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Understanding Fuel Pump Problems


4/13/2010
By Gary Goms

It’s important for the parts professional to understand the basic issues involved in fuel pump diagnosis.
 
If 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.

The 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
Let’s 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.

Because a 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
Many 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 on.

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.

To 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 activated.

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 procedure.

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.

Older 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.

Since the 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.

INSTALLATION ERRORS
Most 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.

Many 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 pump replacement.

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).
  Previous Comments
avatar   MARGARET CROSSETT   star   4/24/2010   11:55 AM

thank you so much , out of two hours of searching your site is the only one that

really helped us , a great deal.

and didnt ask for a deposit first .

thanks again




avatar   Wedge Antilles   star   4/22/2010   8:47 PM

I personally wish my store would stop selling fuel pumps. I know 1 out of every 2 pumps sold will be back with a year, usually because the customer was too cheap to buy a fuel filter or wiring harness at the time of purchase, but you can't tell them that.



avatar   Dan   star   4/21/2010   6:40 PM

Very good article, Gary. I printed it off and gave it to my manager with the suggestion that all employees should read it, and maybe send copies to some of ouer customers.

















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