the engine runs but displays driveability symptoms that may
or may not be fuel-related (hard starting, hesitation, misfiring,
power loss), first atempt to eliminate other possible causes
of the problem.
1.Check the vehicle's on-board diagnostic system; correct any
2.Check the ignition system: spark plugs, wires, coil, timing.
Observe ignition patterns on a scope to identify ignition and
3.Check for vacuum leaks at hoses, throttle body, intake manifold,
power brake booster, vacuum-powered accessories.
4.Check the EGR and PCV systems. A leaky EGR valve can imitate
a vacuum leak, leaning out the air/fuel mixture. A clogged PVC
valve or hose can cause a rich mixture, while an air leak may
create a lean condition.
5.Run a power balance test to reveal weak cylinders, a power
loss caused by leaky valves or clogged injectors.
Electrical --Check the fuse first. If it's blown, the most likely
reason is a hot short in the relay, or the wiring between the
relay and the pump. Next, check other electrical components
using a voltmeter and wiring diagram.
Especially make sure the pump relay is receiving the correct
voltage from the control module and is sending the correct voltage
to the fuel pump. Attach voltmeter leads to the hot and ground
terminals, turn the key on and check for correct voltage. Turn
the key off, move the hot lead to the relay's output terminal,
and turn the key on. The relay should click, and the voltmeter
should read the specified voltage. You have only seconds to
check this before the ECM shuts down the relay. If nothing happens
(no click, no needle), test voltage output at the relay driver
terminal on the engine control module. A voltage signal here
shows the ECM is supplying power, but the relay isn't responding.
(No voltage at the ECM would indicate a defective driver circuit.)
The final check is continuity in the wiring between the ECM
and the relay. If it's okay, replace the relay.
To isolate an electrical problem other than the relay, bypass
other components one at a time using a jumper wire. If the jumper
restores power to the fuel pump, replace the bypassed electrical
Pressure --If the engine runs but exhibits driveability problems
that suggest fuel delivery problems (too much or too little),
the only way to know for sure is by checking fuel-system pressures.
Hook up a pressure gauge, start the engine, and compare readings
to the manufacturer's specs.
Usually the operating pressure specification is at idle with
the vacuum hose attached to the regulator.Maximum system pressure
(wide-open throttle operation) is read with the hose disconnected
or pinched off.
If the engine hesitates when accelerating, monitor fuel pressure
during a test drive by fitting an EFI-approved hose (200 PSI)
long enough to reach the pressure gauge wherever it can be observed
from the driver's seat.
Read "deadhead" or maximum pump pressure (higher than
maximum fuel-system operating pressure) by momentarily pinching
off the return line while the engine is running, or by connecting
the gauge directly to the fuel line (no tee).
Check the system's ability to hold pressure after shutdown by
turning off the key and watching the gauge. If the pressure
drops suddenly, the pump check valve may be stuck open. If the
pressure bleeds off slowly, it may indicate a leaky check valve/fuel
line/ fuel injector, or a faulty regulator.
Too High . The mostly likely cause is a faulty regulator or
a clogged return line. Disconnect the return line at the regulator,
install another hose and reroute the return fuel to an appropriate
container. Start the engine and read the fuel pressure. If it's
now within specifications, check for a blocked or kinked return
line or a clogged or stuck roller valve in the line.
If fuel pressure is still too high with the return line disconnected,
check the fuel volume flowing through the bypass hose. If it's
little or none, pull a vacuum on the regulator with a hand pump.
No increase indicates a faulty regulator. But if the flow increases,
and the system pressure comes back down to spec, look for a
leaky, blocked or misrouted vacuum line.
T oo Low. The many possibilities include: a clogged in-tank
pump strainer; a plugged fuel filter; a blocked or kinked fuel
line; a leaky pulsator, fuel line or hose; a leaky pump check
valve; a defective fuel-pressure regulator; or simply the wrong
fuel pump for the application.
Check for filter and fuel line blockages by teeing a gauge into
the tank outlet line and reading fuel pressure there. A number
at least several PSI higher than at the engine indicates a restricted
filter or fuel line. If there's no difference in pressure between
outlet and engine, the pump is weak or its strainer is clogged.
Either way, you must pull to pump to find out.
Causes of low pump output include low voltage, a bad ground
wire and a leaky or defective pump check valve. Usually the
check valve is integral to the pump. If it's faulty you must
replace the pump. High levels of alcohol in fuel can cause rubber
check balls to swell and stick. A leaking coupling hose or the
pump's pulsator also may cause low pressure in the system.
Flow .Some manufacturer's specs for checking pump output require
measuring the volume of fuel delivered at idle over time. Do
this quickly and safely by disconnecting the return line at
the regulator and installing a bypass to a safe container. Check
the system's pressure at the same time.