Fuel System Description (LDD or LUJ)
Fuel System Overview
The fuel system is a returnless on-demand design. The fuel
pressure regulator is a part of the fuel pump module, 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 to the fuel tank. Reducing the
internal temperature of the fuel tank results in lower evaporative
emissions.
An electric turbine style fuel pump is attached to the fuel pump
module inside the fuel tank. The fuel pump supplies high pressure
fuel through 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, a part of
the fuel pump module, maintains the correct fuel pressure to the
fuel injection system. The fuel pump module contains a reverse flow
check valve. The check valve and the fuel pressure regulator
maintain fuel pressure in the fuel feed pipe and the fuel rail in
order to prevent long cranking times.
Fuel Tank
The fuel tank stores the fuel supply. The fuel tank is located
in the rear of the vehicle. The fuel tank is held in place by 2
metal straps that are attached to the underbody. The fuel tank is
molded from high-density polyethylene.
Fuel Filler Cap
Note: If a fuel tank filler
cap requires replacement, use only a fuel tank filler cap with the
same features. Failure to use the correct fuel tank filler cap can
result in a serious malfunction of the fuel and Evaporative
Emission (EVAP) system.
The fuel fill pipe has a tethered fuel filler cap. A
torque-limiting device prevents the cap from being overtightened.
To install the cap, turn the cap clockwise until the cap clicks
audibly. 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 Pump Module
The fuel pump module consists of the following major
components:
• |
The fuel pressure
regulator |
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 ECM sends the fuel level information via the serial
data circuit to the instrument panel cluster. This information is
used for the instrument panel cluster fuel gauge and the low fuel
warning indicator, if applicable. The ECM also monitors the fuel
level input for various diagnostics.
Fuel Pump
The fuel pump is mounted in the fuel pump module reservoir. The
fuel pump is an electric high-pressure pump. Fuel is pumped to the
fuel injection system at specified rates of 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
ECM 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 Strainer
The fuel strainer is attached to the lower end of the fuel pump
module. The fuel strainer is made of woven plastic. The functions
of the fuel strainer are to filter contaminants and to wick away
fuel. Normally, the fuel strainer does not require maintenance.
Fuel stoppage at this point indicates that the fuel tank contains
an abnormal amount of sediment or contamination.
Fuel Pressure Regulator
The fuel pressure regulator is contained in the fuel pump module
near the fuel pump outlet. 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. The fuel pressure
regulator is not vacuum biased. Fuel pressure is controlled by a
pressure balance across the regulator. The fuel system pressure is
constant.
Fuel Feed Pipes
The fuel feed pipe carries fuel from the fuel tank to the fuel
injection system. The fuel pipe consists of 3 sections:
• |
The rear fuel pump fuel feed
hose runs from the top of the fuel tank to the chassis fuel pipe.
The rear fuel hose is constructed of nylon. |
• |
The fuel feed intermediate
pipe is located under the vehicle and connects the rear fuel pump
fuel feed hose to the front fuel pump fuel feed hose. The
intermediate fuel pipe is constructed of a combination of nylon and
steel pipes. |
• |
The front fuel pump fuel feed
hose connects the fuel feed intermediate pipe to the fuel rail. The
front fuel hose contains the fuel pulse dampener and is constructed
of a combination of nylon and steel pipes. |
Nylon Fuel Pipes
Warning: Refer to
Fuel and Evaporative Emission Pipe
Warning .
Nylon pipes are constructed to withstand maximum fuel system
pressure, exposure to fuel additives, and changes in
temperature.
Heat resistant rubber hose or corrugated plastic conduit protect
the sections of the pipes that are exposed to chafing, high
temperature, or vibration.
Nylon fuel pipes are somewhat flexible and can be shaped around
gradual turns under the vehicle. However, if nylon fuel pipes are
forced into sharp bends, the pipes may kink and restrict the flow
of fuel. Also, once exposed to fuel, nylon pipes may become stiffer
and are more likely to kink if bent too far. Exercise special care
when working on a vehicle with nylon fuel pipes.
Quick-Connect Fittings
Nylon fuel pipes are somewhat flexible and can be shaped around
gradual turns under the vehicle. However, if nylon fuel pipes are
forced into sharp bends, the pipes may kink and restrict the flow
of fuel. Also, once exposed to fuel, nylon pipes may become stiffer
and are more likely to kink if bent too far. Exercise special care
when working on a vehicle with nylon fuel pipes.
Fuel Pulse Dampener
The fuel pulse dampener is a part of the front fuel pump fuel
feed hose. The fuel pulse dampener is diaphragm-operated with fuel
pump pressure on one side and with spring pressure on the other
side. The function of the dampener is to dampen the fuel pump
pressure pulsations.
Fuel Rail Assembly
The fuel rail assembly is attached to the cylinder head. The
fuel rail assembly performs the following functions:
• |
Positions the injectors in the
intake ports of the cylinder head |
• |
Distributes fuel evenly to the
injectors |
Fuel Injectors
The fuel injector assembly is a solenoid device controlled by
the ECM that meters pressurized fuel to a single engine cylinder.
The ECM energises the high-impedance, 12 Ω, injector solenoid
to open a ball valve, normally closed. This allows fuel to flow
into the top of the injector, past the ball valve, and through a
director plate at the injector outlet. The director plate has
machined holes that control the flow of fuel, generating a spray of
finely atomized fuel at the injector tip. Fuel from the injector
tip is directed at the intake valve, causing the fuel to become
further atomized and vaporized before entering the combustion
chamber. This fine atomization improves fuel economy and emissions.
The fuel pressure regulator compensates for engine load by
increasing fuel pressure as the engine vacuum drops.
Fuel Metering Modes of Operation
The ECM monitors voltages from several sensors in order to
determine how much fuel to feed to 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.
Starting Mode
When the ECM detects reference pulses from the crankshaft
position 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
accelerator pedal position (APP) 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 referred to as open loop and
closed loop. When the engine is first started and the engine speed
is above a predetermined RPM, the system begins open loop
operation. The ECM ignores the signal from the heated oxygen sensor
(HO2S). The engine ECM calculates the air/fuel ratio based on
inputs from the engine coolant temperature (ECT), the manifold
absolute pressure (MAP), and accelerator pedal position (APP)
sensor. The system stays in open loop until meeting the following
conditions:
• |
The heated oxygen sensor
(HO2S) has varying voltage output, showing that the heated oxygen
sensor (HO2S) is hot enough to operate properly. |
• |
The ECT sensor is above a
specified temperature. |
• |
A specific amount of time has
elapsed after starting the engine. |
Specific values for the above conditions exist for each
different engine, and are stored in the programmable read-only
memory (EEPROM), which may be erased electrically. The system
begins closed loop operation after reaching these values. In closed
loop, the ECM calculates the air/fuel ratio, injector ON time,
based upon the signal from various sensors, but mainly from the
heated oxygen sensor (HO2S). This allows the air/fuel ratio to stay
very close to 14.7:1.
Acceleration Mode
The ECM monitors the changes in the accelerator pedal position
(APP) sensor. and the manifold absolute pressure (MAP) sensor
signal 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 increased performance.
Deceleration Mode
The ECM monitors changes in accelerator pedal position (APP)
sensor and manifold absolute pressure (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
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 |
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 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 heated oxygen sensor (HO2S) signal voltage. These changes
fine-tune the engine fueling. The long term fuel trim makes rough
adjustments to fueling in order to recenter 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).
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