Halderman J.D., Linder J. Automotive Fuel and Emissions Control Systems
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WIDE-BAND OXYGEN SENSORS 231
ATMOSPHERE
PLATINUM
ELECTRODES
ZIRCONIA
ELECTROLYTE
SENSOR
SIGNAL
CIRCUIT
EXHAUST
STREAM
HEATER
CONTROL
CIRCUIT
FIGURE 18–4 A typical heated zirconia oxygen sensor, showing
the sensor signal circuit that uses the outer (exhaust) electrode
as negative and the ambient air side electrode as the positive.
HEATER
PLATINUM
ELECTRODES
ZIRCONIA
ELEMENT
CUP TYPE HEATED OXYGEN SENSOR
ATMOSPHERE
FIGURE 18–3 Most conventional zirconia oxygen sensors
and some wide-band oxygen sensors use the cup-type design.
CONSTRUCTION A typical zirconia oxygen sensor has the
sensing element in the shape of a thimble and is often referred to
as follows:
Thimble design
Cup design
Finger design
SEE FIGURE 18–3 .
A typical zirconia oxygen sensor has a heater inside the
thimble and does not touch the inside of the sensor. The sensor
is similar to a battery that has two electrodes and an electrolyte.
The electrolyte is solid and is the zirconia (zirconium dioxide).
There are also two porous platinum electrodes, which have the
following functions:
Exhaust side electrode —This electrode is exposed to
the exhaust stream.
Ambient side electrode —This electrode is exposed to
outside (ambient) air and is the signal electrode, also
called the reference electrode or ambient air electrode.
SEE FIGURE 18–4 .
The electrolyte (zirconia) is able to conduct electrons as
follows:
If the exhaust is rich, O
2
from the reference (inner) elec-
trode wants to flow to the exhaust side electrode, which
results in the generation of a voltage.
If the exhaust is lean, O
2
flow is not needed, and, as a
result, there is little, if any, electron movement, and there-
fore no voltage is being produced.
HEATER CIRCUITS The heater circuit on conventional
oxygen sensors requires 0.8 to 2 amperes, and it keeps the
sensor at about 600°F (315°C).
A wide-band oxygen sensor operates at a higher tempera-
ture than a conventional HO2S from 1,200°F to 1,400°F (650°C
to 760°C). The amount of electrical current needed for a wide-
band oxygen sensor is about 8 to 10 amperes.
PLANAR DESIGN In 1998, Bosch introduced a wide-band
oxygen sensor that is flat and thin (1.5 mm, or 0.006 in.),
known as a planar design and not in the shape of a thimble, as
previously constructed. Now several manufacturers produce a
similar planar design wide-band oxygen sensor. Because it is
thin, it is easier to heat than older styles of oxygen sensors and
as a result can achieve closed loop in less than 10 seconds.
This fast heating, called light-off time (LOT), helps improve
fuel economy and reduces cold-start exhaust emissions. The
type of construction is not noticed by the technician, nor does
it affect the testing procedures.
A conventional oxygen sensor can be constructed using
a planar design instead of the thimble-type design. A planar
design has the following features:
The elements including the zirconia electrolyte and the
two electrodes and heater are stacked together in a flat-
type design.
The planar design allows faster warm-up because the
heater is in direct contact with the other elements.
Planar oxygen sensors are the most commonly used.
Some planar designs are used as a conventional narrow-
band oxygen sensor.
The sandwich-type design of the planar style of oxygen sen-
sor has the same elements and operates the same but is stacked
in the following way from the exhaust side to the ambient air side:
Exhaust stream
Outer electrode
Zirconia (ZiO
2
) (electrolyte)
Inner electrode (reference or signal)
Outside (ambient) air
Heater
SEE FIGURE 18–5
232 CHAPTER 18
OUTER ELECTRODE (EXHAUST)
INNER ELECTRODE (ATMOSPHERE)
ZIRCONIUM DIOXIDE
REFERENCE AIR (ATMOSPHERE)
HEATER
(-) SIGNAL
(+) SIGNAL
PCM
HEATER POWER
HEATER GROUND
FIGURE 18–5 A planar design zirconia oxygen sensor places
all of the elements together, which allows the sensor to reach
operating temperature quickly.
NOTE: Another name for a conventional oxygen sensor
is a
Nernst cell. The Nernst cell is named for Walther
Nernst, 1864–1941, a German physicist known for his
work in electrochemistry.
