it seems like you’re processing more fuel pump warranty returns than
ever from your professional customers, it’s probably because many
simply don’t understand modern automotive fuel delivery technology.
Although electronic fuel injection has become the norm since the late
1980s, many technicians still don’t understand the value of having a
good diagnostic procedure for testing electric fuel pump functions.
spin-off, of course, is that it’s often the jobber store that’s asked
to take up the slack when an allegedly defective fuel pump is returned
for credit or exchange.
Too often, neither party really
understands the issues involved in fuel delivery systems testing.
Because being able to speak the language is always part of the
solution, it’s important for the parts professional to understand the
basic issues involved in fuel pump diagnosis.
PROCEDURE, PROCEDURE, PROCEDURE
begin our discussion of fuel pump diagnostics with a typical cranking,
no-starting complaint. The most efficient way to diagnose a
fuel-related cranking, no-start engine complaint on any OBD II vehicle
produced since 1996 is to attach a professional-level scan tool to the
vehicle’s diagnostic connector.
A good technician can use the scan
tool to activate the fuel pump circuit. If he activates the pump and
hears it running, he knows the powertrain control module’s (PCM’s) fuel
pump electrical circuit is functioning correctly.
crankshaft position (CKP) sensor signal is required to activate both
the ignition and fuel delivery systems, a technician should also check
for the presence of a CKP signal.
Of course, some vehicles won’t
show a cranking speed or CKP signal on some scan tools, but if the
engine doesn’t produce an engine speed signal, the CKP and ignition
system function can be verified by simply removing a spark plug wire
and testing for the presence of spark. If the engine lacks spark, the
tech should then use a professional automotive lab scope to evaluate
the CKP signal.
But let’s say the CKP, cranking speed and spark
availability pass their initial tests. If the electrical part of the
fuel system is functioning correctly, the next step might be simply to
remove the engine’s air inlet and squirt some aerosol throttle body
cleaner into the air intake. If the engine starts momentarily, the fuel
delivery system is likely at fault.
SYSTEMS, SYSTEMS, SYSTEMS
fuel pump warranty returns could be eliminated if the technician simply
followed a prescribed diagnostic procedure and understood how the
various operating systems in the modern vehicle affect fuel pump
operation. The basic operating system for an electric fuel pump is
relatively simple. When the key is turned on, the powertrain control
module (PCM) closes the fuel pump relay, which activates the fuel pump
for about three seconds to pressurize the fuel injectors. If you listen
carefully, you can hear the characteristic whine of the fuel pump
momentarily activating on your vehicle when you turn the ignition key
When the engine is started, the crankshaft position sensor
(CKP) indicates to the PCM that the engine is cranking and the fuel
pump is again activated to supply fuel to the engine. When the engine
starts, the CKP signals the PCM to keep the fuel pump and fuel delivery
system running. Always keep in mind that neither the fuel nor ignition
system will function without a valid CKP signal.
Now let’s add the
anti-theft or security system function to the fuel delivery system
function. The ignition keys on most modern vehicles contain a resistor
or “identification chip” that identifies the key to the PCM or
anti-theft module. If the anti-theft system malfunctions on many
General Motors vehicles, the PCM or anti-theft module will deactivate
the fuel pump after about five seconds of operation.
The PCMs on
many Ford Motor Co. vehicles simply won’t allow the fuel pump to
activate if the key can’t be identified. Unfortunately for parts
distribution, many fuel pumps have been replaced because of problems in
the anti-theft or security systems.
Last, all vehicles have
mechanisms that deactivate the fuel pump if the engine stalls or if the
vehicle is involved in an impact collision.
Ford vehicles, for
example, install an impact inertia switch between the fuel pump and
pump relay. Occasionally, the inertia switch will accidentally
disengage on a rough road, causing the pump to shut off. In this case,
the inertia switch must be reset by pressing the button on the switch.
shut off the fuel pump in the event of a stall, the PCM on some imports
must “see” an engine running signal from the air flow sensor to
maintain fuel pump operation. On all other vehicles, the PCM must see a
valid signal from the crankshaft position sensor to keep the fuel pump
This quick review of operating systems should
illustrate why scan tool diagnostics should be used to diagnose all
suspected fuel pump failures. If the anti-theft or security system is
defeating fuel pump operation, a trouble code will be stored in the
PCM’s diagnostic memory. In many cases, a fuel pump warranty return can
be attributed to something as innocent as a missing chip on the
ignition key or an inertia switch that must be reset per owner’s manual
FUEL PRESSURE TESTS
Fuel pressure testing is
relatively simple on conventional fuel delivery systems. An accurate
fuel pressure gauge is attached to the fuel line or fuel injector rail
and the pressure is tested key-on, engine off and key-on, engine
running. The pressure must meet manufacturer’s specifications.
A low-pressure reading can be caused by a faulty fuel pressure regulator or worn pump.
vehicles with two-line fuel systems have the fuel pressure regulator
located on the engine while more modern single fuel line systems have
the regulator built into the fuel pump module itself. Because a fuel
pump can produce pressure but fail a volume test, a professional
mechanic will also test fuel pump volume to see if the pump will
produce at least one pint (preferably two) per minute.
popular introduction of pulse-modulated (PM) electric fuel pumps early
in this decade, fuel pump pressure testing procedures have changed in
recent years. Pulse-modulated fuel pumps control fuel pressure by
changing the speed of the fuel pump. In brief, the PCM or the fuel pump
control module changes the fuel pump speed by rapidly switching the
fuel pump on and off. Because fuel pressure varies widely according to
driving conditions on PM systems, most pulse-modulated systems do not
have fuel pressure testing ports. Instead, a professional-level scan
tool and technical information system is required to accurately monitor
and diagnose the fuel pump’s pulse modulation cycle.
auto repair shops are driven by the flat-rate labor system in which a
mechanic is paid by the job rather than by the hour. Consequently, in
his haste to produce as many jobs per day as possible, it’s very easy
for a mechanic to skip some essential steps in fuel pump installation.
mechanics, for example, forget to check the fuel tank for dirt and
water contamination, either of which can cause a repeat fuel pump
failure. In other cases of repeat fuel pump failure, mechanics forget
to make sure that the plastic fuel tank baffle is securely attached to
the inside of the fuel tank. Because a loose baffle will tend to batter
the fuel pump until it suffers a mechanical failure, the mechanic
should also inspect the pump for a missing filter sock or other
indications of physical damage.
As for electrical failures, keep
in mind that the fuel pump relay usually wears at the same rate as the
fuel pump. Too many mechanics inadvertently create a fuel pump warranty
complaint by forgetting to replace the relatively inexpensive fuel pump
relay as a precautionary measure.
Many General Motors
applications also have experienced burned or corroded fuel pump
connectors at the fuel tank. These connectors are readily available or
included with the fuel pump and should be installed as part of the fuel
Lastly, many fuel pumps are grounded directly to
the vehicle’s frame. Cleaning the frame ground and coating it with
corrosion inhibitor should be standard operating procedure for
servicing these systems.
Gary Goms is a former educator and
shop owner who remains active in the aftermarket service industry. Gary
is an ASE-certified Master Automobile Technician (CMAT) and has earned
the L1 advanced engine performance certification. He is also a graduate
of Colorado State University and belongs to the Automotive Service
Association (ASA) and the Society of Automotive Engineers (SAE).