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Home Asian Diagnostic Solutions: Fuel System Pressure & Volume Checks

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For maximum efficiency, modern import fuel injection systems require that fuel be delivered at a specified volume and pressure. Because the fuel injector’s job is to mix fuel with the intake air charge, it must produce a finely misted, conical spray pattern through the fuel injector nozzle. Lower-than-specified fuel pressures not only fail to provide enough fuel for combustion at larger throttle openings, but may also introduce an uneven air/fuel charge distribution into the combustion chamber itself, which usually results in hard starting, hesitation and reduced power output.

Of course, fuel volume is required to maintain fuel injection pressure at the fuel injector nozzle. While a marginally performing fuel pump might maintain pressure at idle, its effective operating pressure will decrease as engine load increases. Outside factors such as higher ambient air temperatures, ethanol gasoline and inlet filter or screen restrictions may further reduce fuel volume by aggravating the tendency of any fuel pump to cavitate or form bubbles in the pump inlet assembly.

Last, it’s important to recognize that excessively high or low fuel pressure limits the PCM’s ability to maintain a chemically correct or stoichiometric air/fuel mixture ratio. If low fuel pressure causes a post-1996 model PCM to increase injector pulse width to maintain a chemically stoichiometric air/fuel ratio, a positive short- and long-term fuel trim number will appear in the PCM’s data stream. If high fuel pressure caused by a sticking external pressure regulator or restricted fuel return line forces the PCM to reduce injector pulse width to maintain “stoic,” the short- and long-trim numbers become negative.

FUEL PUMP CONFIGURATIONS
Modern electric fuel pump systems are produced in conventional dual-line and, most recently, single-line configurations. The conventional two-line fuel pump consists of an in-tank mounted pump, an in-line fuel filter mounted on the chassis and a fuel pressure regulator mounted at the end of the fuel injector rail assembly. Excess fuel is returned through a second fuel line leading from the pressure regulator to the fuel tank.

High intake manifold vacuum effectively increases the pressure differential between the fuel rail and fuel injector nozzle. Consequently, most external fuel pressure regulators use intake manifold vacuum to reduce fuel pressure during high intake manifold vacuum conditions. In contrast, single-line fuel pumps with integral fuel pressure regulators rely entirely on the PCM modulating fuel injector pulse width to compensate for high vacuum operating conditions.

In addition, some applications employ a two-speed fuel pump system to reduce fuel pump noise at low vehicle speeds. Two-speed systems supply power to the fuel pump through a resistor for low-speed operation and directly to the fuel pump for high-speed operation. In some diagnostic scenarios, a two-speed pump may not switch to high-speed operation and may fail to supply adequate fuel pressure at high engine loads.

FUEL PRESSURE TESTING
Because the tolerances for fuel pressure variations are small, it’s extremely important to use accurate, professional-grade test equipment for testing fuel pressure and volume. Most professional test gauges incorporate a volume test release valve that can be used to release fuel pressure or to provide an approximate indication of fuel volume.

When testing conventional two-line systems, the modulated pressure specification is tested by leaving the intake vacuum hose attached. Removing the vacuum hose with the engine running delivers an unmodulated fuel pressure reading. Both results should equal specified values. Because single-line systems aren’t vacuum-modulated, only one pressure value is specified. Most systems must also maintain a rest or residual pressure that must be maintained with the key off for a specified length of time. Residual pressure essentially keeps the fuel injectors primed for the next starting cycle. Although many manufacturers publish a residual pressure specification, some experts insist that the lack of residual fuel pressure appears to cause few noticeable driveability problems.

FUEL VOLUME TESTING
Although many auto manufacturers don’t include a volume specification in their test data, volume testing remains an extremely important part of fuel system diagnostics. As mentioned above, a rough indication of volume can be obtained by using the fuel pressure tester’s release valve to bleed excess fuel into a graduated plastic container for a specific length of time.

The most accurate method of testing volume on systems with external fuel pressure regulators is to measure the flow exiting the pressure regulator with the pump activated. For most applications, the fuel pump should produce at least three pints of fuel in 30 seconds. On single-line systems, the volume can be measured only from a disconnected fuel line.

Because either test requires a fuel return line to be removed, use a scan tool or test connector to activate the fuel pump relay or have a helper ready to shut off the engine. Whenever performing any fuel system test, have a fire extinguisher at hand and make sure that any potential ignition sources, such as an incandescent light bulb, are removed from the test area.

CURRENT RAMP TESTING
Because current ramp testing requires advanced lab scope and current probe skills, space doesn’t allow but a summary of this process. In addition, test results are very application-specific and, therefore, require nameplate experience to interpret the results.

With that said, many advanced diagnostic technicians use oscilloscope-based current ramp testing to measure the amperage draw of the fuel pump and the wear in the pump’s armature and brush assembly. Most current ramps show three to five amperes current draw and an even, undulating current flow activity at the motor’s armature from a pump in good operating condition. Current ramping is most often used to reveal intermittent electrical failures in the fuel pump relay, inertia switch (if so equipped), power supply, motor armature or ground connection.

DIAGNOSTIC TIPS
A logical series of diagnostic tests must be performed to ensure an accurate fuel pump diagnosis. To illustrate, adding several gallons of fuel before testing will eliminate problems caused by an erroneous fuel level gauge. Before adding fuel, however, always inspect for a badly dented or collapsed fuel tank. Either type of damage might restrict fuel flow into the fuel pump inlet or from the fuel pressure return line.

Next, remember that fuel filters occasionally cause intermittent fuel pump volume failures if the filter media is saturated with dirt and debris. During a volume test, the regulated fuel should flow smoothly and without large bubbles. An erratic fuel flow might indicate a mechanical problem with the pump assembly or inlet screen. While some bubbling is normal due to fuel agitation, large bubbles indicate that air is entering the pump inlet.