CONSTRUCTION In a conventional zirconia oxygen sensor,
a bias or reference voltage can be applied to the two platinum
electrodes, and then oxygen ions can be forced (pumped) from
the ambient reference air side to the exhaust side of the sensor.
If the polarity is reversed, the oxygen ion can be forced to travel
in the opposite direction.
A dual cell planar-type wide-band oxygen sensor is made
like a conventional planar O2S and is labeled Nernst cell. Above
the Nernst cell is another zirconia layer with two electrodes,
which is called the pump cell. The two cells share a common
ground, which is called the reference.
DUAL-CELL PLANAR WIDE-
BAND SENSOR OPERATION
ZIRCONIA
DIFFUSION CHAMBER
REFERENCE CHAMBER
EXHAUST
HEATER
7
8
6
2
3
4
CALIBRATION
RESISTOR
PCM
REFERENCE
PUMP
CELL
NERNST
CELL
HEATER CONTROL (PWM)
GROUND
5V
TO HEATER RELAY (+)
FIGURE 18–6 The reference electrodes are shared by the Nernst cell and the pump cell.
There are two internal chambers:
The air reference chamber is exposed to ambient air.
The diffusion chamber is exposed to the exhaust
gases.
Platinum electrodes are on both sides of the zirconia elec-
trolyte elements, which separate the air reference chamber and
the exhaust-exposed diffusion chamber.
OPERATION The basic principle of operation of a typical
wide-band oxygen sensor is that it uses a positive or nega-
tive voltage signal to keep a balance between two sensors.
Oxygen sensors do not measure the quantity of free oxygen in
the exhaust. Instead, oxygen sensors produce a voltage that is
based on the ion flow between the platinum electrodes of the
sensor to maintain a stoichiometric balance.
An example follows:
If there is a lean exhaust, there is oxygen in the exhaust,
and the ion flow from the ambient side to the exhaust
side is low.
If there were rich exhaust, the ion flow is increased to
help maintain balance between the ambient air side and
the exhaust side of the sensor.
The PCM can apply a small current to the pump cell elec-
trodes, which causes oxygen ions through the zirconia into or
out of the diffusion chamber. The PCM pumps O
2
ions in and
out of the diffusion chamber to bring the voltage back to 0.45,
using the pump cell.
The operation of a wide-band oxygen sensor is best de-
scribed by looking at what occurs when the exhaust is stoichio-
metric, rich, and lean.
SEE FIGURE 18–6 .
STOICHIOMETRIC
When the exhaust is at stoichiometric (14.7:1 air-fuel
ratio), the voltage of the Nernst cell is 450 mV (0.450 V).
WIDE-BAND OXYGEN SENSORS 233
EXHAUST
LEAN
EXHAUST
2
4
5
HEATER (+)
HEATER (-)
450mv
0
ma
PCM
PUMP CELL CONTROL
NERNST CELL INPUT
FIGURE 18–8 When the exhaust is lean, the PCM applies a positive current into the pump cell.
EXHAUST
RICH
EXHAUST
2
4
5
HEATER (+)
HEATER (-)
450mv
0
ma
PCM
PUMP CELL CONTROL
NERNST CELL INPUT
FIGURE 18–7 When the exhaust is rich, the PCM applies a negative current into the pump cell.
The voltage between the diffusion chamber and the air ref-
erence chamber changes from 0.45 V. This voltage will be
as follows:
Higher if the exhaust is rich
Lower if the exhaust is lean
The reference voltage remains constant, usually at
2.5 volts, but can vary depending on the year, make, and model of
vehicle and the type of sensor. Typical reference voltages include
the following:
2.2
2.5
2.7
3.3
3.6
RICH EXHAUST. When the exhaust is rich, the voltage between
the common (reference) electrode and the Nernst cell electrode
that is exposed to ambient air is higher than 0.45 V. The PCM
applies a negative current in milliamperes to the pump cell elec-
trode to bring the circuit back into balance.
SEE FIGURE 18–7 .
LEAN EXHAUST. When the exhaust is lean, the voltage between
the common (reference) electrode and the Nernst cell electrode
is lower than 0.45 V. The PCM applies a positive current in mil-
liamperes to the pump cell to bring the circuit back into balance.
SEE FIGURE 18–8 .
234 CHAPTER 18
STEP 3 Measure the heater circuit for proper resistance or cur-
rent flow.
