Fuel System Description
Engine Controls and Fuel - 5.3L L84 or 6.2L L87 | Description and Operation | Document ID: 4991559 Fuel System Overview The fuel system is an electronic returnless on-demand design. A returnless fuel system reduces the internal temperature of the fuel tank by not returning hot fuel from the engine to the fuel tank. Reducing the internal temperature of the fuel tank results in lower evaporative emissions. An electric turbine style fuel pump attaches to the fuel tank fuel pump module inside the fuel tank. The fuel pump supplies fuel through the fuel feed pipe to the high pressure fuel pump. The high pressure fuel pump supplies fuel to a variable-pressure fuel rail. Fuel enters the combustion chamber through precision multi-hole fuel injectors. The high pressure fuel pump, fuel rail pressure, fuel injection timing, and injection duration are controlled by the Engine Control Module (ECM). Acceleration Mode When the driver pushes on the accelerator pedal, air flow into the cylinders increases rapidly. To prevent possible hesitation, the ECM increases the pulse-width to the injectors to provide extra fuel during acceleration. This is also known as power enrichment. The ECM determines the amount of fuel required based upon engine speed, Engine Coolant Temperature (ECT), Manifold Absolute Pressure (MAP), Mass Air Flow (MAF), and throttle position sensors. Battery Voltage Correction Mode When the battery voltage is low, the ECM compensates for the weak spark delivered by the ignition system in the following ways: Increasing the amount of fuel delivered Increasing the idle RPM Increasing the ignition dwell time Clear Flood Mode If the engine floods, the engine can be cleared by pressing the accelerator pedal down to the floor and then cranking the engine. When the throttle position sensor is at Wide Open Throttle (WOT), the ECM reduces the fuel injector pulse-width in order to increase the air to fuel ratio. The ECM holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the ECM returns to the starting mode. Deceleration Mode When the driver releases the accelerator pedal, air flow into the engine is reduced. The ECM monitors the corresponding changes in throttle position, MAF, and MAP. The ECM shuts OFF fuel completely if the deceleration is very rapid, or for long periods, such as long, closed-throttle coast-down. The fuel shuts OFF in order to prevent damage to the catalytic converters. Electronic Returnless Fuel System The electronic returnless fuel system is a microprocessor controlled fuel delivery system which transports fuel from the tank to the fuel rail. It functions as an electronic replacement for a traditional, mechanical fuel pressure regulator. The pressure relief regulator valve within the fuel tank provides an added measure of over pressure protection. Desired fuel pressure is commanded by the ECM, and transmitted to the fuel pump power control module via a GMLAN serial data message. A fuel pressure sensor located on the fuel feed pipe provides the feedback the engine control module requires for Closed Loop fuel pressure control. Flexible Fuel Sensor The flexible fuel sensor measures the ethanol-gasoline ratio of the fuel being used in a flexible fuel vehicle. Flexible fuel vehicles can be operated with a blend of ethanol and gasoline, up to 85 % ethanol. In order to adjust the ignition timing and the fuel quantity to be injected, the engine management system requires information about the percentage of ethanol in the fuel. The flexible fuel sensor uses quick-connect style fuel connections, an incoming fuel connection, and an outgoing fuel connection. All fuel passes through the flexible fuel sensor before continuing on to the fuel rail. The flexible fuel sensor measures two different fuel related parameters, and sends an electrical signal to the ECM to indicate ethanol percentage, and fuel temperature. The flexible fuel sensor has a three-wire electrical harness connector. The three wires provide a ground circuit, a power source, and a signal output to the ECM. The power source is ignition positive voltage and the ground circuit connects to a chassis ground. The signal circuit carries both the ethanol percentage and fuel temperature within the same signal, on the same wire. The flexible fuel sensor uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temperature, and changes the output signal accordingly. The electrical characteristic of the flexible fuel sensor signal is a square-wave digital signal. The signal is both variable frequency and variable pulse-width. The frequency of the signal indicates the ethanol percentage, and the pulse-width indicates the fuel temperature. The ECM provides an internal pull-up to 5 V on the signal circuit, and the flexible fuel sensor pulls the 5 V to ground in pulses. The output frequency is linear to the percentage of ethanol content in the fuel. The normal range of operating frequency is between 50 and 150 Hz, with 50 Hz representing 0 % ethanol, and 150 Hz representing 100 % ethanol. The normal pulse-width range of the digital pulses is between 1 and 5 ms, with 1 ms representing −40° C (−40° F), and 5 ms representing 151.25° C (304.25° F). The microprocessor inside the sensor is capable of a certain amount of self-diagnosis. An output frequency of 180 Hz indicates either that the fuel is contaminated, or that an internal sensor electrical fault has been detected. Certain substances dissolved in the fuel can cause the fuel to be contaminated, raising the