When testing fuel pressure, cycle the fuel pump relay with the key on, engine off several times to see if the fuel pressure remains the same. Similarly, snap the throttle open several times with the engine running to detect an intermittently sticking fuel pressure regulator. In any case, the modulated fuel pressure must maintain a consistent reading at both idle and wide-open throttle conditions.

PULSE-MODULATED FUEL PUMPS
Some vehicles incorporate a pulse-modulated fuel pump electrical system in which the PCM modulates fuel pressure according to operating conditions. Rather than operating on a steady flow of current, the modulated fuel pump is designed to operate on a series of electrical pulses supplied by a fuel pump module. The PCM monitors fuel pressure through a sensor mounted on the fuel rail. Using this input, the PCM can command the fuel pump module to increase or decrease fuel pressure by modulating the pulse width to the fuel pump.

The diagnostic procedures of pulse-modulated fuel pumps are very application-specific and should never be confused with the pressure and volume testing of conventional pump systems. Pulse-modulated pump diagnostics are scan-tool based, with the conventional pressure test being used only to confirm the calibration of the fuel pressure sensor. Pulse-modulated fuel pumps are but another example of how modern fuel delivery technology is changing.

FUEL PUMP MANUFACTURERS COUNCIL TACKLES FUEL PUMP WARRANTY ISSUES

By Larry Carley, Technical Editor

Believe it or not, fuel pumps have a higher return rate than almost any other automotive part. According to the Fuel Pump Manufacturers Council (FPMC), about 10% of all the fuel pumps that are sold by jobber parts stores are returned. But it isn’t because there’s an epidemic of defective fuel pumps. FPMC members say that up to 80% or more of the returned pumps work fine when they are tested by the manufacturer. Most of the pumps are coming back because somebody misdiagnosed the fault and replaced the wrong part, or they assumed the vehicle had a bad fuel pump because the engine wouldn’t start.

Parts that are returned for any reason is an ongoing issue in the parts business because it costs everybody time and money. Parts stores don’t like returns because it requires extra work, processing and handling — and it may cost them repeat business if a customer loses confidence in the parts they sell.

Manufacturers don’t like returns because they have to eat the cost of the returned part (plus shipping), plus all the paperwork that goes with it. Returns can also hurt their relationship with the warehouse and jobber customers, and possibly undermine their reputation as a supplier of quality parts.

Do-It-Yourselfers don’t like returns because it means replacing the same part twice, and making another trip back to the parts store for another pump, or yet another part if the second (or third, or fourth) pump doesn’t fix their problem. Today’s tank-mounted fuel pumps are hard to reach, and can take up to several hours of hard work to replace.

Professional installers are less apt to make these types of mistakes, so fuel pump return rates are usually lower — but not always. Some technicians are very good diagnosticians and correctly identify the fault the first time. Others who are not as skilled might make a guess and hope for the best. The hard part is convincing the latter group of technicians that they need more training — or that they need to use the knowledge they already have to rule out other possibilities before they replace a “bad” fuel pump.

The only way to know if a fuel pump is good or bad is to test it. On-vehicle testing requires measuring two things: fuel pump pressure and fuel pump volume. If a fuel pump can’t deliver the required pressure or adequate volume, the engine may not start or may not run right.

Testing fuel pressure requires a fuel pressure gauge and some adapters. Many technicians have these tools, but don’t always use them to save time or effort. Fuel pressure is easy to read on vehicles that have a fuel pressure service fitting on the injector rail, but not so easy on engines that require splicing a tee fitting into the fuel supply line. If fuel pressure is below the minimum specifications, the pump may be weak. Or, the fuel filter may be clogged, the fuel pressure regulator may be defective or the fuel pump may not be receiving enough voltage to run at normal speed. All of these possibilities need to be investigated before the pump is replaced.

Electrical problems can also affect fuel delivery. These include a bad fuel pump relay, loose, corroded or burned wiring connections to the fuel pump, or even low charging voltage. A good fuel pump should also be capable of pumping at least 750 ml (3/4 quart) of fuel in 30 seconds. If it can’t, there’s a problem. The pump might be worn, or a clogged fuel filter might be restricting fuel flow to the engine, or the pump might not be getting enough voltage to run at full speed. Again, all of these possibilities need to be investigated before any parts are replaced.

Currently, there is no easy way to bench-test a fuel pump in a parts store. But that may soon change. Several companies are developing easy-to-use fuel pump testers that parts store employees can use to check a customer’s old fuel pump. The tester can also be used to verify the operation of a new pump before it leaves the store, and to resolve any warranty issues that arise after a pump has been replaced. The tester cycles a nonflammable solvent through the pump to measure both pressure and volume. The results are compared to OEM specifications, and if the pump meets the specifications, it removes all doubt as to whether the pump is good or bad. The new equipment should be available later this year, and should go a long way toward reducing unnecessary fuel pump replacements and returns.

The FPMC is also looking at things their members can do to reduce fuel pump returns. The FPMC has hosted several jobber and installer focus groups to better understand the issues, and to discuss ideas for solving these issues. One thing everyone agrees upon is that parts store employees, DIY customers and professional technicians all need access to more service information.

Several FPMC members say they plan to package more detailed diagnosis and installation instructions with their replacement pumps (if anybody will read them). Others say they are stepping up their educational efforts by providing more in-store and in-shop training — especially in “problem” locations where return rates are unusually high.

The FPMC also plans to launch a new fuel pump information website where anyone can go for detailed fuel pump application, diagnosis and installation information. The goal is to have the website up later this year (we’ll keep you posted).

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Gary Goms

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 also belongs to the Automotive Service Association (ASA) and the Society of Automotive Engineers (SAE).
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