Fuel System Description Pickup
Fuel System Overview
The fuel tank stores the fuel supply. The electric fuel pump supplies
fuel through an in-line fuel filter 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. A separate pipe returns unused fuel to the fuel
tank.
Fuel Tanks
The fuel tanks store the fuel supply. The primary fuel tank is located
in the rear of the vehicle on the left side. On dual-tank applications, the
secondary fuel tank is located in the rear of the vehicle above the spare
tire. The fuel tanks are each held in place by 2 metal straps that
attach to the frame. The fuel tanks are molded from high density polyethylene.
Fuel Fill Pipe
The fuel fill pipe has a built-in restrictor and deflector in order
to prevent refueling with leaded fuel. When refueling dual-tank applications,
fuel first enters the secondary tank, and then flows to the primary tank.
Fuel Filler Cap
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 clicks. This indicates that
the cap is correctly torqued and fully seated. A built-in device indicates
that the fuel filler cap is fully seated. A fuel filler cap that is not
fully seated may cause a malfunction in the emission system.
Fuel Sender Assembly
The fuel sender assembly on single tank applications consists of
the following major components:
| • | The fuel level sensor (6) |
| • | The fuel tank pressure (FTP) sensor (1) |
| • | The fuel tank fuel pump module (2) |
The primary fuel sender on dual tank applications consists of the following
major components:
| • | The fuel level sensor (2) |
| • | The fuel tank fuel pump module (4) |
The secondary fuel sender on dual tank applications consists of the
following major components:
| • | The fuel level sensor (4) |
| • | The secondary fuel pump (2) |
Fuel Level Sensor
The fuel level sensor consists of a float, a wire float arm, and a ceramic
resistor cord. The position of the float arm indicates the fuel level. The
fuel level sensor contains a variable resistor, which changes resistance
in correspondence to the amount of fuel in the fuel tank. The powertrain
control module (PCM) sends the fuel level information via the class 2
circuit to the instrument panel (IP) cluster. This information is used for
the IP fuel gauge and the low fuel warning indicator, if applicable. The
PCM also monitors the fuel level input for various diagnostics.
Primary Fuel Pump
The primary fuel pump is mounted in the fuel sender assembly reservoir.
The primary fuel pump is an electric high pressure pump. Fuel is pumped to
the fuel rail at a specified flow and pressure. Excess fuel from the
fuel rail assembly returns to the fuel tank through the fuel return
pipe. The fuel pump delivers a constant flow of fuel to the engine
even during low fuel conditions and aggressive vehicle maneuvers. The
powertrain control module (PCM) 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.
Secondary Fuel Pump (Dual Tank Applications Only)
On dual tank applications, the secondary fuel pump is located in the
secondary fuel tank. The secondary fuel pump is powered by a secondary fuel
pump relay when the fuel level drops below a predetermined value. Fuel is
transferred from the secondary fuel tank to the primary fuel tank in order
to ensure all of the usable fuel volume is available to the primary fuel
pump. The secondary fuel pump relay supply voltage is received from the primary
fuel pump relay when the primary fuel pump is energized.
Fuel Strainer
The fuel strainer attaches to the lower end of the fuel sender. The
fuel strainer is made of woven plastic. The functions of the fuel strainer
are to filter contaminants and to wick fuel. The fuel strainer is
self-cleaning and normally requires no maintenance. Fuel stoppage at
this point indicates that the fuel tank contains an abnormal amount
of sediment or water.
In-Line Fuel Filter
The fuel filter is located on the fuel feed pipe, between the fuel pump
and the fuel injectors. The paper filter element (2) traps particles
in the fuel that may damage the injection system. The filter housing (1)
is made to withstand maximum fuel system pressure, exposure to fuel additives,
and changes in temperature.
