The components of a typical
gasoline delivery system are fuel tanks, fuel lines, fuel filters, and fuel
Modern fuel tanks include devices
that prevent vapors from leaving the tank. For example, to contain vapors and
allow for expansion, contraction, and overflow that results from changes in the
temperature, the fuel tank has a separate air chamber dome at the top. Another
way to contain vapors is to use a separate internal expansion tank within the
main tank. All fuel tank designs provide some control of fuel height when the
tank is filled. Frequently, this is achieved by using vent lines within the
filler tube or tank. With tank designs such as this, only 90 percent of the tank
is over filled, leaving 10 percent for expansion in hot weather. Some vehicles
have an overfill limiting valve to prevent overfilling of the tank. If a tank is
filled to capacity, it overflows whenever the temperature of the fuel increases.
Fuel tanks can be constructed of
pressed corrosion-resistant steel, aluminum, or molded reinforced polyethylene
plastic. Aluminum and molded plastic fuel tanks are becoming more common as
manufacturers attempt to reduce the overall weight of the vehicle. Metal tanks
are usually ribbed to provide added strength. Seams are welded, and heavier
gauge steel is often used on exposed sections for added strength.
Most tanks have slosh baffles or
surge plates to prevent the fuel from splashing around inside the unit. In
addition to slowing down fuel movement, the plates tend to keep the fuel pick-up
or sending assembly immersed in the fuel during hard braking and acceleration.
The plates or baffles also have holes or slots in them to permit the fuel to
move from one end of the tank to the other. Except for rear engine vehicles, the
fuel tank in a passenger car is located in the rear of the vehicle for improved
The fuel tank is provided with an
inlet filler tube and cap. The location of the fuel inlet filler tube depends on
the tank design and tube placement. It is usually positioned behind the filler
cap or a hinged door in the center of the rear panel or in the outer side of
either rear fender panel. Vehicles designed for unleaded fuel use have a
restrictor in the filler tube that prevents the entry of the larger leaded fuel
delivery nozzle at the gas pumps. The filler pipe can be a rigid one-piece tube
soldered to the tank or a three-piece unit. The three-piece unit has a lower
neck soldered to the tank and an upper neck fastened to the inside of the body
sheet metal panel.
Filler tube caps are nonventing and
usually have some type of pressure-vacuum relief valve arrangement. Under normal
operating conditions the valve is closed, but whenever pressure or vacuum is
more than the calibration of the cap, the valve opens. Once the pressure or
vacuum has been relieved, the valve closes. Most pressure caps have four
antisurge tangs that lock onto the filler neck to prevent the delivery system's
pressure from pushing fuel out of the tank. By turning such a cap one-half turn,
the pressure in the tank will not be released all at once. Then, with another
quarter turn, the cap can be removed.
Starting with the 1976 model year,
a Federal Motor Vehicle Safety Standard (FMVSS 301) required a control on
gasoline leakage from passenger cars and certain light trucks and buses, after
they were subjected to barrier impacts and rolled over. Tests conducted under
these severe conditions showed the most common gasoline leak path was the
gasoline supply line from the fuel tank to the carburetor.
Most rollover leakage protection
devices used on carburetor-equipped engines are variations of a basic one-way
check valve. These protective check valves are usually installed in the fuel
vapor vent line between the tank and the vapor canister and at the carburetor
fuel feed or return line fitting. In some systems the check valve is part of the
carburetor inlet fuel filter.
Under normal operation, the
mechanical fuel pump pressure is sufficient to open the check valve and supply
fuel to the engine. However, if the vehicle is involved in a rollover accident,
fuel spills out of the carburetor, the engine stalls, and the fuel pump ceases
to operate. This decreases fuel system pressure to the point where the check
valve closes. This prevents fuel from reaching the carburetor where it would
A check valve might also be fitted
in the fuel tank filler cap, and most caps' pressure-vacuum relief valve
settings have been increased so fuel pressure cannot open them in a rollover.
Many electric fuel pumps found on
vehicles with fuel injection systems have an inertia switch that shuts off the
jump if the vehicle is involved in a collision or rolls over. The Ford inertia
switch consists of a permanent magnet, a steel ball inside a conical ramp, a
target plate, and a set of electrical contacts. The magnet holds the steel ball
in the bottom of the conical ramp. In the event of a collision, the inertia of
the ball causes it to break away from the magnet, roll up the conical ramp, and
strike the target plate. The force of the ball striking the plate causes the
electrical contacts in the inertia switch to open, cutting off power to the fuel
pump. The switch has a reset button that must be depressed to close the contacts
before the pump operates again.
Some fuel line systems contain a
fuel return arrangement that aids in keeping the gasoline cool, thus reducing
chances of vapor lock. The return system consists of a special fuel pump
equipped with an extra outlet fitting and necessary fuel line. The fuel return
line generally runs next to the conventional fuel line, except that flow is in
the opposite direction. The fuel return system allows a metered amount of cool
fuel to circulate through the tank and fuel pump, thus reducing vapor bubbles
caused by overheated fuel.
