Fuel System Overview

This vehicle is equipped with a returnless fuel system. The fuel pressure regulator is a part of the fuel sender assembly, eliminating the need for a return pipe from the engine. A returnless fuel system reduces the internal temperature of the fuel tank by not returning hot fuel from the engine. Reducing the internal temperature of the fuel tank results in lower evaporative emissions.

The fuel tank stores the fuel supply. An electric fuel pump attaches to the fuel sender assembly inside the fuel tank. The fuel pump supplies high pressure fuel through the fuel filter, contained in the fuel sender assembly, and the fuel feed pipe to the Fuel Injection System. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pressure regulator maintains the correct fuel pressure to the fuel injection system.

Fuel Filler Cap

Caution: Use a fuel tank filler pipe cap with the same features as the original when a replacement is necessary. Failure to use the correct fuel tank filler pipe cap can result in a serious malfunction of the fuel system.

The fuel fill pipe has a tethered fuel filler cap. A torque-limiting device prevents the cap from being over-tightened. To install the cap, turn the cap clockwise until you hear audible clicks. This indicates that the cap is correctly torqued and fully seated. A fuel filler cap that is not fully seated may cause a malfunction in the emission system.

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 control module sends the fuel level information to the instrument panel cluster (IPC). This information is used for the IPC fuel gage and the low fuel warning indicator, if equipped. The control module also monitors the fuel level input for various diagnostics.

Fuel Pump

The fuel pump is mounted in the fuel sender assembly reservoir. The fuel pump is an electric high-pressure pump. Fuel is pumped to the fuel injection system at a specified flow and pressure. The fuel pump delivers a constant flow of fuel to the engine even during low fuel conditions and aggressive vehicle maneuvers. The control module controls the electric fuel pump operation through a fuel pump relay. The fuel pump flex pipe acts to dampen the fuel pulses and noise generated by the fuel pump.

Fuel Sender Strainer

The fuel strainer acts as a coarse filter to perform the following functions:

Fuel stoppage at the fuel strainer indicates that the fuel tank contains an abnormal amount of sediment or water. The fuel tank will need to be removed and cleaned, and the fuel strainer should be replaced.

Fuel Filter

The fuel filter is contained in the fuel sender assembly inside the fuel tank. The fuel filter paper element traps particles in the fuel that may damage the fuel injection system. The filter housing is made to withstand maximum fuel system pressure, exposure to fuel additives, and changes in temperature. There is no service interval for fuel filter replacement.

Fuel Pressure Regulator

The fuel pressure regulator is contained in the fuel sender assembly. The fuel pressure regulator is a diaphragm relief valve. The diaphragm has fuel pressure on one side and regulator spring pressure on the other side. A software bias compensates the injector on-time because the fuel pressure regulator is not referenced to the manifold vacuum. The fuel pressure regulator keeps fuel available to the injectors at a regulated pressure.

Fuel Injectors

The fuel injectors are pulse width modulated (PWM) solenoids controlled by the engine control module (ECM). The ECM turns ON the fuel injectors in order to open a normally closed ball or pintle valve. This allows fuel to flow into the top of the fuel injectors, past the ball or pintle valve, and through a recessed flow director plate at the injector outlet. The fuel rail is mounted on the intake manifold and distributes the fuel to each cylinder through the individual fuel injectors.

Modes of Operation

Engine Fueling: The engine is fueled by individual fuel injectors, one for each cylinder, that are controlled by the engine control module (ECM). The ECM controls each fuel injector by turning ON the fuel injector coil for a brief period once every other engine revolution. The period of time when the fuel injector is turned ON is called the pulse width and is measured in milliseconds. The pulse width is calculated by the ECM to deliver the correct amount of fuel for proper driveability and emissions control. While the engine is running, the ECM is constantly monitoring the inputs and recalculating the appropriate pulse width for each fuel injector. The pulse width calculation is based on the fuel injector flow rate, the desired air/fuel ratio, and actual air mass in each cylinder. The pulse width is also adjusted for battery voltage, short term, and long term fuel trim. The fuel injector pulse, is timed to occur as each cylinders intake valves are closing to attain largest duration and most vaporization. The fueling system has several automatic adjustments in order to compensate for the differences in fuel system hardware, driving conditions, fuel used, and vehicle aging. The battery voltage adjustment is necessary since the changes in the voltage across the fuel injector affect the flow rate. The short term and the long term fuel trims are fine and gross adjustments to the pulse width that are designed to maximize driveability and emissions control. These fuel trims are based on the feedback from the oxygen sensors in the exhaust stream and are only used when the fuel control system is in Closed Loop operation. Fueling during cranking is slightly different than fueling during an engine run. As the engine begins to turn, a prime pulse may be injected to speed starting. Once the ECM determines where the engine is in the firing order , the ECM begins pulsing the fuel injectors. The pulse width during cranking is based on the coolant temperature and the engine load. Under certain conditions, the fueling system will turn OFF the fuel injectors for a period of time. This is referred to as fuel cut-off. Fuel cut-off is used in order to improve traction, save fuel, improve emissions, and protect the vehicle under certain extreme or abusive conditions. In case of a major internal problem, the ECM may be able to use a back-up fuel strategy for limp in mode that will run the engine until service can be performed.

Starting Mode: When the ECM detects reference pulses from the crankshaft position (CKP) sensor, the ECM will enable the fuel pump. The fuel pump runs and builds up pressure in the fuel system. The ECM then monitors the manifold absolute pressure (MAP), intake air temperature (IAT), engine coolant temperature (ECT), and the throttle position (TP) sensor signals in order to determine the required injector pulse width for starting.

Clear Flood Mode: If the engine is flooded with fuel during starting and will not start, the Clear Flood Mode can be manually enabled. To enable Clear Flood Mode, press the accelerator to wide open throttle (WOT). The ECM will completely turn OFF the fuel injectors and will maintain this mode as long as the ECM detects a WOT condition with engine speed below a predetermined value.

Run Mode: The Run Mode has 2 conditions called Open Loop operation and Closed Loop operation.

Open Loop:  Upon initial engine start up and when the engine speed is more than a predetermined value, the ECM operates the fuel system in Open Loop operation. During Open Loop operation, the ECM ignores the signals from the oxygen sensors and calculates the required fuel injector pulse width based primarily on inputs from the MAP, IAT and ECT sensors. The ECM operates in Open Loop until the following conditions are met:

Closed Loop: During Closed Loop operation, the fuel injector pulse width is based on the oxygen sensor signal. The ECM uses the oxygen sensor signal to keep the air/fuel ratio close to 14.7:1 as possible.

Acceleration Mode: The ECM monitors the changes in the TP and the MAP sensor signals in order to determine when the vehicle is being accelerated. The ECM will then increase the injector pulse width in order to provide more fuel for improved performance.

Deceleration Mode: The ECM monitors changes in TP and MAP sensor signals to determine when the vehicle is being decelerated. The ECM will then decrease injector pulse width or even turn OFF injectors for short periods to reduce exhaust emissions, and for better (engine braking) deceleration.

Battery Voltage Correction Mode: The ECM can compensate to maintain acceptable vehicle driveability when the ECM detects a low battery voltage condition. The ECM compensates by performing the following functions:

Fuel Cut-Off Mode: To prevent dieseling or engine run-on, no fuel is delivered to the fuel injectors when the ignition is OFF. To prevent flooding, no fuel is delivered to the fuel injectors when no reference pulses are received from the CKP sensor.