Fuel Feed and Return Pipes
The fuel feed pipe carries fuel from the fuel tank to the fuel rail
assembly. The fuel return pipe carries fuel from the fuel rail assembly back
to the fuel tank. The fuel pipes consist of 2 sections:
| • | The rear fuel pipe assemblies are located from the top of the
fuel tank to the chassis fuel pipes. The rear fuel pipes are constructed of
nylon. |
| • | The chassis fuel pipes are located under the vehicle and connect
the rear fuel pipes to the fuel rail pipes. These pipes are constructed of
steel with sections of rubber hose covered with braiding. |
Nylon Fuel Pipes
Caution: In order to reduce the risk of fire and personal injury observe the
following items:
| • | Replace all nylon fuel pipes that are nicked, scratched or damaged
during installation, do not attempt to repair the sections of the nylon fuel
pipes |
| • | Do not hammer directly on the fuel harness body clips when installing
new fuel pipes. Damage to the nylon pipes may result in a fuel leak. |
| • | Always cover nylon vapor pipes with a wet towel before using a
torch near them. Also, never expose the vehicle to temperatures higher than
115°C (239°F) for more than one hour, or more than 90°C (194°F)
for any extended period. |
| • | Apply a few drops of clean engine oil to the male pipe ends before
connecting fuel pipe fittings. This will ensure proper reconnection and prevent
a possible fuel leak. (During normal operation, the O-rings located in the
female connector will swell and may prevent proper reconnection if not lubricated.) |
Nylon pipes are constructed to withstand maximum fuel system pressure,
exposure to fuel additives, and changes in temperature. There are 3 sizes
of nylon pipes used: 9.5 mm (3/8 in.) ID for the fuel feed,
7.6 mm (5/16 in.) ID for the fuel return, and 12.7 mm
(1/2 in.) ID for the vent. Heat resistant rubber hose or corrugated
plastic conduit protect the sections of the pipes that are exposed to
chafing, to high temperatures, or to vibration.
Nylon fuel pipes are somewhat flexible and can be formed around gradual
turns under the vehicle. However, if nylon fuel pipes are forced into sharp
bends, the pipes kink and restrict the fuel flow. Also, once exposed to
fuel, nylon pipes may become stiffer and are more likely to kink if bent
too far. Take special care when working on a vehicle with nylon fuel
pipes.
Quick-Connect Fittings
Quick-connect fittings provide a simplified means of installing and
connecting fuel system components. The fittings consist of a unique female
connector and a compatible male pipe end. O-rings, located inside the
female connector, provide the fuel seal. Integral locking tabs inside
the female connector hold the fittings together.
Fuel Pipe O-Rings
O-rings seal the threaded connections in the fuel system. Fuel system
O-ring seals are made of special material. Service the O-ring seals with the
correct service part.
Fuel Rail Assembly
The fuel rail assembly attaches to the engine intake manifold. The fuel
rail assembly performs the following functions:
| • | Positions the injectors (3) in the intake manifold |
| • | Distributes fuel evenly to the injectors |
| • | Integrates the fuel pressure regulator (2) with the fuel
metering system |
Fuel Pressure Regulator Assembly
The fuel pressure regulator is a vacuum-operated diaphragm relief valve
with fuel pump pressure on one side, and with regulator spring pressure
and intake manifold vacuum on the other side. The fuel pressure regulator
maintains a constant pressure differential across the injectors at all
times. The pressure regulator compensates for engine load by increasing
the fuel pressure when the engine vacuum drops.
Fuel Metering Modes of Operation
The powertrain control module (PCM) reads voltages from several sensors
in order to determine how much fuel to give the engine. The fuel is delivered
during one of several conditions called modes. The PCM controls all modes.
| • | The Starting Mode--With the ignition switch in the ON position,
before engaging the starter, the PCM energizes the fuel pump relay for 2 seconds
allowing the fuel pump to build up pressure. The PCM first tests speed
density, then switches to the mass air flow (MAF) sensor. The PCM also
uses the engine coolant temperature (ECT), the throttle position (TP),
and the manifold absolute pressure (MAP) sensors to determine the proper
air/fuel ratio for starting. The PCM controls the amount of fuel delivered
in the starting mode by changing the pulse width of the injectors. This
is done by pulsing the injectors for very short times. |
| • | The Clear Flood Mode--If the engine floods, clear the engine
by pushing the accelerator pedal down to the floor and then crank the engine.
