pump. Before testing a fuel pump, check for the
Restricted fuel filters
Smashed or kinked fuel line or hoses
Air leak into the vacuum side of pump or line
Carburetor or injection system problems
Ignition system problems
Low engine compression
To measure fuel pump pressure, connect a pressure
gauge to the output line of the fuel pump. Start and idle
the engine at the rpm specified by the manufacturer with
a mechanical fuel pump. With an electric fuel pump,
you may only need to activate the pump motor.
Compare your pressure reading to the manufacturer's
specifications. Fuel pressure for a carburetor type
system is approximately 4 to 6 psi. A gasoline injection
system will usually have a high-pressure output,
varying from 15 to 40 psi. If fuel pump pressure is NOT
within specifications. check the pump volume, the
lines, and the filters before replacing the pump.
Fuel pump volume is the amount of fuel the pump
can deliver in a specific amount of time. It is measured
by allowing the fuel to pour into a graduated (marked)
container for a certain amount of time (normally 30
seconds). Route an output line from the fuel pump to a
measuring container. For safety, a valve or clip should
be installed to control fuel flow into the container. With
the engine idling at a set speed, allow the fuel to pour
into the container for the prescribed amount of time.
Close off the clip or the valve and compare volume
output to the specifications. Output should be a
minimum of 1 pint in 30 seconds for carburetor systems.
Fuel injection systems have a slightly higher output
from the supply pump. Always refer to the service
manual specifications for the particular fuel pump and
vehicle. If the fuel pump fails both pressure and volume
test, then check the fuel pump vacuum.
A vacuum test will eliminate possible problems in
the fuel lines, the hoses, the filters, and the pickup
screen in the tank. For example, a clogged fuel pickup
screen could make the fuel pump fail the volume test.
To measure vacuum, connect a vacuum gauge to the
inlet side of the pump, leaving the fuel hose from the
volume test in the graduated container.
control valve on the hose and start the engine and allow
it to run on the fuel in the carburetor, or connect voltage,
to an electric pump. Compare your reading with the
manufacturers specifications. Normally, fuel pump
vacuum should be about 7 to 10 in/hg. A good reading
indicates a good fuel pump. If the pump failed the
pressure or volume test but passed the vacuum test, the
fuel supply lines and filter may be at fault.
Fuel Lines and Hoses
Fuel lines and hoses carry fuel from the tank to the
engine. The main fuel line allows the fuel pump to draw
fuel out of the tank. The fuel is pulled through this line
to the pump and then to the carburetor, or metering
section of the injection system.
Fuel lines are normally made of double wall steel
tubing. For fire safety, a fuel line must be able to
withstand the constant and severe vibration produced
by the engine and road surface. Lines are placed away
from exhaust pipes, mufflers, and manifolds, so that
excessive heat will not cause vapor lock. They are
attached to the frame, the engine, and other units, so the
effects of vibration will be minimized.
Fuel hoses, made of synthetic rubber, are used
where severe movement occurs between parts. A
flexible hose can absorb movement without breaking.
Hose clamps are required to secure fuel hoses to the fuel
lines or to metal fittings.
Faulty fuel lines and hoses are a common source of
fuel leaks. Fuel hoses can become hard and brittle after
being exposed to the engine heat and the elements.
Engine oil can soften and swell them. Always inspect
hoses closely and replace any in poor condition. Metal
fuel lines rarely cause problems; however, they should
be replaced if they become smashed, kinked, rusted, or
leaking. Remember these rules when working with fuel
lines and hoses:
Place a rag around the fuel line fitting during
removal. This action will keep fuel from spraying on
you or on a hot engine. Use a flare nut or tubing wrench
on fuel line fittings.
Use only approved double wall steel tubing for
fuel lines. NEVER use copper or plastic tubing.
Make smooth bends when forming a new fuel
line. Use a bending spring or bending tool.
Form double lap flares on the ends of fuel lines.
A single lap flare is NOT approved for fuel lines.
Reinstall fuel line hold-down clamps and
brackets. If not properly supported, the fuel line can
vibrate and fail.