Part 3
ENGINE OBD II MONITORS
Part 3
Continued From Part 2 Part 2
Fuel System Monitor
The fuel system monitor is an on-board strategy designed to monitor the fuel control system. The fuel control system uses fuel trim tables stored in the PCM keep alive memory (KAM) to compensate for the variability that occurs in fuel system components due to normal wear and aging. Fuel trim tables are based on air mass. During closed-loop fuel control, the fuel trim strategy learns the corrections needed to correct a biased rich or lean fuel system. The correction is stored in the fuel trim tables. The fuel trim has 2 means of adapting: long term fuel trim and a short term fuel trim. See Engine Control (EC) System. Long term fuel trim relies on the fuel trim tables and short term fuel trim refers to the desired air/fuel ratio parameter called LAMBSE. LAMBSE is calculated by the PCM from the heated oxygen sensor (HO2S) inputs and helps maintain a 14.7:1 air/fuel ratio during closed-loop operation. Short term fuel trim and long term fuel trim work together. If the HO2S indicates the engine is running rich, the PCM corrects the rich condition by moving the short term fuel trim into the negative range, less fuel to correct for a rich combustion. If after a certain amount of time the short term fuel trim is still compensating for a rich condition, the PCM learns this and moves the long term fuel trim into the negative range to compensate and allow the short term fuel trim to return to a value near 0%. Inputs from the cylinder head temperature (CHT), intake air temperature (IAT), mass air flow (MAF) sensors are required to activate the fuel trim system, which in turn activates the fuel system monitor. Once activated, the fuel system monitor looks for the fuel trim tables to reach the adaptive clip (adaptive limit) and LAMBSE to exceed a calibrated limit. The fuel system monitor stores the appropriate DTC when a malfunction is detected as described below.
1. The HO2S detects the presence of oxygen in the exhaust and provides the PCM with feedback indicating air/fuel ratio.
2. A correction factor is added to the fuel injector pulse width calculation and the mass air flow calculation, according to the long and short term fuel trims as needed to compensate for variations in the fuel system.
3. When deviation in the LAMBSE parameter increases, air/fuel control suffers and emissions increase. When LAMBSE exceeds a calibrated limit and the fuel trim table has clipped, the fuel system monitor sets a DTC as follows:
The DTCs associated with the monitor detecting a lean shift in fuel system operation are P0171 (Bank 1) and P0174 (Bank 2).
The DTCs associated with the monitor detecting a rich shift in fuel system operation are P0172 (Bank 1) and P0175 (Bank 2).
4. The MIL is activated after a malfunction is detected on 2 consecutive drive cycles.Typical fuel system monitor entry conditions:
- RPM range greater than idle
- Air mass range greater than 5.67 g/sec (0.75 lb/min)
- Purge duty cycle of 0%Typical fuel monitor thresholds:
- Lean Condition Malfunction: LONGFT greater than 25%, SHRTFT greater than 5%
- Rich Condition Malfunction: LONGFT less than 25%, SHRTFT less than 10%
Heated Oxygen Sensor (HO2S) Monitor
The HO2S monitor is an on-board strategy designed to monitor the HO2S sensors for malfunctions or deterioration which can affect emissions. The fuel control or stream 1 HO2S are checked for correct output voltage and response rate. Response rate is the time it takes to switch from lean to rich or rich to lean. Stream 2 HO2S sensors are used for catalyst monitoring, and stream 3 HO2S sensors used for fore-aft oxygen sensor (FAOS) control are also monitored for correct output voltage. Input is required from the camshaft position (CMP), crankshaft position (CKP), cylinder head temperature (CHT), fuel rail pressure (FRP), fuel tank pressure (FTP), intake air temperature (IAT), mass air flow (MAF), manifold absolute pressure (MAP), and throttle position (TP) sensors and the vehicle speed sensor (VSS) to activate the HO2S monitor. The fuel system monitor and misfire detection monitor must also have completed successfully before the HO2S monitor is enabled.
1. The HO2S senses the oxygen content in the exhaust flow and outputs a voltage between 0 and 1.0 volt. Lean of stoichiometric, air/fuel ratio of approximately 14.7:1, the HO2S generates a voltage between 0 and 0.45 volt. Rich of stoichiometric, the HO2S generates a voltage between 0.45 and 1.0 volt. The HO2S monitor evaluates the stream 1 (fuel control), stream 2 (catalyst monitor), and stream 3 (FAOS control) HO2S for correct function.