When the throttle position (TP) sensor is at wide open throttle (WOT) ,
the PCM reduces the injector pulse width in order to increase the air
to fuel ratio. The PCM 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 PCM returns to the starting mode. |
| • | The Run Mode--The run mode has 2 conditions. These
conditions are called 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 PCM ignores the signal from the heated
oxygen sensor (HO2S) and calculates the air/fuel ratio based on inputs
from the ECT, the MAF, the MAP, and the TP sensors. The system stays in
Open Loop until meeting the following conditions: |
| - | Both HO2Ss have varying voltage output, showing that they are
hot enough to operate properly. This depends upon the engine temperature. |
| - | 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. These values are stored in the electrically erasable programmable
read-only memory (EEPROM). The system begins Closed Loop operation after
reaching these values. In Closed Loop, the PCM calculates the air/fuel
ratio, injector ON time, based upon the signal from various sensors,
but mainly from the HO2S. This allows the air/fuel ratio to stay very
close to 14.7:1. |
| • | Acceleration Mode--When the driver pushes on the accelerator
pedal, the air flow into the cylinders increases rapidly, while fuel flow
tends to lag behind. In order to prevent possible hesitation, the PCM
increases the pulse width to the injectors in order to provide extra
fuel during acceleration. The PCM determines the amount of fuel required
based upon the throttle position, the coolant temperature, the MAP,
the MAF, and the engine speed. |
| • | Deceleration Mode--When the driver releases the accelerator
pedal, the air flow into the engine is reduced. The PCM reads the corresponding
changes in the TP, the MAP, and the MAF. The PCM shuts OFF fuel completely
if the deceleration is very rapid, or for long periods, such as during
a long, closed-throttle coast-down. The fuel shuts OFF in order to protect
the TWC. |
| • | Battery Voltage Correction Mode--When the battery voltage
is low, the PCM 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 Cut-off Mode--The PCM 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 the rated tire speed. |
| - | During an extended, high speed, closed throttle coast down--This
reduces the emissions and increases the engine braking. |
| - | During extended deceleration, in order to protect the catalytic
converters |
Short Term Fuel Trim
The short term fuel trim values change rapidly in response to the HO2S
signal voltages. These changes fine-tune the engine fueling. The ideal fuel
trim values are around 0 percent. A positive fuel trim value indicates
that the PCM is adding fuel in order to compensate for a lean condition.
A negative fuel trim value indicates that the PCM is reducing the amount
of fuel in order to compensate for a rich condition.
When the PCM determines that the short term fuel trim is out of the
operating range, the following DTCs will set:
| • | DTC P0171 Bank 1 Too Lean |
| • | DTC P0172 Bank 1 Too Rich |
| • | DTC P0174 Bank 2 Too Lean |
| • | DTC P0175 Bank 2 Too Rich |
Long Term Fuel Trim
The long term fuel trim is a matrix of cells arranged by RPM and manifold
absolute pressure (MAP). Each cell of the long term fuel trim is a register
like the short term fuel trim. When the engine operating conditions change,
the PCM will switch from cell to cell in order to determine which long
term fuel trim factor to use in the base pulse width equation.
While in any given cell, the PCM also monitors the short term fuel trim.
If the short term fuel trim is far enough from 0 percent, the PCM will
change the long term fuel trim value. Once the long term fuel trim value
is changed, that value should force the short term fuel trim back toward
0 percent. If the mixture is still not correct, the short term
fuel trim will continue to have a large deviation from the ideal 0 percent.
When this occurs, the long term fuel trim value will continue to change
until the short term fuel trim becomes balanced. Both the short term fuel
trim and long term fuel trim have limits which vary by calibration. If
the mixture is off enough so that the long term fuel trim reaches the
limit of fuel trim control, and still cannot correct the condition, the
short term fuel trim will also go to the short term fuel limit of control
in the same direction. If the mixture is still not corrected by both the
short term fuel trim and the long term fuel trim at both the short term
fuel trim extreme values, a fuel trim diagnostic trouble code (DTC) will
likely result. When the PCM determines that the long term fuel trim is
out of the operating range, the following DTCs will set:
| • | DTC P0171 Bank 1 Too Lean |
| • | DTC P0172 Bank 1 Too Rich |
| • | DTC P0174 Bank 2 Too Lean |
| • | DTC P0175 Bank 2 Too Rich |
During power enrichment conditions, the PCM sets the short term fuel
trim to 0 percent and freezes the fuel trim there until the power
enrichment is no longer in effect. This is done so the Closed Loop factor
and the long term fuel trim will not try to correct for the power enrichment
condition.
Fuel System Description Cab Chassis
Fuel System Overview
The fuel tank stores the fuel supply. The electric fuel pump sends fuel through
an in-line fuel filter to the fuel rail assembly. The fuel pump provides fuel at
a higher rate of flow than is needed by the fuel injectors. The fuel pressure regulator
keeps fuel available to the injectors at a regulated pressure. A separate pipe returns
unused fuel to the fuel tank.
Fuel Tank
The fuel tanks store the fuel supply. The front fuel tank is located on the
left side of the vehicle. On duel-tank applications, the secondary fuel tank is located
in the rear of the vehicle. The fuel tanks are each held in place by two metal straps
that attach to the frame. The fuel tanks are molded from high density polyethylene.