output frequency higher than the actual ethanol percentage should indicate. Examples of these substances include water, sodium chloride (salt), and methanol. It should be noted that it is likely that the Flexible Fuel Sensor will indicate a slightly lower ethanol percentage than what is advertised at the fueling station. This is not a fault of the sensor. The reason has to do with government requirements for alcohol-based motor fuels. Government regulations require that alcohol intended for use as motor fuel be denatured. This means that 100 % pure ethanol is first denatured with approximately 4½ % gasoline, before being mixed with anything else. When an ethanol gasoline mixture is advertised as E85, the 85 % ethanol was denatured before being blended with gasoline, meaning an advertised E85 fuel contains only about 81 % ethanol. The Flexible Fuel Sensor measures the actual percentage of ethanol in the fuel. Fuel Cutoff Mode The ECM cuts OFF fuel from the fuel injectors when the following conditions are met in order to protect the powertrain from damage and improve driveability: The ignition is OFF. This prevents engine run-on. The ignition is ON but there is no ignition reference signal. This prevents flooding or backfiring. The engine speed is too high, above red line. The vehicle speed is too high, above rated tire speed. During an extended, high speed, closed throttle coast down—This reduces emissions and increases engine braking. During extended deceleration, in order to prevent damage to the catalytic converters Fuel Metering Modes of Operation The ECM monitors voltages from several sensors in order to determine how much fuel to give the engine. The ECM controls the amount of fuel delivered to the engine by changing the fuel injector pulse-width. The fuel is delivered under one of several modes. Fuel Pressure Sensor The fuel pressure sensor is a serviceable 5 V, 3-pin device. It is located on the fuel feed line forward of the fuel tank, and receives power and ground from the engine control module through a vehicle wiring harness. The sensor provides a fuel pressure signal to the engine control module, which is used to provide Closed Loop fuel pressure control. Fuel Pump Power Control Module The fuel pump power control module is a serviceable GMLAN module. The fuel pump power control module receives the desired fuel pressure message from the ECM and controls the fuel pump located within the fuel tank to achieve the desired fuel pressure. The fuel pump power control module controls the in tank fuel pump by providing varying AC voltage to the 3 phase motor inside the fuel pump. Fuel Rail Assembly The fuel rail assembly attaches to each cylinder head. The fuel rail distributes high pressure fuel to the fuel injectors. The fuel rail assembly consists of the following components: The direct fuel injectors The fuel rail pressure sensor Fuel Injectors The fuel injection system is a high pressure, direct injection, returnless on-demand design. The fuel injectors are mounted in the cylinder head beneath the intake ports and spray fuel directly into the combustion chamber. Direct injection requires high fuel pressure due to the fuel injector’s location in the combustion chamber. Fuel pressure must be higher than compression pressure requiring a high pressure fuel pump. The fuel injectors also require more electrical power due to the high fuel pressure. The ECM supplies a separate high voltage supply circuit and a high voltage control circuit for each fuel injector. The injector high voltage supply circuit and the high voltage control circuit are both controlled by the ECM. The ECM energizes each fuel injector by grounding the control circuit. The ECM controls each fuel injector with 65 V. This is controlled by a boost capacitor in the ECM. During the 65 V boost phase, the capacitor is discharged through an injector, allowing for initial injector opening. The injector is then held open with 12 V. The fuel injector assembly is an inside opening electrical magnetic injector. The injector has six precision machined holes that generate a cone shaped oval spray pattern. The fuel injector has a slim extended tip in order to allow a sufficient cooling jacket in the cylinder head. Fuel Rail Pressure Sensor The fuel rail pressure sensor transmits fuel pressure and temperature information by serial data using the Society of Automotive Engineers (SAE) J2716 Single Edge Nibble Transmission (SENT) protocol. The fuel rail pressure sensor internal micro processor allows 4 separate sensor outputs from one 3 wire sensor. The ECM supplies the fuel rail pressure sensor with a 5 V reference circuit, a low reference circuit, and an asynchronous signal/serial data circuit. The asynchronous signal means communication is only going from the fuel rail pressure sensor to the ECM. The ECM decodes the serial data signal into separate values which are displayed on a scan tool as the inputs from the Fuel Rail Temperature Sensor, Fuel Rail Temperature Sensor 2, Fuel Rail Pressure Sensor and Fuel Rail Pressure Sensor 2. Fuel Tank The fuel tank is molded from high density polyethylenes and stores the fuel supply. The fuel tank is located on the left side of the vehicle and held in place by 2 metal straps that attach to the frame. The fuel limit vent valve which is mounted inside the fuel tank, as part of the tank assembly is designed to close the fuel tank vent when the fuel level reaches a calibrated level during fueling. When the fuel limit vent valve is closed