Misfire Monitor
The input to the Misfire Monitor is the signal from the crankshaft position sensor and timing wheel. The signal is acquired and processed by the PCM and provided to the Misfire Monitor as individual tooth period measurements. The Monitor uses the tooth period measurements to calculate crankshaft acceleration signals for misfire detection. All misfire processing is performed in software (separate chips are no longer used except for some vehicle lines that may still be using older style PCMs.)
Two different technologies are used for misfire monitoring. They are the Low Data Rate (LDR), and High Data Rate (HDR) systems. The LDR system is capable of meeting the FTP monitoring requirements on most engines and is capable of meeting "full-range" misfire monitoring requirements on 3 and 4-cylinder engines. It is also used on 6 cylinder engines with rear mounted crank sensors. The HDR system is capable of meeting "full-range" misfire monitoring requirements on 8 and 10 cylinder engines. All software allows for detection of any misfires that occur 6 engine revolutions after initially cranking the engine. This meets the OBD-II requirement to identify misfires within 2 engine revolutions after exceeding the warm drive, idle rpm.
The Monitor includes a diagnostic check on the crank sensor input. The Monitor checks the number of tooth period measurements received on each cylinder event. A P1336 will be set if the Monitor receives an invalid number of tooth period measurements. A P1336 points to noise present on the crank sensor input or a lack of synchronization between the cam and crank sensors.
Low Data Rate System
The LDR Misfire Monitor uses a low-data-rate crankshaft position signal, (i.e. one time measurement signal for each cylinder event). The PCM calculates crankshaft rotational velocity for each cylinder from this crankshaft position signal. The acceleration for each cylinder can then be calculated using successive velocity values. The changes in overall engine rpm are removed by subtracting the median engine acceleration over a complete engine cycle. The crankshaft acceleration is then processed by three algorithms. The first algorithm, called pattern cancellation, is optimized for detection of sporadic patterns of misfire. The algorithm learns the normal pattern of cylinder accelerations from the mostly good firing events and is then able to accurately detect deviations from that pattern. The second algorithm, called pattern cancellation by opposing engine revolution or "pc-rev", is optimized for single cylinder patterns. The algorithm compares the acceleration of a cylinder to its opposite cylinder on the opposing engine revolution. The algorithm learns the normal patterns that repeat every engine revolution and is then able to accurately detect deviations between the paired cylinders. The third algorithm is a non-filtered acceleration signal that is a general purpose signal for all patterns including multi-cylinder patterns. The resulting deviant cylinder acceleration values are used in evaluating misfire in the "General Misfire Algorithm Processing" section below.
High Data Rate System
The High Data Rate (HDR) Misfire Monitor uses a high data rate crankshaft position signal, (i.e. one time measurement signal per each 2 teeth for a total of 36 measurements for one engine cycle on a 36-1 tooth wheel). This high-resolution signal is processed with a digital low pass filter. The low pass filter filters the high-resolution crankshaft velocity signal to remove some of the crankshaft torsional vibrations that degrade signal to noise. Two low pass filters are used to enhance detection capability - a "base" filter and a more aggressive filter to enhance single-cylinder capability at higher rpm. This significantly improves detection capability for continuous misfires on single cylinders up to redline. The high-resolution acceleration can then be calculated using successive velocity values. The changes in overall engine rpm are removed by subtracting the median engine acceleration over a complete engine cycle. The crankshaft acceleration is then processed by three algorithms similar to the LDR system. The final stage is to decimate the high resolution signals by selecting the peak acceleration values from within a window location for each cylinder. The resulting deviant cylinder acceleration values are used in evaluating misfire in the "General Misfire Algorithm Processing" section below.
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 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 CCM section) will trigger the same type of fuel injector disablement.
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.
Rough Road Detection
The Misfire Monitor includes a Rough Road Detection (RRD) system to eliminate false misfire indications due to rough road conditions. The RRD system uses data from ABS wheel speed sensors for estimating the severity of rough road conditions. This is a more direct measurement of rough road over other methods which are based on driveline feedback via crankshaft velocity measurements. It improves accuracy over these other methods since it eliminates interactions with actual misfire.
