Fuel Pump Diagnostics

Fuel Pump Diagnostics

Electric fuel pumps are a vital part of any electronic fuel injection system because achieving an accurate air/fuel mixture ratio depends upon the fuel injector receiving adequate fuel pressure and volume. Because electric fuel pumps have moving parts that wear out, they also have a relatively predictable service life and, therefore, become a predictable profit center for the independent import service specialist. In addition, a fuel pump replacement should include a new in-tank filter, in-line fuel filter and a new fuel pressure regulator if testing indicates the need for replacement.

Most current import vehicle applications incorporate an in-tank, single- or dual-line fuel pump configuration. The single-line configuration regulates the fuel pressure at the fuel pump and operates with no fuel return line. The dual-line configuration, on the other hand, regulates fuel pressure at the end of the fuel injector delivery rail and excess fuel is returned to the tank via a separate fuel line.

The pulse-modulated, single-line electric fuel pump is a more recent configuration that is designed to vary fuel pressure according to engine demand and driving conditions. In the pulse-modulated configuration, a fuel pressure sensor mounted on the fuel injector rail provides a pressure input to the engine’s powertrain control module (PCM). When the PCM calculates the pressure needed to meet a specific driving condition, it signals a separate fuel pump module to electrically pulse the fuel pump until the correct fuel pressure is delivered to the fuel injectors.

Although generally an obsolete configuration, many older imports use a low-pressure fuel pump in the fuel tank to supply fuel to a high-pressure fuel pump located on the vehicle’s frame or underbody. The low-pressure pump produces about 5-6 psi pressure while the high-pressure pump produces approximately 80 psi pressure. Residual fuel or “rest” fuel is often maintained via a separate check valve located in the fuel pressure line. Although these systems have their own test requirements, it’s important to remember that a bad low-pressure pump will cause the high- pressure pump to fail. In addition, some nameplates actually require the inlet screen on the low-pressure pump to be cleaned or replaced at regular intervals.

In any case, when a fuel pump wears to the point that it can no longer deliver a sufficient volume of fuel at a specified pressure, a no-code driveability complaint often develops that may include symptoms like intermittent starting, surging, loss of power or stalling. If these symptoms are present, fuel pump testing should be high on a technician’s diagnostic agenda.

To develop a spray pattern that properly atomizes fuel, a fuel injector needs to operate at a specified pressure. An excessively low fuel pressure causes a lean air/fuel (a/f) mixture. The PCM compensates for a lean a/f mixture by increasing the pulse width of the fuel injector which, in turn, may degrade the spray pattern.

Unfortunately, too many technicians rely solely on fuel pressure to measure fuel pump performance. A static pressure test is a poor indicator of fuel pump performance, since it gives no indication of how much fuel volume a fuel pump will supply during an extended period of maximum fuel consumption, such as during acceleration, towing a trailer or climbing a long hill.

There are several methods of measuring fuel pump performance. Many fuel pump pressure testers, for example, allow the technician to also test volume by flowing fuel into a graduated container. Most OE specifications are rated in pints per minute or its metric equivalent. Of course, the most difficult part of this test is to measure both pressure and volume through the Schrader valve located on the fuel rail. A better method is to use a dedicated pressure/volume tester that connects in series between the fuel pump and fuel injector rail.

If the vehicle can be road-tested, a technician may use an electronic pressure transducer connected to a graphing digital voltmeter or a voltmeter with a minimum and maximum voltage recording feature to record fuel pressure changes under full throttle. Keep in mind that vacuum-modulated fuel pressure regulators will typically increase fuel pressure by 4-6 psi when intake manifold vacuum drops at wide open throttle (WOT). Because high intake manifold vacuum has the net effect of increasing fuel pressure at the injector nozzle, fuel pressure must be reduced on most applications during part-throttle operation.

Another method of road testing fuel pressure is to use a graphing scan tool or the scan tool’s recording feature to record short-term fuel trim numbers during WOT operation. Keep in mind that, if the fuel pressure drops during WOT operation, the PCM will increase the injector pulse width to maintain a stoichiometric, or 14.7:1, a/f ratio. This increase in pulse width will translate into higher-than-normal fuel trim numbers. Generally speaking, fuel trims should read +/-5%. When fuel trim numbers exceed +10% during acceleration, they may indicate that the fuel filter is restricted or that the fuel pump itself is not producing the specified pressure and volume.

The last measure of fuel pump performance is residual or rest fuel pressure. Residual fuel pressure is required to keep the fuel injectors pressurized when the engine is shut off. If residual pressure leaks away while the engine is shut off, air may enter the fuel injectors and cause hard starting after the engine cools off.

Residual pressure specifications usually include a minimum pressure that should be maintained for a specific time. If residual pressure can’t be maintained for the specified time, the fuel lines, fuel injectors, fuel pressure regulator or check valve in the fuel pump may be leaking fuel. A service manual should be consulted for correct diagnostic procedures.

Many nameplates develop mileage-related fuel pump failure symptoms that can include excessive noise, hard starting, surging, loss of power or stalling issues. All too often, a worn fuel pump will pass a routine pressure and volume test in the shop, but fail at high ambient operating temperatures or during cold starting.

Understanding pattern fuel pump failures is important in diagnosing many intermittent performance problems. In addition, understanding pattern failures is also important in recommending fuel pump replacement as preventive maintenance to customers for whom security is an important issue.

Keeping in mind that even a worn pump can meet minimum standards for pressure and volume, many expert driveability technicians have made a science of fuel pump diagnosis by compiling a database of lab scope waveforms and typical amperage draws for known good and known bad electric fuel pumps. While space doesn’t allow for an in-depth analysis of amperage diagnostics, let me summarize the main points of the technique.

Lab scope waveforms display fuel pump motor amperage by using a low-amperage inductive pickup to detect amperage flow and variations in the power feed wire leading to the fuel pump. The inductive pickup must be sensitive enough to detect amperage variations on the motor’s commutator. Generally, these variations should be displayed as a series of identical “bumps” on a lab scope waveform display.

An irregularity in the display indicates armature wear or the inability of worn motor brushes to properly contact the armature. If the number of commutator segments is known (eight is a typical number), the actual speed of the pump can be calculated by counting the number of armature segments per time division on the scope. Worn motor armatures are commonly indicated by a jagged waveform. A high-amperage draw indicates that the motor bushings are worn out or that the fuel filter, fuel pressure regulator or fuel return line may be restricted. On the other hand, low amperage may indicate that the motor condition is good, but that wear in the mechanical pump may be producing low fuel pressure. This information is covered in more detail in many books and training tapes available to shops via aftermarket automotive publishers.


Because a catastrophic gasoline fire is an ever-present danger when replacing in-tank fuel pumps, it’s extremely important to follow recommended safety procedures. For example, before replacing a fuel filter, always loosen the gas cap to safely vent fuel tank pressure. Next, gently loosen a fuel line or open the fuel pressure test port to vent residual fuel pressure into a folded shop rag.

When removing a fuel tank for a fuel pump replacement, it’s extremely important to prevent fuel spillage by following application-specific, OEM procedures for draining the gasoline into an approved fuel caddy or holding tank. It’s also very important to use “floor-dry” or “kitty litter” to immediately absorb minor spills to prevent spilled fuel from being ignited by a spark or open flame. Above all, never use breakage-prone incandescent bulbs around gasoline. Instead, use more durable, plastic-encased fluorescent lighting fixtures to safely illuminate any fuel pump replacement.

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