Fuel System Monitor
The adaptive fuel strategy uses O2 sensors for fuel feedback. The fuel equation includes short and long term fuel trim modifiers:
| FUEL MASS = AIR MASS * SHRTFT * LONGFT EQUIV_RATIO * 14.64 |
Where:
- Fuel Mass = desired fuel mass
- Air Mass = measured air mass, from MAF sensor
- SHRTFT = Short Term Fuel Trim, calculated
- LONGFT = Long Term Fuel Trim, learned table value, stored in Keep Alive Memory
- EQUIV_RATIO = Desired equivalence ratio, 1.0 = stoich, > 1.0 is lean, < 1.0 is rich
- 14.64 = Stoichiometric ratio for gasoline
A conventional O2 sensor (not a wide-range sensor) can only indicate if the mixture is richer or leaner than stoichiometric. During closed loop operation, short term fuel trim values are calculated by the PCM using oxygen sensor inputs in order to maintain a stoichiometric air/fuel ratio. The PCM is constantly making adjustments to the short term fuel trim, which causes the oxygen sensor voltage to switch from rich to lean around the stoichiometric point. As long as the short term fuel trim is able to cause the oxygen sensor voltage to switch, a stoichiometric air/fuel ratio is maintained.
When initially entering closed loop fuel, SHRTFT starts 1.0 and begins adding or subtracting fuel in order to make the oxygen sensor switch from its current state. If the oxygen sensor signal sent to the PCM is greater than 0.45 volts, the PCM considers the mixture rich and SHRTFT shortens the injector pulse width. When the cylinder fires using the new injector pulse width, the exhaust contains more oxygen. Now when the exhaust passes the oxygen sensor, it causes the voltage to switch below 0.45 volts, the PCM considers the mixture lean, and SHRTFT lengthens the injector pulse width. This cycle continues as long as the fuel system is in closed loop operation.
As components continue to change beyond normal limits or if a malfunction occurs, the long-term fuel trim values will reach a calibratable rich or lean limit where the adaptive fuel strategy is no longer allowed to compensate for additional fuel system changes. Long term fuel trim corrections at their limits, in conjunction with a calibratable deviation in short term fuel trim, indicate a rich or lean fuel system malfunction.
Note that in the PCM, both long and short-term fuel trim are multipliers in the fuel pulse width equation. Scan tools normally display fuel trim as percent adders. If there were no correction required, a scan tool would display 0% even though the PCM was actually using a multiplier of 1.0 in the fuel pulse width equation.
| DTCs | P0171 Bank 1 Lean, P0174 Bank 2 Lean P0172 Bank 1 Rich, P0175 Bank 2 Rich |
| Monitor execution | continuous while in closed loop fuel |
| Monitor Sequence | none |
| Sensors OK | Fuel Rail Pressure (if available), IAT, CHT/ECT, MAF, TP |
| Monitoring Duration | 2 seconds to register malfunction |
| Entry Condition | Minimum | Maximum |
| Engine Coolant Temp | 150 °F | 250 °F |
| Engine Load | 12% | |
| Intake Air Temp | -30% °F | 150 °F |
| Air Mass Range | 0.75 lb/min | |
| Purge Duty Cycle | 0% | 0% |
| Long Term Fuel Trim correction cell currently being utilized in conjunction with Short Term Fuel Trim: Lean malfunction: LONGFT > 25%, SHRTFT > 5% Rich malfunction: LONGFT < 25%, SHRTFT < 5% |
FAOSC (Rear Fuel Trim) Monitor
As the front UEGO sensor ages and gets exposed to contaminants, it can develop a rich or lean bias in its transfer function. The rear bias control (also called FAOSC - Fore/Aft Oxygen Sensor Control) system is designed to compensate for any of these bias shifts (offsets) using the downstream HO2S sensor. The "FAOS" monitor looks for any bias shifts at the stoichiometric point of the front UEGO sensor lambda curve. If the UEGO has developed a bias beyond the point for which it can be compensated for, lean (P2096, P2098) or rich (P2097, P2099) fault codes will be set.
