Fuel System

The 3.0L L81 Dual Overhead Cam (DOHC) engine utilizes Sequential Fuel Injection (SFI). SFI allows the ECM to individually control each fuel injector, which optimizes fuel economy, lowers tailpipe emissions and increases performance. The ECM Pulse Width Modulates (PWM) each fuel injector by individually grounding each fuel injector circuit.

The ECM bases its fuel injector pulse width (the amount of fuel the engine needs) on three main parameters:
^ Temperature of the air/fuel mixture at the intake valve. Calculation is based on the Engine Coolant Temperature (ECT) sensor and Intake Air Temperature (IAT) sensor. The IAT sensor is integral to the Mass Airflow (MAF) sensor.
^ Engine speed from the Crankshaft Position (CKP) sensor.
^ Engine load. Calculation is based on the MAF sensor. If the MAF sensor signal is lost, the ECM will calculate airflow based on engine speed and throttle angle.

These parameters allow the ECM to calculate a base fuel injector pulse width when the system is in open loop. Open loop is when the ECM is not using the heated oxygen sensor 1 to modify fuel. The system is in open loop anytime the engine is running and the heated oxygen sensor 1 is not varying above 600 mV or below 300 mV. The heated oxygen sensor 1 signal on Scan tool will read 450 mV when cold.

The ECM will go into open loop fuel control if a fault exists in the heated oxygen sensor 1 circuit. If a fault exists in one of the sensors necessary to calculate the base pulse width, the ECM will substitute a modeled value for the calculation. However, the Scan tool will still read the failed sensor signal value. The ECM will remain in closed loop as long as a valid heated oxygen sensor 1 signal is present. NOTE: The ECM will go into open loop under heavy acceleration and decelerations due to the inability of the heated oxygen sensor 1 to accurately identify the amount rich or amount lean from a 14.7 to 1 air/fuel ratio.

When the heated oxygen sensor 1 is above 600°F, it is fully functional and will produce a voltage based on the amount of oxygen in the exhaust gas. The ECM can make fuel corrections to achieve a desired 14.7 to 1 air/fuel ratio by using the heated oxygen sensor 1. Normal readings vary between 60 and 1080 mV. When the ECM detects the heated oxygen sensor 1 voltage above 600 mV or under 300 mV for a short period of time, it will go into closed loop fuel control. When the exhaust gas has high oxygen content, the air/fuel mixture is lean and the heated oxygen sensor 1 signal voltage will be low. To compensate, the ECM will command rich or increase the fuel injector pulse width. When the exhaust gas has low oxygen content, the air/fuel mixture is rich and the heated oxygen sensor 1 signal voltage will be high. To compensate, the ECM will decrease the amount of fuel by reducing the injector pulse width.

The ECM has the ability to adapt fuel control based on previous heated oxygen sensor 1 signals. The Short Term Fuel Trim (STFT) value is used to adapt fuel control over a short period of time. A value of 0% is the nominal STFT value. If the engine is running at 0% in closed loop, the ECM does not have to modify fuel to obtain a 14.7 to 1 air/fuel ratio. The 0% value is based off of the calculation from the three main parameters. If for instance the vehicle is running rich, the STFT value will decrease causing the ECM to decrease the injector pulse width. The ECM will continue to do this until the heated oxygen sensor 1 indicates a lean condition. The ECM will toggle from rich to lean to improve catalytic converter efficiency.

The Long Term Fuel Trim (LTFT) values are based on the STFT values. There are two different LTFT ranges that encompass light decelerations, idle, cruise and accel, these ranges are called the additive fuel adaptation range (idle and low engine loads for only an additional amount of fuel) and multiplicative fuel adaptation range (higher engine loads for a multiplying amount of fuel). When the vehicle is in one of these conditions, the ECM will use the LTFT value it has stored. For instance, the vehicle could be running lean at idle, but be OK while cruising. Therefore, if the vehicle is cruising then comes down to idle, the ECM will automatically increase the injector pulse width according to the additive LTFT value.

The ECM will NOT update the LTFT values when in EVAP purge mode. Instead, the ECM will continuously cycle the EVAP purge solenoid On and Off, while updating the LTFT values during the purge Off time.

To obtain a reading of how the vehicle is running overall, the LTFT IDLE/DECEL value on Scan tool for each back should be used while in idle or very light decel state. the LTFT CRUISE/ACCEL value on Scan tool for each bank should be used while in a cruise or light acceleration state. To obtain a reading of how the vehicle is running at a particular instant, the STFT value should be used. The STFT and LTFT values can significantly aid in diagnosing a driveability concern if used properly.

NOTE: The ECM will use the heated oxygen sensor 2 (post catalyst oxygen sensor) from each bank to add or subtract the time it is holding the fuel control system rich or lean (this does NOT increase or decrease the amount of fuel). This technique is called fuel trim biasing which is used to maintain a 600 mV output voltage from the heated oxygen sensor 2. This is performed to improve tailpipe emissions.