In the event of an RRD system failure, the RRD output will be ignored and the Misfire Monitor will remain active. An RRD system failure could be caused by a failure in any of the input signals to the algorithm. This includes the ABS wheel speed sensors, Brake Pedal sensor, or CAN bus hardware failures. Specific DTCs will indicate the source of these component failures.
A redundant check is also performed on the RRD system to verify it is not stuck high due to other unforeseen causes. If the RRD system indicates rough road during low vehicle speed conditions where it is not expected, the RRD output will be ignored and the Misfire Monitor will remain active.
Profile Correction
"Profile correction" software is used to learn and correct for mechanical inaccuracies in the crankshaft position wheel tooth spacing. Since the sum of all the angles between crankshaft teeth must equal 360°, a correction factor can be calculated for each misfire sample interval that makes all the angles between individual teeth equal. The LDR misfire system learns one profile correction factor per cylinder (ex. 4 correction factors for a 4 cylinder engine), while the HDR system learns 36, 40 or 60 correction factors depending on the number of crankshaft wheel teeth (ex. 35 for some V6/V8 engines, 39 for V10 engines, 58 for some I4/V6 engines).
The corrections are calculated from several engine cycles of misfire sample interval data. The correction factors are the average of a selected number of samples. In order to assure the accuracy of these corrections, a tolerance is placed on the incoming values such that an individual correction factor must be repeatable within the tolerance during learning. This is to reduce the possibility of learning bad corrections due to crankshaft velocity disturbances.
Since inaccuracies in the wheel tooth spacing can produce a false indication of misfire, the misfire monitor is not active until the corrections are learned. Two methods of learning profile correction are used:
- Neutral Profile Correction and Non Volatile Memory
- Customer Drive Cycle for Profile Correction (60-40 MPH Deceleration)
Neutral Profile Correction and Non-Volatile Memory
Neutral profile learning is used at End of Line to learn profile correction via a series of one or more neutral engine rpm throttle snaps. This allows the Misfire Monitor to be activated at the Assembly Plant. A Test Tool command is required to enable this method of learning, so this method will only be performed by a Plant or Service technician. Learning profile correction factors at high-speed (3, 000 rpm) neutral conditions versus during 60-40 mph decels optimizes correction factors for higher rpms where they are most needed and eliminates driveline/transmission and road noise effects. This improves signal to noise characteristics which means improved detection capability.
The profile correction factors learned at the Assembly Plant are stored into non-volatile memory. This eliminates the need for specific customer drive cycles. However, misfire profiles may need to be relearned in the Service Bay using a service procedure if major engine work is done or the PCM is replaced. (Re-learning is not required for a reflash.)
On selected vehicles, the neutral profile correction strategy is the only method used for profile correction learning. In the event of a loss of non-volatile memory contents (new PCM installed), the correction factors are lost and must be relearned. DTC P0315 is set until the misfire profile is relearned using a scan tool procedure.
The neutral profile correction strategy is available on most gasoline engine vehicles. It is not available on HEV and diesel engine vehicles.
Customer Drive Cycle for Profile Correction (60-40 MPH Deceleration)
This method was the traditional method for profile correction learning until the introduction of Neutral Profile Correction. It is now only used as a backup method
To prevent any fueling or combustion differences from affecting the correction factors, learning is done during deceleration fuel shut off (DFSO). This can be done during closed throttle, non-braking, defueled decelerations in the 97 to 64 km/h (60 to 40 MPH) range after exceeding 97 km/h (60 MPH) (likely to correspond to a freeway exit condition). In order to minimize the learning time for the correction factors, a more aggressive DFSO strategy may be used when the conditions for learning are present. The corrections are typically learned in a single 97 to 64 km/h (60 to 40 MPH) deceleration, but may take up to 3 such decelerations or a higher number of shorter decelerations. If the software is unable to learn a profile after three, 97 to 64 km/h (60 to 40 MPH) deceleration cycles, DTC P0315 is set.