| DTCs | P2096 - Post catalyst fuel trim system too lean (Bank 1) P2097 - Post catalyst fuel trim system too rich (Bank 1) P2098 - Post catalyst fuel trim system too lean (Bank 2) P2099 - Post catalyst fuel trim system too rich (Bank 2) |
| Monitor execution | Continuous while in closed loop fuel |
| Monitor Sequence | > 30 seconds time in lack of movement test, > 30 seconds time in lack of switch test |
| Sensors OK | ECT, IAT, MAF, MAP, VSS, TP, ETC, FRP, FVR, DPFE EGR, VCT, VMV/EVMV, CVS, CPV, EVAPSV, FTP, CKP, CMP, ignition coils, injectors, no misfire DTCs, no system failures affecting fuel, no EVAP gross leak failure, UEGO heaters OK, rear HO2S heaters OK, no "lack of switching" malfunction, no "lack of movement" malfunction, no UEGO circuit malfunction, no rear stream 2 HO2S circuit malfunction, no rear stream 2 HO2S functional DTCs, no rear stream 2 HO2S response rate malfunction. |
| Monitoring Duration | 5 seconds to register a malfunction |
| Entry Conditions | Minimum | Maximum |
| Closed loop stoich fuel control | ||
| Time since engine start | 20 seconds | |
| Engine Coolant Temp | 160 °F | 250 °F |
| Time since entering closed loop fuel | 20 seconds | |
| Fuel Level | 15% | |
| Short Term Fuel Trim Range | -13% | 18% |
| Air mass range | 2 lbm/min | 8 lbm/min |
| Learning conditions stability time (based on air mass) | 15 seconds | |
| Injector fuel pulse width (not at minimum clip) | 650 usec | |
| Inferred HO2S 2 Heated Tip Temperature | 1100 °F | |
| No excessive movement between currently utilized long term fuel trim cells (1 = complete change from one cell to adjacent cell) | 0.5 | |
| UEGO sensor within +/- 2 % from the fuel control target | ||
| UEGO ASIC not in recalibration mode | ||
| Stream1 UEGO response test not running | ||
| Intrusive UEGO catalyst monitor not running | ||
| Not performing intrusive UEGO Lack-of-Movement fuel control defib | ||
| No air passing through during valve overlap (scavenging). | ||
| Battery Voltage | 11.0 Volts | 18.0 Volts |
| >= 5 seconds since reaching the FAOSC lean or rich limits while system bias maturity is met. Lean malfunction: -0.083 rear bias trim limit Rich malfunction: 0.087 rear bias trim limit |
Air Fuel Ratio Imbalance Monitor
The Air Fuel Imbalance Monitor is designed to monitor the cylinder-to-cylinder air fuel imbalance per engine bank. When an Air Fuel (A/F) imbalance is present, the front UEGO signal becomes noisier. The monitor uses the high frequency component from the UEGO signal as an indicator of A/F imbalance. "Hash" is the difference between two consecutive front UEGO voltage samples. The UEGO signal is monitored continuously and a differential or "hash" value is continuously calculated. When the hash is below a threshold, it is indicative of normal operation. If the hash exceeds the threshold, an A/F imbalance is assumed which increments a hash error counter. The counter accumulates hash during series of calibratable rpm windows. Typically, a single window consists of 50 engine revolutions. A total rpm window counter calculates number of completed rpm windows. Monitor completion typically requires 30 rpm windows. When the monitor completes, an A/Fuel imbalance index is calculated. The monitor index is defined as the ratio of the failed rpm windows over the total rpm windows required to complete monitor. If the monitor imbalance ratio index exceeds the threshold value, an A/F imbalance DTC is set.
| DTCs | P219A - Bank 1 Air-Fuel Ratio Imbalance P219B - Bank 2 Air-Fuel Ratio Imbalance |
| Monitor execution | Once per driving cycle during closed loop |
| Monitor Sequence | Monitor runs after fuel monitor has adapted |
| Sensors OK | ECT, IAT, MAF, VSS, TP, ETC, FRP, DPFE EGR, VCT, VMV/EVMV, CVS, FTP, CKP, CMP, ignition coils, injectors, no misfire DTCs, no system failures affecting fuel, no EVAP gross leak failure, UEGO heaters OK, rear HO2S heaters OK, no "lack of switching" malfunction, no "lack of movement" malfunction, no UEGO circuit malfunction, no rear stream 2 HO2S circuit malfunction, no rear stream 2 HO2S functional DTCs, no rear stream 2 HO2S response rate malfunction. |
| Monitoring Duration | Time to complete monitor ranges from 300 to 700 seconds |
| Entry Condition | Minimum | Maximum |
| Closed Loop Fuel Control | ||
| Engine Air Mass | 2 lb/min | 10 lb/min |
| Engine RPM | 1250 rpm | 3000 rpm |
| Engine Load | 40% | 70% |
| Engine Coolant Temp | 150 °F | 250 °F |
| Intake Air Temp | 20 °F | 150 °F |
| Throttle Position Rate of Change | 0.122 v/100 msec | |
| Fuel percentage from purge | 40% | |
| Fuel Level | 15% | |
| Fuel monitor has adapted | ||
| No purge on/off transition | ||
| Fuel type leaning is complete (FFV only) |
| Imbalance Ratio Bank 1 >.75 Imbalance Ratio Bank 2 >.75 |
| Monitor ID | Test ID | Description | |
| $81 | $80 | Bank 1 imbalance-ratio and max. limit (P219A/P219B) | unitless |
| $82 | $80 | Bank 2 imbalance-ratio and max. limit (P219A/P219B) | unitless |