2. The time between HO2S switches is monitored after vehicle startup and during closed loop fuel conditions. Excessive time between switches or no switches since startup indicates a malfunction. Since lack of switching malfunctions can be caused by HO2S sensor malfunctions or by shifts in the fuel system, DTCs are stored that provide additional information for the lack of switching malfunction. Different DTCs indicate whether the sensor always indicates lean/disconnected (P2195 or P2197), or always indicates rich (P2196 or P2198). The HO2S signal is also monitored for high-voltage, in excess of 1.1 volts and stores a unique DTC (P0132 or P0152). An over-voltage condition is caused by a HO2S heater or battery power short to the HO2S signal line.
A functional test of the rear HO2S (stream 2 or stream 3) sensors is done during normal vehicle operation. The peak rich and lean voltages are continuously monitored. Voltages that exceed the calibrated rich and lean thresholds indicate a functional sensor. If the voltages have not exceeded the thresholds after a long period of vehicle operation, the air/fuel ratio may be forced rich or lean in an attempt to get the rear sensor to switch. This situation normally occurs only with a green, less than 804.7 km (500 mi), catalyst. If the sensor does not exceed the rich and lean peak thresholds, a malfunction is indicated. Also, a rear HO2S response test is done during a deceleration fuel shut-off (DFSO) event. Carrying out the HO2S response test during a DFSO event helps to isolate a sensor concern from a catalyst concern. The response test monitors how quickly the sensor switches from a rich to lean voltage. It also monitors if there is a delay in the response to the rich or lean condition. If the sensor responds very slowly to the rich to lean voltage switch or is never greater than a rich voltage threshold or less than a lean voltage threshold, a concern is indicated.
3. The MIL is activated after a concern is detected on 2 consecutive drive cycles.
4. The MIL is activated after a malfunction is detected on 2 consecutive drive cycles.The HO2S monitor DTCs can be categorized as follows:
- P0030, P0050 - HO2S heater control (universal HO2S)
- P0040, P0041 - Swapped HO2S connectors
- P0053, P0054, P0055, P0059, P0060 - HO2S heater resistance
- P0130, P0150 - HO2S circuit concerns (universal HO2S)
- P0132, P0138, P0152, P0158 - HO2S circuit high-voltage
- P0133, P0139 - HO2S slow/delayed response rate
- P0153, P0159 - HO2S slow response rate
- P0134, P0154 - HO2S circuit no activity detected (universal HO2S)
- P0135, P0141, P0155, P0161 - HO2S heater circuit
- P013A, P013C, P013E, P0144A - Rear HO2S DFSO response test
- P1127 - Downstream HO2S not running in on-demand self-test
- P2096, P2097, P2098, P2099 - Post-catalyst fuel trim (universal HO2S)
- P2195, P2196, P2197, P2198, P2270, P2271, P2272, P2273, P2274, P2275 - HO2S lack of switching For applications using a universal HO2S in the upstream or stream 1 position, there are additional DTCs such as heater temperature control, additional circuit diagnostics, lack of movement, and fore/aft sensor catalyst optimization.For applications using a universal HO2S in the upstream or stream 1 position, there are additional DTCs such as heater temperature control, additional circuit diagnostics, lack of movement, and fore-aft oxygen sensor catalyst optimization.During deceleration fuel shut-off (DFSO) the PCM monitors the integrity of the upstream universal heated oxygen sensor (HO2S) UO2SPCT circuit by comparing the actual oxygen sensor voltage to an expected oxygen sensor voltage. The associated DTCs are:
- P2626 - O2 Sensor Positive Current Trim Circuit/Open (Bank 1 Sensor 1)
- P2629 - O2 Sensor Positive Current Trim Circuit/Open (Bank 2 Sensor 1)
Misfire Detection Monitor
The misfire detection monitor is an on-board strategy designed to monitor engine misfire and identify the specific cylinder in which the misfire has occurred. Misfire is defined as lack of combustion in a cylinder due to absence of spark, poor fuel metering, poor compression, or any other cause. The misfire detection monitor is enabled only when certain base engine conditions are first satisfied. Input from the cylinder head temperature (CHT), intake air temperature (IAT), mass air flow (MAF) sensors is required to enable the monitor. The misfire detection monitor is also carried out during an on-demand self-test.