Some form of liquid vapor separator
is incorporated into most modern vehicles to stop liquid fuel or bubbles from
reaching the vapor storage canister or the engine crankcase. It can be located
inside the tank, on the tank, in fuel vent lines, or near the fuel pump. Check
the service manual for the exact location of the liquid vapor separator and line
Inside the fuel tank there is also
a sending unit that includes a pick-up tube and float-operated fuel gauge. The
fuel tank pick-up tube is connected to the fuel pump by the fuel line. Some
electric fuel pumps are combined with the sending unit. The pick-up tube extends
nearly, but not completely, all the way to the bottom of the tank. Rust, dirt,
sediment, water cannot be drawn up into the fuel tank filter, which can cause
clogging. The ground wire is often attached to the fuel tank unit.
Inspecting the Fuel Tank A
fuel tank should be inspected for leaks, road damage, corrosion, and rust on
metal tanks, loose, damaged, or defective seam, loose mounting bolts, and
damaged mounting straps. Leaks in the fuel tank, lines, or filter may cause
gasoline odor in and around the vehicle, especially during low-speed driving and
idling. In most cases, the fuel tank must be removed for servicing.
Fuel lines can be made of either
metal tubing, flexible nylon, or synthetic rubber hose. The latter must be able
to resist gasoline. It must also be nonpermeable, so gas and gas vapors cannot
evaporate through the hose. Ordinary rubber hose, such as that used for vacuum
lines, deteriorates when exposed to gasoline. Only hoses made for fuel systems
should be used for replacement. Similarly, vapor vent lines must be made of
material that resists attack by fuel vapors. Replacement vent hoses are usually
marked with the designation EVAP to indicate their intedned use. The inside
diameter of a fuel deliver hose is generally larger than that of a fuel return
Many fuel tanks have vent hoses to
allow air in the fuel tank to escape when the tank is being filled with fuel.
Vent hoses are usually installed alongside the filler neck.
The fuel lines carry fuel from the
fuel tank to the fuel pump, fuel filter, and carburetor or fuel injection
assembly. These lines are usually made of rigid metal, although some sections
are constructed of rubber hose to allow for car vibrations. This fuel line,
unlike filler neck or vent hoses, must work under pressure or vacuum. Because of
this, the flexible synthetic hoses must be stronger. This is especially true for
the hoses on fuel injection systems, where pressures reach 50 psi or more. For
this reason, flexible fuel line hose must also have special resistance
properties. Many auto manufacturers recommend that flexible hose be used only as
a delivery hose to the fuel metering unit in a fuel injection system. It should
not be used on the pressure side of the injector systems. This application
requires a special high-pressure hose.
All fuel lines should occasionally
be inspected for holes, cracks, leaks, kinks, or dents. Many fuel system
troubles can occur in the lines are blamed on the fuel pump or carburetor. For
instance, a small hole in the fuel line admits air but does not necessarily show
any drip marks under the car. Air can then enter the fuel line, allowing the
fuel to gravitate back into the tank. Then, instead of drawing fuel from the
tank, the fuel pump sucks only air through the hole in the fuel line. When this
condition exists, the fuel pump is frequently tested, and if there is
insufficient fuel, it is considered faulty, when in fact there is nothing wrong
with it. If a hole is suspected, remove the coupling at the tank and the pump
and pressurize the line with air. The leaking air is easily spotted.
Since the fuel is under pressure,
leaks in the line between the pump and carburetor or injectors are relatively
easy to recognize. When a damaged fuel line is found, replace it with one of
similar construction - steel with steel, and the flexible with nylon or
synthetic rubber. When installing flexible tubing, always use new clamps. The
old ones lose some of their tension when they are removed and do not provide an
effective seal when used on the new line.
Fuel supply lines from the tank to
the carburetor or injectors are routed to follow the frame along the
underchassis of vehicles. Generally, rigid lines are used extending from near
the tank to a point near the fuel pump. To absorb engine vibrations, the gaps
between the frame and tank or fuel pump are joined by short lengths of flexible
Steel tubing should inspected for
leaks, kinks, and deformation. This tubing should also be checked for loose
connections and proper clamping to the chassis. If the fuel tubing threaded
connections are loose, they must be tightened to the specified torque.
Any damaged or leaking fuel line -
either a portion or the entire length - must be replaced. To fabricate a new
fuel line, select the correct tube and fitting dimension and start with a length
that is slightly longer than the old line. With the old line as a reference, use
a tubing bender to form the same bends in the new line as those that exist in
the old. Although steel tubing can be bent by hand to obtain a gentle curve, any
attempt to bend a tight curve by hand usually kinks the tubing. To avoid
kinking, always use a bending tool.