| DTCs | P0300 to P0310 (general and specific cylinder misfire) P1336 (noisy crank sensor, no cam/crank synchronization) P0315 (unable to learn profile) P0316 (misfire during first 1, 000 revs after start-up) P0313 (misfire detected with low fuel level) |
| Monitor execution | Continuous, misfire rate calculated every 200 or 1000 revs |
| Monitor Sequence | None |
| Sensors OK | CKP, CMP, MAF, ECT/CHT |
| Monitoring Duration | Entire driving cycle (see disablement conditions below) |
| Entry Condition | Minimum | Maximum |
| Time since engine start-up | 0 seconds | 0 seconds |
| Engine Coolant Temperature | 20 °F | 250 °F |
| RPM Range (Full-Range Misfire certified, with 2 rev delay) | 2 revs after exceeding 150 rpm below "drive" idle rpm | redline on tach or fuel cutoff |
| Profile correction factors learned in NVRAM | Yes | |
| Fuel tank level | 15% |
| Temporary disablement conditions: |
| Closed throttle decel (negative torque, engine being driven) > -100 ft lbs |
| Fuel shut-off due to vehicle-speed limiting or engine-rpm limiting mode |
| High rate of change of torque (heavy throttle tip-in or tip out) > -450 deg/sec or 250 deg/sec ; > -200 ft lbs/sec or > 250 ft lbs/sec |
| Rough Road conditions present |
| Type A (catalyst damaging misfire rate): misfire rate is an rpm/load table ranging from 40% at idle to 4% at high rpm and loads |
| Type B (emission threshold rate): 0.9% to 1.5% |
| Monitor ID | Test ID | Description | |
| A1 | $80 | Total engine misfire and catalyst damage misfire rate (updated every 200 revolutions) (P030x) | percent |
| A1 | $81 | Total engine misfire and emission threshold misfire rate (updated every 1, 000 revolutions) (P030x) | percent |
| A1 | $82 | Highest catalyst-damage misfire and catalyst damage threshold misfire rate (updated when DTC set or clears) (P030x) | percent |
| A1 | $83 | Highest emission-threshold misfire and emission threshold misfire rate (updated when DTC set or clears) (P030x) | percent |
| A1 | $84 | Inferred catalyst mid-bed temperature (P030x) | °C |
| A2 - AD | $0B | EWMA misfire counts for last 10 driving cycles (P030x) | events |
| A2 - AD | $0C | Misfire counts for last/current driving cycle (P030x) | events |
| A2 - AD | $80 | Cylinder X misfire rate and catalyst damage misfire rate (updated every 200 revolutions) (P030x) | percent |
| A2 - AD | $81 | Cylinder X misfire rate and emission threshold misfire rate (updated every 1, 000 revolutions) (P030x) | percent |
The profile learning operation includes DTC P0315 if profile correction factors are not learned. On selected vehicles, this code is set immediately after a new PCM is installed until the scan tool procedure for Neutral Profile Correction is completed. On all other vehicles, this code is set if profile learning does not complete during the Customer Drive Cycle for Profile Correction.
| DTCs | P0315 - unable to learn profile in three 60 to 40 mph decels |
| Monitor Execution | Once per profile learning sequence. |
| Monitor Sequence | Profile must be learned before misfire monitor is active. |
| Sensors OK | CKP, CMP, CKP/CMP in synch |
| Monitoring Duration | 10 cumulative seconds in conditions (a maximum of three 60-40 mph defueled decels) |
| Entry Condition | Minimum | Maximum |
| Engine in decel-fuel cutout mode for 4 engine cycles | ||
| Brakes applied (Brake On/Off Switch) | No | No |
| Engine RPM | 1300 rpm | 3700 rpm |
| Change in RPM | 600 rpm/background loop | |
| Vehicle Speed | 30 mph | 75 mph |
| Learning tolerance | 1% |
| Entry Condition | Minimum | Maximum |
| Engine in decel-fuel cutout mode for 4 engine cycles | ||
| Park/Neutral gear | ||
| Engine RPM | 2000 rpm | 3000 rpm |
| Learning tolerance | 1% |