STEP 4 Measure the reference voltage relative to ground. This
can vary but is generally 2.4 to 2.6 volts.
STEP 5 Using jumper wires, connect an ammeter and measure
the current in the pump cell control wire.
RICH EXHAUST (LAMBDA LESS THAN 1.00) When
the exhaust is rich, the Nernst cell voltage will move higher than
0.45 volt. The PCM will pump oxygen from the exhaust into the
diffusion gap by applying a negative voltage to the pump cell.
LEAN EXHAUST (LAMBDA HIGHER THAN 1.00)
When the exhaust is lean, the Nernst cell voltage will move
lower than 0.45 volt. The PCM will pump oxygen out of the
diffusion gap by applying a positive voltage to the pump cell.
The pump cell is used to pump oxygen into the diffusion
gap when the exhaust is rich. The pump cell applies a negative
voltage to do this:
Positive current lean exhaust
Negative current rich exhaust
SEE FIGURE 18–9 .
DUAL-CELL DIAGNOSIS
SCAN TOOL DIAGNOSIS Most service information speci-
fies that a scan tool be used to check the wide-band oxygen sensor.
This is because the PCM performs tests of the unit and can identify
faults. However, even wide-band oxygen sensors can be fooled if
there is an exhaust manifold leak or other fault that could lead to
false or inaccurate readings. If the oxygen sensor reading is false,
the PCM will command an incorrect amount of fuel. The scan data
shown on a generic (global) OBD-II scan tool will often be different
than the reading on the factory scan tool.
SEE CHART 18–1 for
an example of a Toyota wide-band oxygen sensor being tested
using a factory scan tool and a generic OBD-II scan tool.
SCAN TOOL DATA (PID) The following information will be
displayed on a scan tool when looking at data for a wide-band
oxygen sensor:
HO2S1 _________ mA
If the current is positive, this
means that the PCM is pumping
current in the diffusion gap
because of a rich exhaust.
If the current is negative, the PCM
is pumping current out of thediffu-
sion gap because of a lean exhaust.
Air-fuel ratio _________
Usually expressed in lambda.
One means that the exhaust is
at stoichiometric (14.7:1 air-fuel
ratio), and numbers higher than
1 indicate a lean exhaust, and
numbers lower than 1 indicate a
rich exhaust.
CHART 18–1
A comparison showing what a factory scan tool and a generic
OBD-II scan tool might display at various air-fuel ratios.
MASTER TECH TOYOTA
(FACTORY SCAN TOOL) OBD-II SCAN TOOL AIR-FUEL RATIO
2.50 V 0.50 V 12.5:1
3.00 V 0.60 V 14.0:1
3.30 V 0.66 V 14.7:1
3.50 V 0.70 V 15.5:1
4.00 V 0.80 V 18.5:1
When testing a wide-band oxygen sensor for proper operation,
perform the following steps:
STEP 1 Check service information and determine the circuit
and connector terminal identification.
STEP 2 Measure the calibration resistor. While the value of
this resistor can vary widely, depending on the type of
sensor, the calibrating resistor should still be checked
for opens and shorts.
NOTE: The calibration resistor is usually located
within the connector itself.
•
If open, the ohmmeter will read OL (infinity ohms).
•
If shorted, the ohmmeter will read zero or close to
zero.
DIGITAL MULTIMETER
TESTING
CONSTRUCTION A typical single-cell wide-band oxygen
sensor looks similar to a conventional four-wire zirconia oxygen
sensor. The typical single-cell wide-band oxygen sensor, usu-
ally called an air-fuel ratio sensor, has the following construc-
tion features:
It can be made using the cup or planar design.
Oxygen (O
2
) is pumped into the diffusion layer similar to
the operation of a dual-cell wide-band oxygen sensor.
SEE FIGURE 18–10 .