1. The PCM synchronized ignition spark is based on information received from the CKP sensor. The CKP signal generated is also the main input used in determining cylinder misfire.
2. The input signal generated by the CKP sensor is derived by sensing the passage of teeth from the crankshaft position wheel mounted on the end of the crankshaft.
3. The input signal to the PCM is then used to calculate the time between CKP edges and also crankshaft rotational velocity and acceleration. By comparing the accelerations of each cylinder event, the power loss of each cylinder is determined. When the power loss of a particular cylinder is sufficiently less than a calibrated value and other criteria is met, then the suspect cylinder is determined to have misfired.
4. The MIL is activated after one of the above tests fail on 2 consecutive drive cycles.
Misfire Monitor Operation
There are 2 different misfire monitoring systems used. They are low data rate (LDR) and a high data rate (HDR). The LDR system is capable of meeting the federal test procedure monitoring requirements on most engines and is capable of meeting the full-range of misfire monitoring requirements on 4-cylinder engines. The HDR system is capable of meeting the full-range of misfire monitoring requirements on 6 cylinder engines. The HDR on these engines meets the full-range of misfire phase-in requirements specified in the OBD regulations. The PCM software allows for detection of any misfires that occur 6 engine revolutions after initially cranking the engine. This meets the OBD requirement to identify misfires within 2 engine revolutions after exceeding the warm drive, idle RPM.
Low Data Rate System (LDR)
The LDR misfire monitor uses a low data rate crankshaft position signal, one position reference signal at 10 degrees before top dead center (BTDC) for each cylinder event. The PCM uses the CKP signal to calculate the crankshaft speed and acceleration for each cylinder. The crankshaft acceleration is then processed to detect a sporadic, single-cylinder misfire patterns. The changes in overall engine RPM are removed by subtracting the median engine acceleration over a complete engine cycle. The resulting deviant cylinder acceleration values are used in evaluating misfire. See Generic Misfire Processing section below.
High Data Rate System (HDR)
The HDR misfire monitor uses a high data rate crankshaft position which indicates 18 position references per crankshaft revolution. This high resolution signal is processed using 2 different algorithms. The first algorithm is optimized to detect hard misfires on one or more continuously misfiring cylinders. 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 second algorithm, called pattern cancellation, is optimized to detect low rates 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. Both the hard misfire algorithm and the pattern cancellation algorithm produce a deviant cylinder acceleration value, which is used in evaluating misfire in the Generic Misfire Processing section below. Due to the high data processing requirements, the HDR algorithms may be implemented by the PCM in a separate chip. The chip carries out the HDR algorithm calculations and sends the deviant cylinder acceleration values to the PCM microprocessor for additional processing as described below. The chip requires correct operation of the CKP and camshaft position (CMP) sensor inputs. DTC P1336 will be set if the chip detects noise on the CKP sensor input or if the chip is unable to synchronize with the missing tooth location. A DTC P1336 points to noise present on the CKP sensor input or a lack of synchronization between the CMP and CKP sensors.
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 899°C (1,650°F) for Pt/Pd/Rh conventional advanced washcoat, 982°C (1,800°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 determined to be consistently misfiring in excess of the catalyst damage criteria, the fuel injector to that cylinder shuts off to prevent catalyst damage for a calibrated period of time, typically 30 to 60 seconds. Up to 2 cylinders may be disabled at the same time on 6- and 8-cylinder engines and one cylinder on 4 cylinder engines. After the calibrated period of time has elapsed, the injector is re-enabled. If misfire on that cylinder is detected again after 200 revs (about 5 to 10 seconds), the fuel injector is shut off again and the process repeats until the misfire is no longer present. Note that ignition coil primary circuit failures trigger the same type of fuel injector disablement. See Comprehensive Component Monitor (CCM). The misfire rate is also evaluated every 1,000 revolution period and compared to a single (type B) threshold value to indicate an emission-threshold concern, 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.
Continued in Part 4 Part 4