Fuel Control System

The 3.0L L81 dual overhead cam (DOHC) engine utilizes sequential fuel injection (SFI). SFI allows the engine control module (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 3 main parameters:

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 4 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 during cold engine starting and extended decelerations. When the HO2S 1 is above 135°C (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 14.7:1. 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 14.7:1 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 time. A value of 0 percent is the nominal STFT value the engine should be running. If the engine is running at 0 percent in Closed Loop, the ECM does not have to modify fuel to obtain the desired air/fuel ratio. The 0 percent value is based off of the calculation from the 3 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 2 different engine load ranges: idle/decel and cruise/accel that the ECM uses for fuel adaptation. When the vehicle is in 1 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. 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.

Important: The LTFT values are not updated in purge mode. The ECM will cycle the EVAP purge solenoid OFF when updating the LTFT values.

Important: The ECM will use the HO2S 2, post catalyst oxygen sensor, from each bank to add or subtract a slight amount of fuel to keep the HO2S 2 at a target of 660 mV. This technique is called fuel trim biasing which is used to improve tailpipe emissions.

Important: The ECM will only update the LTFT cruise/accel value on the scan tool under moderate to heavy engine loads.

To obtain a reading of how the vehicle is running overall, the LTFT value should be used while in 1 of the 2 driving states: idle/decel or cruise/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.

Misfire Diagnostics

The ECM receives 58 A/C voltage pulses every crankshaft revolution from the crankshaft position sensor to determine the position of the engine at various degrees of rotation. The ECM bases misfire diagnostics on the principle that the crankshaft velocity will increase as each cylinder fires. To detect misfire, the ECM monitors the time between the cylinders 120 degree firing interval, which equates to 20 crankshaft reluctor wheel notches. A double space between notches is used to identify cylinder #1. A misfire on a cylinder occurs when the time to pass 20 notches takes too long. The time is based on what the maximum time between combusting cylinders should be at a given engine speed and load.

Important: The maximum amount of misfires the scan tool can display for the current counter is 255.

The ECM begins misfire detection as soon as the engine is started. The ECM will simultaneously perform 200 and 1,000 crankshaft revolution tests using 2 different internal counters. The 200 revolution tests, 600 combustion events, are used to determine if a catalyst damaging misfire, based on speed and load, has occurred. The 1000 revolution tests are used to determine if an emission related misfire, 1.5 times the emission level standard, has occurred. During this time, if any misfires are occurring, the scan tool will display the number of misfires under parameter MISFIRE CURRENT CYLs. #1-6. The scan tool will display whatever value the ECM has stored for the 1,000 revolution counter.

Once every second, the current misfire counters will be moved to the MISFIRE HISTORY CYLs  #1-6. The current counters will be reset to zero once the 1,000 revolution test is completed. The history counters will only be reset when a clear DTC command has been issued to the ECM.

If an emission related misfire occurs, the ECM will command the MIL ON steady and DTC P0300 will set. This will usually take 30 misfires during a 1,000 revolution test OR a sum of 45 or greater, not exceeding 30 per 1,000 revolutions, during 4 consecutive 1,000 revolution tests. If a catalyst damaging misfire occurs, the ECM will command the MIL to flash.

Important: The higher the speed and load, the quicker the ECM will command the MIL to flash when a catalyst damaging misfire is occurring.

Important: If a catalyst damaging misfire occurs on a specific cylinder for a certain length of time, the ECM will command that cylinder fuel injector OFF for the remainder of the ignition cycle regardless if the misfire condition no longer exists.

The ECM controls the MIL by sending a message over the controller area network (CAN) link to the body control module (BCM), which in turn sends a message over the class II link to the I/P cluster.

Crankshaft Relearn: In order to correctly detect misfires, the ECM must learn the variation between the 58 crankshaft notches. This notch learn variation is done automatically by the ECM for 24 ranges determined by engine speed and load.

The ECM continuously updates the notch variation whenever misfire is enabled and no misfires exist in the particular range. The ECM will take about 20-30 seconds to learn the notch variation for each speed/load range. If a speed/load range has not been learned, misfire diagnostics will still be enabled, but misfire criteria will be relaxed.

Theory of Operation -- Fuel Filling and Onboard Refueling and Vapor Recovery (ORVR)

The onboard refueling and vapor recovery (ORVR) system limits the amount of fuel vapors released into the atmosphere during a vehicle refueling event.

Design of system components reduces the available path for these vapors into the atmosphere and forces them into an EVAP canister where they are stored for later consumption when the vehicle is running.

All these system components are designed to meet the legislated requirements for emissions during refueling without impacting the fuel fill process. The vehicle can be refueled from any available fuel dispensing equipment, at standard fuel nozzle dispensing rates (4-12 gallons/min), with any commercially blended grades of fuel, without problems. Changes to the system are transparent to the customer during refueling - no changes from what he or she has been used to.

As fuel is dispensed into the fuel filler pipe and into the fuel tank, vapors are generated. The gasoline pumped up from a cold underground storage tank into a warmer fuel tank expands as it warms up. The fuel tank is designed to have room for expansion of fuel and the collection of this vapor. This vapor is expanding since it is at a higher pressure than the atmosphere.

The primary flow path for the vapor is past an orifice at the top of the fill limit vent valve (FLVV). From here it flows into the EVAP canister where it is absorbed by the bed of activated carbon. Two other secondary paths for vapor include: 1) grade vent valve orifice located on top of the fuel tank and 2) an orifice in the recirculation line. The vapor passing through the grade vent valve is routed into the same path as the FLVV and into the EVAP canister. The vapor from the recirculation line is drawn to the top of the filler pipe and is directed into the stream of the fuel dispensed from the fill nozzle and is pulled (venturi action) back into the fuel tank.

The actions described thus far continue throughout the refueling process. The vapors generated travel into the EVAP canister, and back into the fuel stream, as they vent through the EVAP vent solenoid (normally open). As the fuel fill process approaches the tank capacity, the FLVV and orifice in the grade vent valve began to function to limit the fuel dispensed.

The FLVV has a buoyant float which rises on the increasing amount of fuel in the tank and closes off the main vapor path from the tank into the canister. The vapor path is now much smaller, only a small orifice in the FLVV and the grade vent valve admit vapor into the canister. Since the vapor path is now restricted, pressure in the tank rises. This pressure increase raises the level of fuel in the filler pipe back up to the fill nozzle and causes it to shut off. At the same time that the fill nozzle shuts off, the check valve at the end of the fuel filler pipe closes and prevents fuel in the tank from rushing back up the fill pipe and causing fuel "spitback".

The fuel tank is now essentially full. The fill nozzle can be operated several more times, at lower flow rates, and flow a few tenths of a gallon more until no more fuel can be dispensed. At this time, the FLVV is completely closed, to prevent liquid fuel from flowing in to the EVAP canister, and the only vapor path is through the grade vent valve into the canister. Once the fuel cap is reinstalled, the refueling cycle is completed.