Part 4
ENGINE OBD II MONITORS
Part 4 of 4
Continued From Part 3 Part 3
Generic Misfire 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. 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 at high RPM with light load conditions, will produce symmetrical acceleration variations. Cylinder events that indicate excessive deviant accelerations of this type are considered noise. Noise-free deviant acceleration exceeding a given threshold is labeled a misfire. The number of misfires are counted over a continuous 200 revolution and 1,000 revolution period. The revolution counters are not reset if the misfire monitor is temporarily disabled such as for negative torque mode. 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 871°C (1,600°F) for Pt/Pd/Rh conventional wash coat, 899°C (1,650°F) for Pt/Pd/Rh advanced washcoat and 982°C (1,800°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 threshold is again exceeded on a subsequent driving cycle, the MIL is illuminated. If a single cylinder is indicated to be consistently misfiring in excess of the catalyst damage criteria, the fuel injector to that cylinder may be shut off for a period of time to prevent catalyst damage. Up to 2 cylinders may be disabled at the same time. This fuel shut-off feature is used only on some 6-cylinder engines and is never used on 4-cylinder engines. The misfire rate is also evaluated every 1,000 revolution period and compared to a single (type B) threshold value to indicate if the emission-threshold exceeded, which can be either a single 1,000 over-rev event from startup or 4 subsequent 1,000 over-rev events on a drive cycle after start-up. Many vehicles will set DTC P0316 if the type B threshold is exceeded during the first 1,000 revolutions after engine startup. This DTC is stored in addition to the normal P03xx DTC that indicates the misfiring cylinder. If the misfire is detected but it cannot be attributed to a specific cylinder, a P0300 is stored.
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 the crankshaft teeth must equal 360 degrees, a correction factor can be calculated for each misfire sample interval that makes all the angles between individual teeth equal. To prevent any fueling or combustion differences from affecting the correction factors, learning is done during deceleration-fuel cutout. The correction factors are learned during closed-throttle, non-braking, de-fueled 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 deceleration-fuel cutout strategy may be used when the conditions for learning are present. The corrections are typically learned in a single deceleration, but can be learned during up to 3 such decelerations. The mature correction factors are the average of a selected number of samples. A low data rate misfire system will typically learn 4 such corrections in this interval, while a high data rate system will learn 36 or 40 in the same interval (data is actually processed in the AICE chip). 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 corrections on rough road conditions which could limit misfire detection capability and to help isolate misfire diagnoses from other 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. In the event of battery disconnection or loss of keep alive memory (KAM), the correction factors are lost and must be relearned. If the software is unable to learn a profile after 2, 97 to 64 km/h (60 to 40 mph) deceleration cycles, DTC P0315 is set.
Misfire Monitor Specifications
Misfire monitor operation: DTCs P0300 to P0310 (random and specific cylinder misfire), P1336 (crankshaft/camshaft sensor range/performance), P0606 (control module processor), P0315 (crankshaft position system variation not learned), P0316 (misfire detected on startup [first 1000 revolutions]). The monitor execution is continuous, misfire rate calculated every 200 or 1,000 revolutions. The monitor does not have a specific sequence. The CKP and CMP sensors have to be operating correctly to run the monitor. The monitoring duration is the entire driving cycle (see disablement conditions below).
Typical misfire monitor entry conditions: Entry condition minimum/maximum time since engine start-up is 0 seconds, engine coolant temperature is -7°C to 121°C (20°F to 250°F), RPM range is (full-range misfire certified, with 2 revolution delay) 2 revolutions after exceeding 150 RPM below drive idle RPM to redline on tach or fuel cutoff, profile correction factors are learned in KAM are Yes, and the fuel tank level is greater than 15%
Typical misfire temporary disablement conditions: Closed throttle deceleration (negative torque, engine being driven), Fuel shut-off due to vehicle-speed limiting or engine-RPM limiting mode, and a high rate of change of torque (heavy throttle tip-in or tip out)
The profile learning operation includes DTC P0315 - unable to learn profile in 3, 97 to 64 km/h (60 to 40 mph) decelerations; monitor execution is once per KAM reset; The monitor sequence: profile must be learned before the misfire monitor is active; The CKP and CMP sensors are required to be OK; no AICE communication errors, CKP/CMP signals must be synchronized. The monitoring duration is 10 cumulative seconds in conditions (a maximum of 3, 97 to 64 km/h (60 to 40 mph) de-fueled decelerations).