SINGLE-CELL WIDE-BAND
OXYGEN SENSORS
WIDE-BAND OXYGEN SENSORS 235
SENSOR 1
SENSOR 2
DIFFUSION
CHAMBER
EXHAUST
STREAM
EXHAUST
STREAM
SENSOR 1 INPUT
GROUND REFERENCE
SENSOR 2 INPUT
PCM
COM
A
mA A
mV
V
V
mA
A
A
V
DIGITAL MULTIMETER
RECORD
MAX MIN
HZ
%
10
2
345
6
7
8
9
0
HZ
MAXMIN
MENU
RECORD
SAVE
RECALL
AUTO
RANGE
AUTOMOTIVE SCOPMETER
DMM
SCOPE
0.450V
FIGURE 18–9 Testing a dual-cell wide-band oxygen sensor can be done using a voltmeter or a scope. The meter reading is at-
tached to the Nernst cell and should read stoichiometric (450 mV) at all times. The scope is showing activity to the pump cell with
commands from the PCM to keep the Nernst cell at 14.7:1 air-fuel ratio.
Current flow reverses positive and negative.
It consists of two cell wires and two heater wires (power
and ground).
The heater usually requires 6 amperes and the ground
side is pulse-width modulated.
TESTING WITH A MILLIAMMETER The PCM controls
the single-cell wide-band oxygen sensor by maintaining a
voltage difference of 300 mV (0.3 V) between the two sensor
leads. The PCM keeps the voltage difference constant under
all operating conditions by increasing or decreasing current
between the element of the cell:
Zero (0 mA) represents lambda or stoichiometric air-fuel
ratio of 14.7:1
10 mA indicates a lean condition
10 mA indicates a rich condition
TESTING USING A SCAN TOOL A scan tool will display a
voltage reading but can vary depending on the type and maker
of scan tool.
SEE FIGURE 18–11 .
236 CHAPTER 18
2.9V
3.3V
AIR–FUEL
RATIO
SENSOR
A/F (-)
A/F (+)
ECM
FIGURE 18–10 A single-cell wide-band oxygen sensor has
four wires with two for the heater and two for the sensor itself.
The voltage applied to the sensor is 0.4 volt (3.3 2.9 0.4)
across the two leads of the sensor.
2.0
2.5
3.0
3.5
4.0
4.5
VOLTAGE AS DISPLAYED ON SCAN TOOL
AIR–FUEL RATIO
12 13 14 15 16 17 18 19
14.7:1 = 3.3 VOLTS
FIGURE 18–11 The scan tool can display various voltages but
will often show 3.3 volts because the PCM is controlling the sen-
sor by applying a low current to the sensor to achieve balance.
Wide-band oxygen sensors have a long life but can fail. Most
of the failures will cause a diagnostic trouble code (DTC) to set,
usually causing the malfunction indicator (check engine) lamp
to light.
WIDE-BAND OXYGEN
SENSOR PATTERN
FAILURES
However, one type of failure may not set a DTC when the
following occurs:
1. Voltage from the heater circuit bleeds into the Nernst cell.
2. This voltage will cause the engine to operate extremely
lean and may or may not set a diagnostic trouble code.
3. W hen testing indicates an extremely lean condition, un-
plug the connector to the oxygen sensor. If the engine
starts to operate correctly with the sensor unplugged, this
is confirmation that the wide-band oxygen sensor has
failed and requires replacement.
6. The heater used on a conventional zirconia oxygen sensor
uses up to 2 amperes and heats the sensor to about 600°F
(315°C). A broadband sensor heater has to heat the sensor
to 1,200°F to 1,400°F (650°C to 760°C) and requires up to
8 to 10 amperes.
7. A typical dual-cell wide-band oxygen sensor uses the
PCM to apply a current to the pump cell to keep the Nernst
cell at 14.7:1:
•
When the exhaust is rich, the PCM applies a negative
current to the pump cell.
•
When the exhaust is lean, the PCM applies a positive
current to the pump cell.
8. Wide-band oxygen sensors can also be made using a
single-cell design.
9. Wide-band oxygen sensors can be best tested using a
scan tool, but dual-cell sensors can be checked with a
voltmeter or scope. Single-cell sensors can be checked
using a milliammeter.
1. Wide-band oxygen sensors are known by many different
terms, including the following:
•
Broadband oxygen sensor
•
Wide-range oxygen sensor
•
Air-fuel ratio (AFR) sensor
•
Wide-range air-fuel (WRAF) sensor
•
Lean air-fuel (LAF) sensor
•
Air-fuel (AF) sensor
2. Wide-band oxygen sensors are manufactured using a cup
or planar design and are dual-cell or single-cell design.
3. A wide-band oxygen sensor is capable of furnishing the
PCM with exhaust air-fuel ratios as rich as 10:1 and as lean
as 23:1.
4. The use of a wide-band oxygen sensor allows the engine
to achieve more stringent exhaust emission standards.
