and the misfire monitor is suspended for a brief interval. Noise-free deviant acceleration exceeding a given threshold is labeled a misfire.
The number of misfires is counted over a continuous 200 revolution and 1000 revolution period. (The revolution counters are not reset if the misfire monitor is temporarily disabled such as for negative torque mode, etc.) At the end of the evaluation period, the total misfire rate and the misfire rate for each individual cylinder is computed. The misfire rate is evaluated every 200 revolution period (Type A) and compared to a threshold value obtained from an engine speed/load table. This misfire threshold is designed to prevent damage to the catalyst due to sustained excessive temperature (1650°F for Pt/Pd/Rh advanced washcoat and 1800°F for Pd-only high tech washcoat). If the misfire threshold is exceeded and the catalyst temperature model calculates a catalyst mid-bed temperature that exceeds the catalyst damage threshold, the MIL blinks at a 1 Hz rate while the misfire is present. If the misfire occurs again on a subsequent driving cycle, the MIL is illuminated.
At high engine speed and load operating conditions the Monitor continuously evaluates the misfire rate during each 200 revolution period. If a sufficient number of misfire events have been accumulated within a 200 revolution block such that the misfire threshold is already exceeded before the end of the block has been reached, the Monitor will declare a fault immediately rather than wait for the end of the block. This improves the capability of the Monitor to prevent damage to the catalyst.
If a single cylinder (i.e. > 90% of misfires attributed to one cylinder) is determined to be consistently misfiring in excess of the catalyst damage criteria, the Monitor will initiate failure mode effects management (FMEM) to prevent catalyst damage. The fuel injector to that cylinder will be shut off for a minimum of 30 seconds. Up to two cylinders may be disabled at the same time on 6 and 8 cylinder engines and one cylinder is disabled on 4 cylinder engines. Fuel control will go open loop and target lambda slightly lean (~1.05). The software may also use the throttle to limit airflow (limit boost) on GTDI engines for additional exhaust component protection. After 30 seconds, the injector is re-enabled and the system returns to normal operation. On some vehicles, the software may continue FMEM beyond 30 seconds if the engine is operating at high speed or load at the end of the 30 second period. The software will wait for a low airflow condition (~1 to 5 second tip-out) to exit from FMEM. This protects the catalyst should the misfire fault still be present when the fuel injector is turned back on. If misfire on that cylinder is again detected after 200 revs (about 5 to 10 seconds), the fuel injector will be shut off again and the process will repeat until the misfire is no longer present. Note that ignition coil primary circuit failures (see COMPREHENSIVE COMPONENT MONITOR - ENGINE information) will trigger the same type of fuel injector disablement. If a particular cylinder cannot be determined, fuel control will still go open loop and target lambda slightly lean (~1.05) to limit catalyst temperatures.
If fuel level is below 15%, the misfire monitor continues to evaluate misfire over every 200 revolution period to determine if catalyst damaging misfire is present so that the fuel shut-off FMEM can be utilized to control catalyst temperatures. If this is the case, a P0313 DTC will be set to indicate that misfire occurred at low fuel levels. The P0313 DTC is set in place of engine misfire codes (P030x) if a misfire fault is detected with low fuel level.
The misfire rate is also evaluated every 1000 revolution period and compared to a single (Type B) threshold value to indicate an emission-threshold malfunction, which can be either a single 1000 revolution exceedence from startup or four subsequent 1000 revolution exceedences on a drive cycle after start-up. Some vehicles will set a P0316 DTC if the Type B malfunction threshold is exceeded during the first 1, 000 revs after engine startup. This DTC is normally stored in addition to the normal P03xx DTC that indicates the misfiring cylinder(s). If misfire is detected but cannot be attributed to a specific cylinder, a P0300 is stored. This may occur on some vehicles at higher engine speeds, for example, above 3, 500 rpm.