The two most-used tubing fittings
are the compression fittings and the double-flare. The double-flare, which is
the most common, is made with a special tool that has an anvil and a cone. The
double-flaring process is performed in two steps. First, the anvil begins to
fold over the end of the tubing. Then, the cone is used to finish the flare by
folding the tubing back on itself, doubling the thickness, and creating two
The angle and size of the flare are
determined by the tool. Careful use of the double-flaring helps to produce
strong, leakproof connections.
The flare tool can also be used to
make sure nylon and synthetic rubber hoses stay in place. that is, to make sure
the connection is secure, put a partial double-lip flare on the end of the
tubing over which the hose is installed. This can be done quickly, with the
proper flaring tool, by starting out as if it was going to be a double-flare but
stopping halfway through the procedure. This provides an excellent sealing ridge
that does not cut into the hose. A clamp should be placed directly behind the
ridge on the hose caused by the raised section on the metal line.
There are a variety of clamps used
on fuel system lines, including the spring and screw types. The crimp clamps are
used most for metal tubing, but they require a special tool to install.
To control the rate of vapor flow
from the fuel tank to the vapor storage tank, a plastic or metal restrictor may
be placed in either the end of the vent pipe or in the vapor-vent hose itself.
When the latter hose must be replaced, the restrictor must be removed from the
old vent hose and installed in the new one.
Automobiles and light trucks
usually have an in-tank strainer and a gasoline filter. The strainer, located in
the gasoline tank, is made of a finely woven fabric. The purpose of this
strainer is to prevent large contaminant particles from entering the fuel system
where they could cause excessive fuel pump wear or plug fuel metering devices.
It also helps to prevent passage of any water that might be present in the tank.
Servicing of the fuel tank strainer is seldom required.
The gasoline filter is usually
located in the engine compartment and is the one this section examines because
it is replaceable and might require service on a regular basis. The most common
types of gasoline filters are in-carburetor filters and in-line filters.
In-Carburetor Filters There
are three basic types of in-carburetor gasoline filters. Pleated paper filters
use pleated paper as the filtering medium. Paper elements are more efficient
than screen-type elements, such as nylon or wire mesh, in removing and trapping
small particles, as well as large-size contaminants. Sintered bronze filters are
often referred to as a stone or ceramic filter. Screw-in filters are designed to
screw into the carburetor fuel inlet. The fuel line attaches to a fitting on the
filter. This filter has a magnetic element to remove metallic contamination
before it reaches the carburetor.
In-Line Filters In-line
gasoline filters are installed in the fuel line. In carbureted engines, the
in-line gasoline filter is usually installed between the fuel pump and the
carburetor. In vehicles with a fuel injection system, the location of the fuel
filter is determined by the manufacturer. Fitted with a pleated paper element,
the in-line filter is sometimes installed as an extra protective measure. The
optionally installed in-line filter then works in conjunction with the in-tank
and in-carburetor gasoline filters. Because of its large capacity, an in-line
filter is often the most economical solution to a fuel system's contamination
problems. An arrow on the filter shows the direction of fuel flow.
Servicing Filters Fuel
filters and elements are serviced by replacement only. Replacing the gasoline
filter or element at the intervals recommended by the vehicle or engine
manufacturer is the most effective method of minimizing fuel starvation and
other carburetor problems. On occasions when the fuel system has been subjected
to excessive amounts of contaminants, more frequent filter changes may be
The fuel pump is the device that
draws the fuel from the fuel tank through the fuel lines to the engine's
carburetor or injectors. Basically, there are two types of fuel pumps:
mechanical and electrical. The latter is the most commonly used today.
Mechanical Fuel Pump The
mechanical fuel pump has a synthetic rubber diaphragm inside the unit that is
actuated by an eccentric located on the engine's camshaft. As the camshaft
rotates during engine operation, a shaft or rocker arm in the pump is moved up
and down or back and forth, depending on the fuel pump's position on the engine.
This causes the diaphragm to move back and forth, drawing fuel from the fuel
The mechanical fuel pump is located
on the engine block near the front of the engine. Normally, it is a sealed unit
that cannot be repaired. If the pump leaks from either the vent hole or from a
seam, it must be replaced. If engine performance indicates inadequate fuel, the
pump, while mounted on the engine, should be tested for pressure and volume.
Incorrect fuel pump pressure and
low volume (capacity of flow rate) are the two most likely fuel pump troubles
that affect engine performance. Low pressure causes a lean mixture. Excessive
pressure causes high fuel consumption and an overly rich mixture with fuel
injection or carburetor flooding. And, low volume causes fuel starvation at high
To determine if the fuel pump is in
satisfactory operating condition, tests for both fuel pump pressure and fuel
pump capacity (volume) should be performed. These tests are performed with the
fuel pump installed on the engine and the engine at normal operating temperature
and at idle speed.