5. A conventional zirconia oxygen sensor can be made in a
cup shape or planar design and is sometimes called a nar-
row band or two-step sensor.
SUMMARY
WIDE-BAND OXYGEN SENSORS 237
4. How does a wide-range oxygen sensor work?
5. How can a wide-band oxygen sensor be tested?
1. What type of construction is used to make wide-band
oxygen sensors?
2. Why are wide-band oxygen sensors used instead of con-
ventional zirconia sensors?
3. How is the heater different for a wide-band oxygen sensor
compared with a conventional zirconia oxygen sensor?
REVIEW QUESTIONS
6. A wide-band oxygen sensor needs to be heated to what
operating temperature?
a. 600°F (315°C)
b. 800°F (427°C)
c. 1,400°F (760°C)
d. 2,000°F (1,093°C)
7. The two internal chambers of a dual-cell wide-band oxygen
sensor include ______________ .
a. Single and dual
b. Nernst and pump
c. Air reference and diffusion
d. Inside and outside
8. When the exhaust is rich, the PCM applies a ______________
current into the pump cell.
a. Positive
b. Negative
9. When the exhaust is lean, the PCM applies a ______________
current into the pump cell.
a. Positive
b. Negative
10. A dual-cell wide-band oxygen sensor can be tested using
a ______________ .
a. Scan tool
b. Voltmeter
c. Scope
d. All of the above
1. A wide-band oxygen sensor was first used on a Honda in
what model year?
a. 1992
b. 1996
c. 2000
d. 2006
2. A wide-band oxygen sensor is capable of detecting the
air-fuel mixture in the exhaust from ______________ (rich)
to ______________ (lean).
a. 12:1 to 15:1
b. 13:1 to 16.7:1
c. 10:1 to 23:1
d. 8:1 to 18:1
3. A conventional zirconia oxygen sensor can be made with
what designs?
a. Cup and thimble
b. Cup and planar
c. Finger and thimble
d. Dual cell and single cell
4. A wide-band oxygen sensor can be made using what design?
a. Cup and thimble
b. Cup and planar
c. Finger and thimble
d. Dual cell and single cell
5. A wide-band oxygen sensor heater could draw how much
current (amperes)?
a. 0.8 to 2 A
b. 2 to 4 A
c. 6 to 8 A
d. 8 to 10 A
CHAPTER QUIZ
238 CHAPTER 19
chapter
FUEL PUMPS, LINES,
AND FILTERS
19
OBJECTIVES: After studying Chapter 19 , the reader should be able to: • Prepare for ASE Engine Performance (A8)
certification test content area “C” (Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair). • Describe how fuel pumps
work. • Describe how to check an electric fuel pump for proper pressure and volume delivery. • Discuss fuel tanks, lines and
filters. • Explain how to check a fuel-pressure regulator.
KEY TERMS: Accumulator 247 • Baffle 238 • Check valve 240 • Delivery system 238 • Filter basket 249 • Gerotor 244
• Hydrokinetic pump 244 • Inertia switch 240 • Onboard refueling vapor recovery (ORVR) 239 • Peripheral pump 244
• Residual or rest pressure 243 • Roller cell 243 • Rotary vane pump 243 • Side-channel pump 244 • Turbine pump 244
• Vacuum lock 240 • Vapor lock 240 • Volatile organic compound (VOC) 243
Creating and maintaining a correct air-fuel mixture requires a
properly functioning fuel and air
delivery system . Fuel delivery
(and return) systems use many if not all of the following com-
ponents to make certain that fuel is available under the right
conditions to the fuel-injection system:
Fuel storage tank, filler neck, and gas cap
Fuel tank pressure sensor
Fuel pump
Fuel filter(s)
Fuel delivery lines and fuel rail
Fuel-pressure regulator
Fuel return line (if equipped with a return-type fuel
delivery system)
Regardless of size and shape, all fuel tanks incorporate
most if not all of the following features:
Inlet or filler tube through which fuel enters the tank
Filler cap with pressure holding and relief features
An outlet to the fuel line leading to the fuel pump or fuel
injector
Fuel pump mounted within the tank
Tank vent system
Fuel pickup tube and fuel level sending unit
FUEL DELIVERY SYSTEM
FUEL TANKS
A vehicle fuel tank is made of corrosion-resistant steel or poly-
ethylene plastic. Some models, such as sport utility vehicles
(SUVs) and light trucks, may have an auxiliary fuel tank.