Fuel Fill Pipe
The fuel fill pipe has a built-in restrictor in order to prevent refueling with
leaded fuel. Once the fill vent is obstructed, fuel backs up the fill pipe and trips
the dispensing nozzle. The front fuel tank vent runs into the rear tank to the top
of the filler pipe assembly, which in turn vents to atmosphere. The fuel tank vent
valves are connected and route to the canister to collect hydrocarbon emissions during
operation of the vehicle.
Fuel Sender Assembly
The fuel sender assembly consists of the following major components:
| • | The fuel sender assembly (2) |
Fuel Level Sensor
The fuel level sensor is a part of the fuel sender assembly. 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 amount of fuel in the
fuel tank. On models that are equipped with a single fuel tank, the fuel level sensor
is wired directly to the instrument panel (I/P) cluster. On models equipped with
dual fuel tanks, both fuel level sensors are wired to the powertrain control module
(PCM). The PCM calculates the total fuel level in both tanks. The instrument panel
(I/P) fuel gage displays the total fuel level in both fuel tanks.
Fuel Pump
The fuel pump is mounted in the fuel sender assembly. The fuel pump is an electric
high pressure pump. The fuel pump provides fuel at a higher rate of flow than is
needed by the fuel injectors. Excess fuel from the fuel rail assembly returns to the
fuel tanks through the fuel return pipe. The fuel pump delivers a constant flow of
fuel to the engine even during low fuel conditions. The powertrain control module
(PCM) 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.
Secondary Fuel Pump - Dual Tanks
The secondary fuel pump is a low pressure electric transfer pump. The secondary
fuel pump is mounted outside the fuel tank on the left side of the body crossmember.
The secondary fuel pump transfers fuel from the secondary fuel tank to the primary
fuel tank when commanded by the powertrain control module (PCM). The PCM grounds
the secondary fuel pump relay control circuit. Fuel level is continuously monitored
in both tanks by the PCM. When the fuel level in the primary tank is approximately
2 gallons lower than that in the secondary tank, the secondary fuel pump is energized
and approximately 4 gallons of fuel is transferred to the primary tank. The above
actions continue until the primary tank fuel level is low enough that it will accept
all of the remaining fuel in the secondary tank, at which time the secondary fuel
pump is energized until the secondary tank is empty.
Fuel Strainer
The fuel strainer attaches to the lower end of the fuel sender. The fuel strainer
is made of woven plastic. The functions of the fuel strainer are to filter contaminants
and to wick fuel. The fuel strainer is self-cleaning and normally requires no maintenance.
Fuel stoppage at this point indicates that the fuel tank contains an abnormal amount
of sediment or water.
Fuel Filter
The fuel filter is located on the fuel feed pipe, between the fuel pump and
the fuel injectors. The paper filter element (2) traps particles in the fuel
that may damage the fuel injection system. The filter housing (1) 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. Replace
a restricted fuel filter.
Fuel Feed and Return Pipes
The fuel feed pipe carries fuel from the fuel tank to the fuel rail assembly.
The fuel return pipe carries fuel from the fuel rail assembly back to the fuel tanks.
The fuel pipes consist of 2 sections:
| • | The rear fuel pipe assemblies are located from the top of the fuel tank
to the chassis fuel pipes. The rear fuel pipes are constructed of nylon. |
| • | The chassis fuel pipes are located under the vehicle and connect the rear
fuel pipes to the fuel rail pipes. The chassis fuel pipes are constructed of steel. |
Nylon Fuel Pipes
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 formed around gradual turns
under the vehicle. However, if nylon fuel pipes are forced into sharp bends, the
pipes kink and restrict the fuel flow. Also, once exposed to fuel, nylon pipes may
become stiffer and are more likely to kink if bent too far. Take special care when
working on a vehicle with nylon fuel pipes.
Quick-Connect Fittings
Quick-connect fittings provide a simplified means of installing and connecting
fuel system components. The fittings consist of a unique female connector and a compatible
male pipe end. O-rings, located inside the female connector, provide the fuel seal.
Integral locking tabs inside the female connector hold the fittings together.
Fuel Pipe O-Rings
O-rings seal the connections in the fuel system. Fuel system O-ring seals are
made of special material. Service the O-ring seals with the correct service part.
