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Fuel System

FUEL PUMP PLAYS CRITICAL ROLE
IN TODAY’S SOPHISTICATED FUEL DELIVERY SYSTEMS

Edited from an article by Gary Goms, ImportCar Magazine, February 2003

Whether diagnosing a fuel-related driveability complaint or selling the service itself, it’s important to remember how a fuel pump fits into today’s sophisticated fuel delivery systems. For starters, a conventional two-line system includes the fuel tank, pump, filter, fuel injectors, pressure regulator and return line. A single line system integrates the fuel pressure regulator into the fuel pump, which eliminates the need for a fuel return line. To keep the fuel pump cool and well lubricated, the pump is submerged near the bottom of the fuel tank. A screen installed on the fuel pump inlet protects the fuel pump from ingesting abrasive particles that accumulate at the bottom of the tank.

After the gasoline leaves the fuel pump, it passes through a fuel filter installed near the fuel tank or in the engine compartment. From the tank, it’s transported through the high-pressure fuel line to the fuel injector rail, which is a tube-like assembly that distributes fuel to the fuel injectors. When the Powertrain Control Module (PCM) opens the fuel injectors, a timed injection of gasoline is sprayed into the intake manifold. As the engine cranks, the air and fuel mixture is drawn into the cylinders and ignited by the spark plugs.

In order to control the fuel pressure available at the fuel injector nozzles, a fuel pressure regulator is installed at the end of the fuel injector fuel delivery tube or "rail." The excess fuel is returned to the fuel tank via the fuel return line. Some newer vehicles using the single-fuel line system vent excess fuel directly from the fuel pump into the fuel tank. Another feature of the single-line system is that the fuel pump and fuel filter may be built into a fuel pump "module" assembly.

FUEL PUMP SAFETY FEATURES
All fuel pumps use an electrical signal from the ignition system or the air flow sensor to indicate to the PCM that the engine is cranking before the fuel pump is activated. To illustrate, when the engine is cranked on a speed density fuel metering system, the ignition reference sensor signals the PCM to activate the fuel pump relay. If the ignition module, wiring or PCM is defective, the fuel pump will not be activated. Similarly, the air flow sensor fuel metering system activates the fuel pump relay when it indicates to the PCM that air is flowing into the engine.

Obviously, the electric fuel pump isn’t a stand-alone component and shouldn’t be treated as such. During the diagnostic and sales process, fuel pumps should be treated as only one of a very sophisticated assembly of interdependent parts that are designed to deliver high-pressure fuel in a safe and reliable manner.

DIAGNOSTIC INDICATORS
When a customer complains of hard starting, engine stalling, intermittent loss of power or poor cold engine performance, the fuel pump and its related parts such as the fuel filter and fuel pressure regulator should be the first items to be tested. More to the point, the fuel systems should be tested for key on-engine running (KOER) pressure, residual pressure or key on-engine off (KOEO) pressure, and total volume in pints per minute of fuel discharge from the fuel pump.

Depending upon the application, the KOER specifications may range from 13 psi to 80 psi of fuel pressure. The KOEO pressure usually is about 30 percent of KOER pressure. Most volume measurements through the fuel pressure test ports should average at least 1-2 pints per minute with the engine running. A fuel pump that fails one or more of these tests should be replaced.

In addition to these tests, particular attention should be paid to short- and long-term fuel trim datastream numbers on 1996 and newer OBD II-equipped import vehicles. Positive long-term fuel trim percentages over 10 percent may indicate that the PCM is holding the fuel injectors open a few milliseconds longer than normal to compensate for lean air/fuel mixtures. A long-term fuel trim (LFT) number of +25, for example, might indicate that the fuel pump is struggling to keep the engine supplied with gasoline.

Since fuel pump reliability depends largely upon fuel quality and fuel filter maintenance, it’s difficult to estimate how long a fuel pump should last in normal service. But pattern fuel pump failures do begin to appear at the 70,000-mile mark on some nameplates, while other nameplates hum merrily along for more than 200,000 miles with no problems. Regardless of the mileage, the possibility of a fuel pump failure should always be considered when the vehicle begins to develop fuel-related driveability symptoms.

When suggesting fuel pump replacements, it’s important to remember that most imports require a fuel filter replacement at 30,000-mile inspection intervals or at 60,000 "tune-up" intervals. If the vehicle has an easily accessible fuel pressure test port, it’s easy to test the fuel pump pressure at the time of the fuel filter replacement.

When mechanical fuel tests aren’t practical because of accessibility issues, a skilled diagnostic technician also may evaluate the condition of the fuel pump by measuring the amperage draw on the fuel pump or by observing an oscilloscope waveform generated by the fuel pump’s rotating armature. In many cases, electrical access to the pump can be obtained at the fuse box or fuel pump relay.

No matter how the fuel pump is tested, a routine fuel pressure test is an easy way of ensuring that your customer isn’t going to be stranded because your shop overlooked a worn fuel pump. Moreover, if the engine stalls because the fuel filter is completely clogged, it’s mandatory that fuel pressure and volume be tested because, in many cases, a clogged fuel filter may have damaged an otherwise sound fuel pump.

It’s important to keep in mind that low fuel pressure also may indicate a clogged fuel pump strainer, fuel filter or defective fuel line. To illustrate, the fuel pressure regulator can be stuck open, which will mimic a clogged fuel filter, crimped fuel line or worn fuel pump. In other cases, the fuel pressure regulator can be stuck closed, which will cause exceedingly high fuel pressure and a subsequent loss of fuel economy and performance.

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