Typical profile learning entry conditions: Entry conditions from minimum to maximum: Engine in deceleration-fuel cutout mode for 4 engine cycles, the brakes are not applied, the engine RPM is 1,300 to 3,700 RPM, the change is less than 600 RPM, the vehicle speed is 48 to 112 km/h (30 to 70 mph), and the learning tolerance is 1%.
Positive Crankcase Ventilation (PCV) System Monitor
The PCV monitor consists of a modified PCV system design. The PCV valve is installed into the rocker cover using a quarter-turn cam-lock design to prevent accidental disconnection. High retention force molded plastic lines are used from the PCV valve to the intake manifold. The diameter of the lines and the intake manifold entry fitting are increased so that inadvertent disconnection of the lines after a vehicle is repaired causes either an immediate engine stall or does not allow the engine to be restarted. In the event that the vehicle does not stall if the line between the intake manifold and PCV valve is inadvertently disconnected, the vehicle has a large vacuum leak that causes the vehicle to run lean at idle. This illuminates the MIL after 2 consecutive driving cycles and stores one or more of the following DTCs: Lack of HO2S sensor switches, bank 1 (P2195), Lack of HO2S sensor switches bank 2 (P2197), fuel system lean, bank 1 (P0171) or fuel system lean, bank 2 (P0174).
For additional PCV information, see Electronic Engine Control (EEC) System.
Thermostat Monitor
The thermostat monitor is designed to verify correct thermostat operation. This monitor is executed once per drive cycle and has a monitor run duration of 300-800 seconds. If a malfunction is present, DTC P0125 or P0128 is set and the malfunction indicator lamp (MIL) is illuminated.
The monitor checks the engine coolant temperature (ECT) or cylinder head temperature (CHT) sensor to warm up in a predictable manner when the engine is generating sufficient heat. A timer is initialized while the engine is at moderate load and the vehicle speed is above a calibrated limit. The target timer value is based on ambient air temperature at start-up. If the timer exceeds the target time and ECT or CHT has not warmed up to the target temperature, a malfunction is indicated. The test runs if the start-up intake air temperature from the intake air temperature (IAT) sensor is at, or below the target temperature. A 2-hour engine off soak time is also required to enable the monitor and to prevent erasing of any pending DTCs during a hot soak. This soak time feature also prevents false-passes of the monitor when the engine coolant temperature rises after the engine is turned off during a short engine off soak period.The target temperature is calibrated to -11°C (20°F) the thermostat regulating temperature. For a typical 90°C (195°F) thermostat, the target temperature would be calibrated to 79°C (175°F). Some vehicle calibrations may lower the target temperature to less than 27°C (50°F) for vehicles that do not warm-up to thermostat regulating temperatures in the 11°C (20°F) to 27°C (50°F) ambient temperature range.
1. Inputs: ECT or CHT, IAT, engine LOAD (from MAF sensor) and vehicle speed input.
a. Typical monitor entry conditions:
i. vehicle speed greater than 24 km/h (15 mph)
ii. intake air temperature at start-up is between -7°C (20°F) and target thermostat temperature
iii. engine load greater than 30%
iv. engine off (soak) time greater than 2 hours
2. Output: MIL.
Malfunction Indicator Lamp (MIL)
The MIL notifies the driver that the PCM has detected an on board diagnostic (OBD) emission-related component or system malfunction. When this occurs, an OBD DTC sets.
- The MIL is located in the instrument cluster and is labeled CHECK ENGINE, SERVICE ENGINE SOON or the international standards organization (ISO) standard engine symbol.
- The MIL is illuminated during the instrument cluster prove out for approximately 4 seconds.
- The MIL remains illuminated after instrument cluster prove out if:
- If the MIL remains on after the bulb check:
- an emission-related malfunction and DTC exists.
- the PCM does not send a control message to the instrument cluster (applications with the MIL controlled through the communication link).
- the PCM is operating in the hardware limited operation strategy (HLOS).
- The MIL remains off during the instrument cluster prove out if an indicator or instrument cluster malfunction is present.
- To turn off the MIL after a repair, a reset command from the scan tool must be sent, or 3 consecutive drive cycles must be completed without a malfunction.
- For all MIL malfunctions, GO to SECTION 3, Symptom Charts
- If the MIL flashes at a steady rate, a severe misfire condition may exist.
- If the MIL flashes erratically, the PCM can reset while cranking if the battery voltage is low.