Tank design and capacity are a compromise between
available space, filler location, fuel expansion room, and fuel
movement. Some later-model tanks deliberately limit tank ca-
pacity by extending the filler tube neck into the tank low enough
to prevent complete filling or by providing for expansion room.
SEE FIGURE 19–1 . A vertical baffle in fuel tanks limits fuel
sloshing as the vehicle moves.
FUEL
TANK
FILLER
TUBE
EXPANSION
DOME
VENT TUBE TO
CHARCOAL CANISTER
FLOOR PAN
(CUTAWAY)
PRESSURE VACUUM
FILLER CAP
STRAP
FUEL LINE
FRONT
"J" BOLT
INSULATOR
FIGURE 19–1 A typical fuel tank installation.
FUEL PUMPS, LINES, AND FILTERS 239
TANK LOCATION AND MOUNTING Most vehicles
use a horizontally suspended fuel tank, usually mounted
below the rear of the floor pan, just ahead of or behind the
rear axle. Fuel tanks are located there so that frame rails
and body components protect the tank in the event of a
crash. To prevent squeaks, some models have insulated
strips cemented on the top or sides of the tank wherever it
contacts the underbody.
Fuel inlet location depends on the tank design and filler
tube placement. It is located behind a filler cap and is often a
hinged door in the outer side of either rear fender panel.
Generally, a pair of metal retaining straps holds a fuel
tank in place. Underbody brackets or support panels hold
the strap ends using bolts. The free ends are drawn under-
neath the tank to hold it in place, then bolted to other support
brackets or to a frame member on the opposite side of the
tank.
FILLER TUBES Fuel enters the tank through a large tube
extending from the tank to an opening on the outside of the
vehicle.
SEE FIGURE 19–2 .
Effective in 1993, federal regulations require manufac-
turers to install a device to prevent fuel from being siphoned
through the filler neck. Federal authorities recognized metha-
nol as a poison, and methanol used in gasoline is a definite
health hazard. Additionally, gasoline is a suspected carcinogen
(cancer-causing agent). To prevent siphoning, manufactur-
ers welded a filler-neck check-ball tube in fuel tanks. To drain
check-ball-equipped fuel tanks, a technician must disconnect
the check-ball tube at the tank and attach a siphon directly to
the tank.
SEE FIGURE 19–3 .
Onboard refueling vapor recovery (ORVR) systems
have been developed to reduce evaporative emissions during
refueling.
SEE FIGURE 19–4 . These systems add compo-
nents to the filler neck and the tank. One ORVR system utilizes
a tapered filler neck with a smaller diameter tube and a check
valve. When fuel flows down the neck, it opens the normally
closed check valve. The vapor passage to the charcoal can-
ister is opened. The decreased size neck and the opened air
passage allow fuel and vapor to flow rapidly into the tank and
the canister respectively. When the fuel has reached a pre-
determined level, the check valve closes, and the fuel tank
pressure increases. This forces the nozzle to shut off, thereby
preventing the tank from being overfilled.
PRESSURE-VACUUM FILLER CAP Fuel and vapors
are sealed in the tank by the safety filler cap. The safety cap
must release excess pressure or excess vacuum. Either con-
dition could cause fuel tank damage, fuel spills, and vapor
escape. Typically, the cap will release if the pressure is over
1.5to 2 PSI (10 to 14 kPa) or if the vacuum is 0.15 to 0.3PSI
(1 to 2 kPa).
FUEL PICKUP TUBE The fuel pickup tube is usually a
part of the fuel sender assembly or the electric fuel pump
assembly. Since dirt and sediment eventually gather on the
bottom of a fuel tank, the fuel pickup tube is fitted with a
filter sock or strainer to prevent contamination from entering
the fuel lines. The woven plastic strainer also acts as a water
separator by preventing water from being drawn up with the
fuel. The filter sock usually is designed to filter out particles
that are larger than 70 to 100 microns, or 30 microns if a
gerotor-type fuel pump is used. One micron is 0.000039 in.
SEE FIGURE 19–5 .
GROUND
WIRE
CHECK BALL
FIGURE 19–3 A view of a typical filler tube with the fuel tank
removed. Notice the ground strap used to help prevent the
buildup of static electricity as the fuel flows into the plastic
tank. The check ball looks exactly like a ping-pong ball.
