Fuel Delivery and Air Induction: Description and Operation

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, which is integral to the mass airflow (MAF) sensor.
^ Engine speed from the crankshaft position (CKP) sensor
^ Engine load. Calculation is based on the mass airflow (MAF) sensor. The MAP sensor will be used as a default if the MAF sensor signal is not valid.

These parameters allow the ECM to calculate a base fuel injector pulse width when the system is in open loop. Open loop (for a specific bank) is when the ECM is not using the heated oxygen sensor 1 (HO2S- 1, pre catalyst oxygen sensor) to modify fuel.

The L81 HO2S-1 (bank 1 or 2) are air/fuel ratio sensors that are not of the traditional switching type used in the four cylinder Saturn engines. These sensors allow a wider range of fuel control (8:1 up to 18:1) accuracy, which allow the ECM to remain in closed loop under all engine running conditions (except extended decelerations). When the HO2S 1 is above 275°F, it will begin to allow O2 ions to pass across its diffusion plates, which will allow the supplied output pump current to start flowing through the sensor. Some of the output pump current is returned to the ECM through the input pump current circuit to limit the current through the sensor. As the air/fuel ratio changes, the sensor current draw (resistance) will vary. The ECM will attempt to maintain a fixed voltage on the signal line at a certain air/fuel ratio by varying the output pump current. When the output current and signal voltage reach a certain level for a commanded air/fuel ratio, the ECM will have reached its A/F ratio target.

The Scan tool displays a lambda value for the HO2S bank 1 sensor 1 and HO2S bank 2 sensor 1 used to denote the actual air/fuel ratio of each bank. Lambda signifies the actual air/fuel ratio measured by the sensor divided by 14.7. A lambda value of 1.00 means that a specific bank of cylinders is running at 14.7 to 1. When the exhaust gas has high oxygen content, the air/fuel mixture is lean and the HO2S-1 lambda value will be high (around 1.2). 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 HO2S-1 lambda value will be low (around 0.8). To compensate, the ECM will decrease the amount of fuel by reducing the injector pulse width. The ECM will normally fluctuate rich to lean around 14.7 to 1 for improved catalytic converter efficiency. This is not as drastic as the traditional switching type sensors, however.

The ECM has the ability to adapt fuel control based on previous HO2S-1 signals. The short term fuel trim (STFT) value is used to adapt fuel control over a short period of ti me. A value of 0% is the nominal STFT value the engine should be running at. If the engine is running at 0% in closed loop, the ECM does not have to modify fuel to obtain the desired 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 HO2S-1 indicates a lean condition The same is true for the lean running condition.

The long term fuel trim (LTFT) values are based on the STFT values. There are three different engine load ranges; idle, medium load and high load that the ECM uses for fuel adaptation. When the vehicle is in one of these conditions, it will use the LTFT adaptive fuel correction value that it has stored. For instance, the vehicle could be running lean at idle, but be rich while cruising under medium load. So if the vehicle is cruising then comes down to idle, the ECM will automatically increase the injector pulse width according to the idle LTFT value. When in fuel adaptation, the ECM will take into account that a richer mixture will result when the EVAP purge solenoid is commanded On to purge fuel vapors from the EVAP canister to the intake manifold.

To obtain a reading of how the vehicle is running overall, the LTFT values should be used while in one of the three driving states: idle, cruise and accel. 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 HO2S-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 improve tailpipe emissions.