- If the MIL flashes erratically, the PCM can reset while cranking if the battery voltage is low.
- The MIL flashes after a period of time with the key in the RUN position (engine not running) if DTC P1000 is set.
Powertrain Control Software
The first 4 bits of a DTC do no convert directly into hex digits. The conversion into different types of DTCs (P, B, C and U) is defined by SAE J2012. This standard contains DTC definitions and formats.
ISO 14229 sends 2 additional bytes of information with each DTC, a failure-type byte and a status byte.
All ISO 14229 DTCs are 4 bytes long instead of 3 or 2 bytes long. Additionally, the status byte for ISO 14229 DTCs is defined differently than the status byte for previous applications with 3 byte DTCs.
Failure -Type Byte
The failure-type byte is designed to describe the specific failure associated with the basic DTC. For example, an failure-type byte of 1C means circuit voltage out of range, 73 means actuator stuck closed. When combined with a basic component DTC, it allows one basic DTC to describe many types of failures.
For example, DTC P0110:1C-AF means intake air temperature sensor circuit voltage out of range. The base DTC P0110, means intake air temperature circuit, while the failure-type byte 1C means circuit voltage out of range. This DTC structure was designed to allow manufactures to more precisely identify different kinds of faults without always having to define new DTC numbers.
The PCM does not use failure-type bytes and always sends a failure-type byte of 00 (no sub-type information). This is because OBD II regulations require manufacturers to use 2 byte DTCs for generic scan tool communications. Additionally, the OBD II regulations require the 2 byte DTCs to be very specific, so there is no additional information that the failure-type byte could provide.
A list of failure-type bytes is defined by SAE J2012 but is not described here because the PCM does not use the failure-type byte.
Status Byte
The status byte is designed to provide additional information about the DTC, such as when the DTC failed, when the DTC was last evaluated, and if any warning indication has been requested. Each of the 8 bits in the status byte has a precise meaning that is defined in ISO 14229.
The protocol is that bit 7 is the most significant bit and is the left-most bit while bit 0 is the least significant bit and is the right-most bit.
DTC status Bit Definitions
Refer to the following status bit descriptions:
Bit 7
- 0 - The ECU is not requesting warning indicator to be active
- 1 - The ECU is requesting warning indicator to be active
Bit 6
- 0 - The DTC test completed this monitoring cycle
- 1 - The DTC test has not completed this monitoring cycle
Bit 5
- 0 - The DTC test never failed since last code clear
- 1 - The DTC test failed at least once since last code clear
Bit 4
- 0 - The DTC test completed since the last code clear
- 1 - The DTC test not completed since the last code clear
Bit 3
- 0 - The DTC is not confirmed at the time of the request
- 1 - The DTC is confirmed at the time of the request
Bit 2
- 0 - The DTC was not failed on the current or previous monitoring cycle
- 1 - The DTC failed on the current or previous monitoring cycle
Bit 1
- 0 - The DTC never failed on the current monitoring cycle
- 1 - The DTC failed on the current monitoring cycle
Bit 0
- 0 - The DTC is not failed at the time of request
- 1 - The DTC is failed at the time of request
For DTCs that illuminate the MIL, a confirmed DTC means the PCM has stored a DTC and has illuminated the MIL. If the fault has corrected itself, the MIL may no longer be illuminated but the DTC still shows a confirmed status for 40 warm up cycles at which time the DTC is erased. Bit 7 can be used to determine if the MIL is illuminated for the DTC.For DTCs that do not illuminate the MIL, a confirmed DTC means the PCM has stored a DTC. If the fault has corrected itself, the DTC still shows a confirmed status for 40 warm up cycles at which time the DTC is erased.To determine if a test has completed and passed, for example, after a repair, information can be combined from 2 bits as follows:If bit 6 is 0 (the DTC test completed this monitoring cycle), and bit 1 is 0 (the DTC never failed on the current monitoring cycle), then the DTC has been evaluated at least once this drive cycle and was a pass.If bit 6 is 0 (the DTC test completed this monitoring cycle) and bit 0 is 0 (the DTC is not failed at the time of request), then the most recent test result for that DTC was a pass.The status byte bits can be decoded as a 2-digit hexadecimal number, and can be displayed as the last 2 digits of the DTC, for example for DTC P0110:1C-AF, AF represents the status byte info.
Torque Based Electronic Throttle Control (ETC)
ETC System with a 2-Track APP Sensor Failure Mode and Effects Management:
