Tech Stuff

Fuel Pump 101: The Basics of Fuel Pump Diagnosis and Repair

Electric fuel pumps are among the most reliable parts of a car. Under normal conditions, it’s not uncommon for a pump to last the whole life of the vehicle. When it finally quits, it’s because the motor is worn out and can’t develop enough torque to generate the correct fuel pressure. Here are the basics for what it takes to diagnose a problem and perform a profitable repair.

The most common cause of fuel pump failure is frequently running the tank low on fuel, which causes the motor to overheat. The second most common cause is fuel contamination, usually dirt and rust particles that clog the fuel strainer and prevent the pump from drawing enough fuel under high engine load. If enough contamination gets past the pump’s intake strainer, it can actually jam the pump and stop the engine immediately.

Diagnosing fuel pressure problems is often tricky because the problem can be something other than the fuel pump. To help you avoid misdiagnosis and comebacks, we’re going to review the components of a fuel pump assembly, describe some diagnostic techniques, and discuss how to give replacement fuel pumps a good chance at a long service life. We’ll focus only on port fuel injection systems that use the electric pump to generate all the pressure in the system, but most of this information can also apply to the fuel delivery system of direct-injection engines, too.

Fuel pump

The complete fuel pump assembly might include a fuel gauge sending unit, a pressure regulator valve, a fuel tank pressure sensor, a pump intake strainer and/or the main fuel system filter, and of course the electric fuel pump. An electric fuel pump consists of two parts: a small brush-type DC motor and the pump itself. This assembly is submerged in a reservoir of fuel that keeps the motor cool and prevents air from getting to the motor, thus eliminating the risk of fire or explosion.

The only wear parts on this whole assembly are the motor’s brushes. As electrical current flows through a circuit, it naturally generates heat. Most of that heat is concentrated at the point of highest resistance, and in a brush-type motor, it’s where the carbon brushes touch the rotating copper contacts (commutator bars). Even on a cold day the fuel pump motor typically runs at about 100 degrees Fahrenheit. This heat is carried away by fuel flowing through the pump, so when the low-fuel warning light turns on there’s not much fuel left in the tank to absorb all that heat. Soon the ends of the brushes get so hot that microscopic particles of carbon burn off and stick to the copper contacts.

If the brushes are overheated often enough, a coating of burnt carbon builds up on the contacts. This creates electrical resistance that reduces current flow through the motor and therefore reduces the motor’s power. Eventually the motor becomes too weak to generate proper fuel pressure, and if the resistance gets high enough, the motor won’t even run.

Half the commutator bars are fouled with burnt carbon from the brushes. This causes high electrical resistance that reduces the motor’s torque. Photo courtesy of Jim Morton
<p>Half the commutator bars are fouled with burnt carbon from the brushes. This causes high electrical resistance that reduces the motor&rsquo;s torque. Photo courtesy of Jim Morton</p>

Connectors

As current flows through a circuit, the most common place for resistance to occur (other than the load) is in a connector. If the connection is tight and remains clean, the resistance is almost nothing. However, if the contacts are loose, corroded, contaminated or simply not big enough to handle the current flow, resistance increases. This reduces or “drops” the voltage downstream of the connector, and heat concentrates at the point where the voltages are different; across the point of highest resistance.

When a connector overheats, the plastic housing starts to warp and the problem tends to snowball; the contacts loosen or become corroded or contaminated, increasing resistance that generates even more heat and promotes corrosion that increases resistance even more. Voltage in the circuit is reduced “downstream” of the high resistance, so this reduces the voltage that reaches the pump motor. That’s why heavy-duty connectors are used in a fuel pump circuit.

Connectors are the most likely place to find high resistance and heat damage in a fuel pump circuit.
<p>Connectors are the most likely place to find high resistance and heat damage in a fuel pump circuit.</p>

Check valve

There are two fuel pressure specifications for every vehicle: system pressure and hold pressure. Hold pressure is maintained by the fuel pump’s check valve when the engine is turned off. This makes it easier to start the engine.

If the check valve fails or is jammed open by fuel contamination, the hold pressure will leak down. If that happens while the engine is cold, the starter will crank a bit longer while the fuel pump builds enough pressure to start the engine. If the engine is warm, the liquid fuel in the warm injector rail can evaporate, creating a “vapor lock” that prevents liquid fuel from reaching the injectors. Extended cranking might get the engine started, but not all fuel pumps are able to compress the fuel vapor to a liquid again. In this case, the engine won’t start until the vapor cools and condenses on its own.

Fuel pressure testing

Low fuel pressure or delivery problems can be caused by a worn or damaged fuel pump, a faulty pressure regulator, a clogged fuel filter or by high resistance in the fuel pump circuit. That means there are two ways to test a fuel pump: mechanically and electrically. Both are needed for an accurate diagnosis, but checking fuel system pressure with a mechanical gauge is the most logical place to start. Fuel system pressure is measured with the engine warmed up and running at idle, using a test gauge connected directly to the fuel injector rail or Tee’d into the fuel supply hose.

Sometimes the fuel pump will generate full pressure when the starter is cranking but stop running after start-up, and the engine will stall quickly. This indicates a problem with an engine sensor or with the powertrain control module (PCM). It is not a fuel pump failure, but it is often misdiagnosed as a bad fuel pump.

If fuel system pressure meets specifications at idle, many techs will assume everything is OK and look no further. That’s a mistake, because it’s possible to have correct pressure at idle when demand is low but not enough pressure or flow volume at higher speeds and loads. The pump should be tested under load.

On older vehicles with a return-type fuel system, an easy way to load-test the pump is to connect the gauge directly to the fuel supply line and run the pump to measure its maximum or “deadhead” pressure. Most pumps should be able to generate at least twice the vehicle’s specified fuel pressure this way. If so, the fuel pump and its voltage supply circuit are probably in good condition. With the engine running at idle, the pressure regulator can be checked by simply disconnecting the vacuum hose to see if pressure increases.

On vehicles with a mechanical returnless fuel supply system, you can’t really check deadhead pressure because the pressure regulator is inside the fuel tank. A common technique for load-testing the pump is to drive the vehicle with a pressure gauge connected. If pressure is within spec during cruise but falls during acceleration, that means either the fuel filter is clogged or the pump can’t keep up with demand.

When fuel pressure is low, a scan tool will show long-term fuel trim is significantly positive all the time and short-term fuel trim will reach maximum on acceleration (fuel trims max out at 25%). However, a vacuum leak can produce similar fuel trim readings.

If fuel system pressure is low only under load, that doesn’t automatically mean there’s something wrong with the fuel pump. Yes, a clogged filter can cause low pressure and low volume under load, but low voltage at the fuel pump connector will do the same thing. Now it’s time to test the electrical system. But first, let’s look at newer models with variable speed fuel pumps.

Fuel pump driver module

Most vehicles now have an electronic returnless fuel supply system that has no fuel pressure regulator. Pressure is controlled by a fuel pump driver module (FPDM) or a fuel pump control module (FPCM) in response to commands from the PCM. On these systems, fuel pressure is managed by controlling the speed of the fuel pump.

The PCM decides how much fuel pressure is needed based on demand, from idle to full load, and also based on signals from the fuel rail pressure (FRP) sensor and a fuel rail temperature (FRT) sensor. These sensors are often in the same housing, and their data will show on a scan tool.

When the ignition switch is on, full battery voltage is supplied to the control module and to the pump, either through the control module or through a separate fuel pump circuit.

The control module operates the fuel pump by controlling the motor’s ground circuit on a pulse width modulated duty cycle.

The scan tool will display pump speed commands as percent duty cycle. Depending on the scan tool, you may need to look up those commands in a service information system to interpret what they mean. For instance, on Ford models, the maximum fuel pump duty cycle is 50%. This runs the pump at full speed to generate maximum pressure during wide-open-throttle or start-up. When the PCM commands 25% duty cycle, it’s actually a command to turn the fuel pump off.

The pump control module sends a diagnostic signal back to the PCM indicating actual duty cycle of the fuel pump. This provides a feedback loop so the PCM can determine if actual pump speed matches the commanded speed. These signals are also displayed on a scan tool, and some scan tools allow you to issue pump speed commands with the tool.

If pressure does not increase when the duty cycle is commanded high, either through a scan tool or at wide-open-throttle on a test drive, this could indicate a faulty pump. If fuel pump duty cycle remains high at idle just to maintain normal pressure, this can also indicate a bad fuel pump motor. However, at this point the only thing you know for sure is that the pump control module is working. You still need to test the voltage supply circuit under load before condemning the pump. If the engine won’t start and/or if your scan tool won’t display fuel pump commands, you can still diagnose the driver module with a voltmeter that can read duty cycle.

You’ll also need a service information system that has wiring diagrams and a good description of how the system operates. Just remember there will always be six wires: fuel pump power, control module power, control module ground, fuel pump ground, the command signal from the PCM and the feedback signal. By back-probing the connector and looking for the correct voltage and ground signals, you can determine if the control module is working. By the way, GM has always used the same wire colors for these circuits.

For purposes of illustration, depicted here is a typical fuel pump control unit schematic. Artwork courtesy of Motologic
<p>For purposes of illustration, depicted here is a typical fuel pump control unit schematic. Artwork courtesy of Motologic</p>

Hold and flow

Hold pressure is checked at the same time as fuel system pressure. After the engine is switched off and the fuel pump stops running, fuel system pressure will decrease slightly and then hold steady for at least five minutes. On many vehicles the hold-pressure specification calls for no less than 20 psi after 20 minutes, but most will engines will start if pressure remains positive while the engine is still warm.

Hold pressure can leak down due to a leaking injector, but this is a relatively slow leak. If hold pressure leaks down immediately after the engine is switched off, that’s a bad check valve.

A fuel system can have good pressure but not enough fuel flow to run the engine properly at higher speeds. Flow rate can be tested by disconnecting the fuel supply line at any convenient point and attaching a hose that leads to a measuring container. Activate the fuel pump for 15 seconds and measure the amount of fuel that flows into the container. Some manufacturers publish a fuel flow specification, but some do not. Either way, expect a healthy fuel pump to flow a minimum of one pint in 15 seconds.

Electrical tests

As noted earlier, high resistance in an electrical circuit reduces the amount of voltage downstream of the point of high resistance. Less voltage to the motor means the motor makes less power. Simply checking for voltage with the fuel pump disconnected does not tell you if there is any resistance in the circuit because the resistance only causes the voltage drop while the circuit is working. So before condemning a fuel pump, it’s important to make sure full battery voltage is available at the fuel pump connector while the pump is running. This is the voltage drop test.

When measuring measure voltage drop, it helps to remember that a voltmeter measures the difference in voltage reaching the meter’s positive and negative probes. In this test, you’re not measuring the difference between battery positive (B+) and ground. Instead, you’re measuring the difference between battery positive and the rest of the positive circuit.

When checking voltage drop across a connector, anything greater than zero volts indicates resistance in the connection. Total voltage drop in the whole circuit from battery to fuel pump should be less than 0.5 volts. Artwork courtesy of AGCO Automotive Repair Service
<p>When checking voltage drop across a connector, anything greater than zero volts indicates resistance in the connection. Total voltage drop in the whole circuit from battery to fuel pump should be less than 0.5 volts. Artwork courtesy of AGCO Automotive Repair Service</p>

With the voltmeter’s positive probe connected to the battery through a long jumper wire, use the negative probe to back-probe the fuel pump connector while the pump is running. If there is absolutely no resistance to the flow of current between the battery and that connector, there will be no difference in electrical pressure, no loss of voltage in that circuit, and the voltmeter will show zero volts. In reality, there is no perfect circuit, so the total voltage drop in a good fuel pump circuit might be as high as 0.5 volts. However, some vehicles will suffer driveability problems with a voltage drop as low as 0.165 volts (165 millivolts).

If voltage drop is high, start looking for a corroded or overheated connector, a faulty relay or even a chaffed wire. Some replacement fuel pumps come with a heavy-duty connector that is used to replace the car’s original connector.

Even plastic tanks can have corrosion from metal parts of the fuel pump assembly.
<p>Even plastic tanks can have corrosion from metal parts of the fuel pump assembly.</p>

Pump replacement

A typical fuel pump will last for years if the gas is always clean, but contaminated fuel can kill a pump in just a few weeks. Before installing a new fuel pump, find out if the old one was ruined by something in the fuel.

If possible, start by getting some information from the customer. Do they usually buy the cheapest fuel they can find? Do they frequently drive with less than a quarter tank of fuel? Do they ever pour additives into the tank? How many times has the car been run out of gas? Did the car stop running soon after they bought gas?

When removing the old pump, check the condition of the tank seal. Does it look like dirt or water might be getting past it? Now drain the fuel tank. If you use a fuel caddy with a filter, you can return the gasoline to the tank later. Otherwise plan on putting in a few gallons of fresh gas (always a safer choice). Inspect the tank for rust, corrosion and other solids. These can accumulate even in plastic tanks because metal in the pump unit corrodes, especially in humid climates. Also, it’s inevitable that some dirt will fall in while the tank is being filled, especially in dry, dusty climates.

Finally, examine the fuel itself. Is it cloudy or discolored? Pour at least a cup of it onto a clean white rag or paper towel and see what it looks like after it dries out. Is there an even tan stain or is it a rainbow of colors? Are there solid particles? How long did it take to dry out? Did it dry out completely or is it still wet or oily? If there’s any question at all about what’s in the fuel, clean or replace the tank and have a conversation with the customer about fuel contamination and your warranty.

If a replacement pump fails soon after installation, it’s almost always due to fuel contamination. Sometimes it’s because dirt fell into the tank when replacing the pump, but more often it’s a result of sediment being stirred up when the pump is replaced. That sediment often contains corrosion from metal parts of the pump assembly that react with moisture in the fuel. If the fuel tank is steel, there’s usually corrosion on the “roof” of the tank. This is why a new pump comes with a new intake strainer, and it’s also why the fuel tank should be completely cleaned out when installing a new pump.

The number one reason that replacement fuel pumps fail is failure to clean out the fuel tank.

When installing a new fuel pump, safety and cleanliness are critical. If the fuel tank must be removed to access the fuel pump module, sometimes it’s tempting to lower the tank only as far a necessary rather than removing it completely. This is a mistake because it increases the risk of stirring up sediment inside the tank that will damage the new fuel pump. Any time a new fuel pump is installed, the tank should be removed, drained and cleaned.

After a fuel pump module is installed, the fuel system must be primed by cycling the ignition switch on and off several times. Often you can hear the pump change pitch as it finally begins making pressure. In some fuel tanks, the module’s reservoir does not automatically fill with fuel even when it’s submerged, and fuel must be poured into it manually during or after installation. Sometimes this requires filling the tank.

The information presented here is just the basics. The Fuel Pump Manufacturers Council, part of the Automotive Aftermarket Suppliers Association, has produced a series of videos that provide more information than we could ever fit into a magazine article. If you’ve suffered a lot of fuel pump comebacks, or even if you just want to learn more, search the web for these free training videos.   ■

Current draw

One quick and easy way to test a fuel pump is to measure the current flowing through the circuit with a digital volt/ohm meter (DVOM). This test will only show if current draw is too low, indicating high resistance somewhere in the circuit; it will not pinpoint the problem. However, it only takes a few minutes, and if the current is not too low, the fuel pump circuit is probably OK.

There’s no specification, but a rule-of-thumb says the current flow should be about half the fuse rating with the pump under load. For example, the fuel pump fuse here is rated for 20 amps, and the circuit flows about 10 amps with the DVOM set to capture min/max readings and the throttle snapped wide open.

Jacques Gordon has worked in the automotive industry for 40 years as a service technician, lab technician, trainer and technical writer. His began his writing career writing service manuals at Chilton Book Co. He currently holds ASE Master Technician and L1 certifications and has participated in ASE test writing workshops.

Author's note:

After 27 years of writing for automotive trade journals, I am retiring from the publishing business. To say it’s been a dream job would be an understatement. The places I’ve been, the cars I’ve driven, the things I’ve seen and done and learned are more than most people experience in a lifetime. Of course it’s been made even richer by all the wonderful people I’ve met and worked with over the years. One of them was my first editor at Chilton Book Co. He told me that by the time a guy looks up the service procedure he needs, he’s probably already in trouble, so it’s my job to help him. That’s what I’ve tried to do in every article I’ve ever written, and readers of every publication I’ve worked on have told me that I’ve succeeded. High praise indeed: it’s been an honor to serve you. While I am leaving the publishing industry, there is still some wrench-turnin’ in my life, so I’ll still be checking in with iATN frequently. Maybe I’ll see you there. -- Jacques Gordon

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