Fuel Rail Assembly
The fuel rail assembly attaches to the engine intake manifold. The fuel rail
assembly performs the following functions:
| • | Positions the injectors (3) in the intake manifold |
| • | Distributes fuel evenly to the injectors |
| • | Integrates the fuel pressure regulator (2) with the fuel metering system |
Fuel Injectors
The Multec 2 fuel injector assembly is a solenoid device, controlled
by the powertrain control module (PCM), that meters pressurized fuel to a single engine
cylinder. The PCM energizes the high-impedance (12.0 ohms) injector solenoid (2)
to open a normally closed ball valve (3). 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 2 machined holes that control the fuel flow, generating
a spray of finely atomized fuel at the injector tip. Fuel from the injector tip is
directed at the intake valve, causing it to become further atomized and vaporized
before entering the combustion chamber. This fine atomization improves fuel economy
and emissions.
Fuel Pressure Regulator Assembly
The fuel pressure regulator is a vacuum operated diaphragm relief valve. The
diaphragm has fuel pressure on one side and regulator spring pressure and intake
manifold vacuum on the other side. The fuel pressure regulator compensates for changes
in intake manifold vacuum by changing the fuel pressure. In this way, the fuel pressure
regulator maintains a constant pressure differential across the fuel injectors under
all operating conditions.
Fuel Metering Modes of Operation
The powertrain control module (PCM) monitors voltages from several sensors in
order to determine how much fuel to give the engine. The PCM 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 ignition is first turned ON, the powertrain control module (PCM) energizes
the fuel pump relay for 2 seconds. This allows the fuel pump to build pressure
in the fuel system. The PCM calculates the air/fuel ratio based on inputs from the
engine coolant temperature (ECT), mass air flow (MAF), manifold absolute pressure
(MAP), and throttle position (TP) sensors. The system stays in starting mode until
the engine speed reaches a predetermined RPM.
Clear Flood Mode
If the engine floods, clear the engine by pressing the accelerator pedal down
to the floor and then crank the engine. When the throttle position (TP) sensor is
at wide open throttle, the powertrain control module (PCM) reduces the fuel injector
pulse width in order to increase the air to fuel ratio. The PCM 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 PCM returns to the starting mode.
Run Mode
The run mode has 2 conditions called 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 powertrain control module (PCM) ignores the signal from
the heated oxygen sensor (HO2S). The PCM calculates the air/fuel ratio based on inputs
from the engine coolant temperature (ECT), mass air flow (MAF), manifold absolute
pressure (MAP), and throttle position (TP) sensors. The system stays in Open Loop
until meeting the following conditions:
| • | Both HO2S have varying voltage output, showing that they are 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 electrically erasable programmable read-only memory (EEPROM). The
system begins Closed Loop operation after reaching these values. In Closed Loop,
the PCM calculates the air/fuel ratio (injector on-time) based upon the signal from
various sensors, but mainly from the HO2S. This allows the air/fuel ratio to stay
very close to 14.7:1.
Acceleration Mode
When the driver pushes on the accelerator pedal, air flow into the cylinders
increases rapidly. To prevent possible hesitation, the powertrain control module
(PCM) increases the pulse width to the injectors to provide extra fuel during acceleration.
This is also known as power enrichment. The PCM determines the amount of fuel required
based upon the throttle position, the coolant temperature, the manifold air pressure,
the mass air flow, and the engine speed.
Deceleration Mode
When the driver releases the accelerator pedal, air flow into the engine is
reduced. The powertrain control module (PCM) monitors the corresponding changes in
throttle position, manifold air pressure, and mass air flow. The PCM 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.
Battery Voltage Correction Mode
When the battery voltage is low, the powertrain control module (PCM) 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 PCM 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 powertrain control module (PCM) controls the air/fuel metering system in
order to provide the best possible combination of driveability, fuel economy, and
emission control. The PCM monitors the heated oxygen sensor (HO2S) signal voltage
while in Closed Loop and regulates the fuel delivery by adjusting the pulse width
of the fuel injectors based on this signal. The ideal fuel trim values are around
0 percent for both short term and long term fuel trim. A positive fuel trim
value indicates the PCM is adding fuel in order to compensate for a lean condition
by increasing the pulse width. A negative fuel trim value indicates that the PCM 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 short term and long
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 the fueling in order to re-center and restore
control to short term fuel trim. A scan tool can be used to monitor the short term
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 PCM selects the cells
based on the engine speed and engine load. If the PCM detects an excessive lean or
rich condition, the PCM will set a fuel trim diagnostic trouble code (DTC).