LOWER NECK
HOSE
UPPER
NECK
CLAMPS
FIGURE 19–2 A three-piece filler tube assembly. The main
three parts include the upper neck, hose and lower neck.
FIGURE 19–4 Vehicles equipped with onboard refueling
vapor recovery usually have a reduced-size fill tube.
240 CHAPTER 19
NOTE: The human eye cannot see anything smaller than
about 40 microns.
The filter is made from woven Saran resin (copolymer of
vinylidene chloride and vinyl chloride). The filter blocks any
water that may be in the fuel tank, unless it is completely sub-
merged in water. In that case, it will allow water through the
filter. This filter should be replaced whenever the fuel pump is
replaced.
TANK VENTING REQUIREMENTS Fuel tanks must be
vented to prevent a vacuum lock as fuel is drawn from the
tank. As fuel is used and its level drops in the tank, the space
above the fuel increases. As the air in the tank expands to fill this
greater space, its pressure drops. Without a vent, the air pres-
sure inside the tank would drop below atmospheric pressure,
developing a vacuum which prevents the flow of fuel. Under
extreme pressure variance, the tank could collapse. Venting the
tank allows outside air to enter as the fuel level drops, prevent-
ing a vacuum from developing.
An EVAP system vents gasoline vapors from the fuel tank
directly to a charcoal-filled vapor storage canister and uses an
unvented filler cap. Many filler caps contain valves that open
to relieve pressure or vacuum above specified safety levels.
Systems that use completely sealed caps have separate pres-
sure and vacuum relief valves for venting.
Because fuel tanks are not vented directly to the atmo-
sphere, the tank must allow for fuel expansion, contraction,
FILTER
FUEL
TUBE
COUPLER
FOAM RUBBER
SLEEVE (ISOLATES
HIGH-FREQUENCY
NOISE)
FUEL LEVEL
SENDER
ELECTRIC
FUEL PUMP
RETURN
TUBE
FLOAT
FIGURE 19–5 The fuel pickup tube is part of the fuel sender
and pump assembly.
ROLLOVER LEAKAGE
PROTECTION
and overflow that can result from changes in temperature or
overfilling. One way is to use a dome in the top of the tank.
Many General Motors vehicles use a design that includes a
vertical slosh baffle that reserves up to 12% of the total tank
capacity for fuel expansion.
All vehicles have one or more devices to prevent fuel leaks in
case of vehicle rollover or a collision in which fuel may spill.
Variations of the basic one-way check valve may be in-
stalled in any number of places between the fuel tank and the
engine. The valve may be installed in the fuel return line, vapor
vent line, or fuel tank filler cap.
In addition to the rollover protection devices, some ve-
hicles use devices to ensure that the fuel pump shuts off when
an accident occurs. On some air vane sensors, a microswitch
is built into the sensor to switch on the fuel pump as soon as in-
take airflow causes the vane to lift from its rest position.
SEE
FIGURE 19–6 .
Ford vehicles use an inertia switch .
SEE FIGURE 19–7 .
The inertia switch is installed in the rear of the vehicle between
the electric fuel pump and its power supply. With any sudden
impact, such as a jolt from another vehicle in a parking lot, the
inertia switch opens and shuts off power to the fuel pump. The
switch must be reset manually by pushing a button to restore
current to the pump.
FUEL LINES
Fuel and vapor lines made of steel, nylon tubing, or fuel- resistant
rubber hoses connect the parts of the fuel system. Fuel lines
supply fuel to the throttle body or fuel rail. They also return ex-
cess fuel and vapors to the tank. Depending on their function,
fuel and vapor lines may be either rigid or flexible.
Fuel lines must remain as cool as possible. If any part of
the line is located near too much heat, the gasoline passing
through it vaporizes, and vapor lock occurs. When this hap-
pens, the fuel pump supplies only vapor that passes into the
injectors. Without liquid gasoline, the engine stalls, and a hot
restart problem develops.
The fuel delivery system supplies 10 to 15 PSI (69 to 103 kPa)
or up to 35 PSI (241 kPa) to many throttle-body injection units and
up to 60 PSI (414 kPa) for multiport fuel-injection systems. Fuel-
injection systems retain residual or rest pressure in the lines for
a half hour or longer when the engine is turned off to prevent hot
engine restart problems. Higher-pressure systems such as these
require special fuel lines.