the tank pressure increases during the filling process. This increase in tank pressure causes the filling station fuel pump nozzle to turn OFF. When the fuel limit vent valve is opened fuel vapors in the fuel tank are allowed to vent through the fuel limit vent valve and hose to the EVAP canister. Fuel Tank Fuel Pump Module The electric turbine style fuel pump attaches to the fuel tank fuel pump module inside the fuel tank and supplies fuel through the fuel feed pipe to the high pressure fuel pump. The fuel tank fuel pump module contains a reverse flow check valve. The check valve maintains fuel pressure in the fuel feed pipe in order to prevent long cranking times. The fuel tank fuel pump module is located inside of the fuel tank. The fuel tank fuel pump module consists of the following major components: The fuel level sensor The fuel pump and reservoir assembly The pressure relief regulator valve The fuel strainer The primary jet pump Fuel Level Sensor The fuel level sensor consists of a float, a wire float arm, and a ceramic resistor card. The position of the float arm indicates the fuel level. The fuel level sensor contains a variable resistor which changes resistance in correspondence with the position of the float arm. The Fuel Pump Power Control Module sends the fuel level information via high speed CAN–bus to the ECM. The ECM then sends the fuel level percentage via low speed CAN–bus to the Instrument Panel Cluster (IPC) in order to control the fuel gauge. The control module monitors the signal circuit of the fuel level sensor in order to determine the fuel level. Fuel Pump and Reservoir Assembly The fuel pump is mounted in the primary fuel tank module reservoir. The ECM supplies voltage to the fuel pump driver control module for 30 s when the ignition is turned on and continuously when the engine is running. While this voltage is being received, the ECM sends a serial data signal to the fuel pump driver control module containing the desired fuel pump speed. The fuel pump driver control module actuates the brushless fuel pump by means of a three phase signal and controls the fuel pump speed according to serial data signals received from the ECM containing the desired fuel pump speed. Pressure Relief Regulator Valve The pressure relief regulator valve replaces the typical fuel pressure regulator used on a mechanical returnless fuel system. The pressure relief regulator valve is closed during normal vehicle operation. The pressure relief regulator valve is used to vent pressure during hot soaks and also functions as a fuel pressure regulator in the event of the fuel pump power control module defaulting to 100% commanded output of the fuel pump. Due to variation in the fuel system pressures, the opening pressure for the pressure relief regulator valve is set higher than the pressure that is used on a mechanical returnless fuel system pressure regulator. Primary Jet Pump The primary jet pump is located in the fuel tank fuel pump module. Fuel pump flow loss, caused by vapor expulsion in the pump inlet chamber, is diverted to the primary jet pump through a restrictive orifice located on the pump cover. The primary jet pump fills the reservoir of the fuel tank fuel pump module. Fuel Trim The ECM controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the Heated Oxygen Sensor (HO2S) signal voltage while in Closed Loop and regulates the fuel delivery by adjusting the pulse- width of the injectors based on this signal. The ideal fuel trim values are around 0 % for both short and long term fuel trim. A positive fuel trim value indicates the ECM is adding fuel in order to compensate for a lean condition by increasing the pulse-width. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse-width. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These changes fine tune the engine fueling. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. A scan tool can be used to monitor the short and long term fuel trim values. The long term fuel trim diagnostic is based on an average of several of the long term speed load learn cells. The ECM selects the cells based on the engine speed and engine load. If the ECM detects an excessively lean or rich condition, the ECM will set a fuel trim Diagnostic Trouble Code (DTC). High Pressure Fuel Pump The high fuel pressure necessary for direct injection is supplied by the high pressure fuel pump. The high pressure fuel pump is mounted on the rear of the engine under the intake manifold and is driven by a three-lobe cam on the camshaft. This high pressure fuel pump also regulates the fuel pressure using an actuator in the form of an internal solenoid-controlled valve. In order to keep the engine running efficiently under all operating conditions, the ECM requests pressure ranging from 2 to 15 MPa (290 to 2,176 PSI), depending on engine speed and load. Output drivers in the ECM provide the high pressure fuel pump control circuit with a 12 V pulse-width modulated (PWM) signal, which regulates fuel pressure by opening and closing the control valve at specific times during pump strokes. This effectively regulates the portion of each pump stroke that is delivered to the fuel rail. The high pressure fuel pump is normally closed and will not deliver an increase in fuel pressure when the control solenoid is NOT powered. In the event of pump control failure, the high pressure system is protected by a relief valve in the pump that prevents the pressure from exceeding 21.5 MPa (3,118 PSI). Nylon Fuel Pipes