DIY

How to Diagnose a Faulty Fuel Pump

Reading Time: 7 minutes

So, you think you may have a faulty fuel pump? Troubleshooting the problem is usually pretty straightforward. But occasionally, a tricky diagnosis will have you pulling out your hair and throwing wrenches. That’s why it’s helpful to have a few extra diagnostic tricks up your sleeve.

Before beginning, you’ll want to have the vehicle’s repair information handy. Manuals, such as those from Chilton, are good but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.

Also, keep in mind, other issues can mimic a faulty fuel pump. You’ll want to perform a thorough diagnosis to ensure the pump is the problem before replacing it. And consult the previous article in this series regarding fuel pump symptoms to understand how both continuous and returnless fuel systems work.

Note: The following is for informational purposes only. Consult the factory information for repair instructions and recommended safety procedures.

Other issues can mimic a faulty fuel pump. Perform a thorough diagnosis to ensure the pump is the problem before replacing it.

Part 1: Check whether the problem is fuel-related

The first step is to verify the problem is fuel related. Then, you can do additional testing to find out if the pump is to blame.

If the engine won’t start:

If you decide to attempt the test, try to start the vehicle by removing the air intake and spraying starter fluid into the throttle body. Have an assistant crank the engine while you spray – if it starts and runs momentarily, the problem is fuel-related and may indicate a bad pump. You’ll want to check fuel pressure as your next step.

Make sure to wear safety glasses while attempting any procedure described in this guide. Use proper safety equipment and have a fire extinguisher on hand.

If the engine won’t start OR it exhibits performance problems:

If fuel trim is above 10, that means the engine is running lean and the fuel pump may not be delivering. Keep in mind, however, other factors, such as vacuum leaks, can cause a lead running condition as well. So, you’ll want to check fuel pressure as your next step.

Check fuel pressure and/or volume

Performing a static fuel pressure test is pretty straightforward. Simply connect the gauge as outlined in your repair manual.

Checking fuel pressure and/or volume will tell you whether or not enough fuel is making it from the tank to the engine. But that doesn’t necessarily mean the pump is bad. It’s wise to do further testing to rule out fuel pump circuit issues and other problems, such as a clogged fuel filter or bad pressure regulator, that could limit the fuel supply.

Caution: Before hooking up a fuel pressure gauge, be sure to relieve fuel system pressure as outlined in your repair manual. Have your safety glasses on and a fire extinguisher nearby.

connecting fuel gauge on test port located on fuel rail
Most modern vehicles have a handy test port located on the fuel rail.

Performing a static fuel pressure test is pretty straightforward. Simply connect the gauge as outlined in your repair manual. Most modern vehicles have a handy test port located on the fuel rail.

Next, turn the ignition to the on position (key on, engine off), note the reading on the gauge and compare it to the specification listed in your repair manual. If the reading varies significantly from spec, you may be dealing with a bad fuel pump.

ignition key switched to on position for static fuel pressure test
Turn the ignition to the on position (key on, engine off) for the static fuel pressure test.

Also, when performing a static test, you’ll want to note whether the pressure holds. The factory repair information will tell you how long (usually several minutes) fuel pressure should hold once the pump is turned off. If pressure bleeds off too fast, there is a fuel-related problem such as a bad pump check valve or faulty regulator.

Those looking to perform a more in-depth fuel pressure assessment may choose to check fuel pressure at idle or under load. Also, continuous fuel systems can be checked for “dead head” pressure (pressure with the return line pinched off). But that test does not work on newer returnless systems that have the regulator in the tank.

fuel gauge showing pressure is holding during a static test
When performing a static test, check if the fuel pressure holds. Factory repair information will tell you how long pressure should hold once the pump is turned off.

Start by connecting the gauge and placing the bleed end of its hose into a clean container. Without starting the engine, activate the pump for 15 seconds while you hold down the relief valve on the tool. Typically, the pump should deliver about a pint of fuel during that time. Consult the repair information for the exact procedure and specifications for your vehicle.

Part 2: Before condemning the pump…

So, you’ve verified a lack of fuel pressure and/or volume and now, you think you can condemn the pump. But wait – not so fast. In many cases, you’ll want to check the following beforehand:

In many newer, returnless fuel systems, the fuel pump control module (FPCM) directly controls the fuel pump. The device uses information from sensors, such as the fuel pressure sensor and fuel temperature sensor, to determine control of the pump’s ground circuit and vary the pump’s speed. It’s a good idea to make sure everything within the control system is working properly before blaming the pump.

It’s a good idea to check the integrity of the wiring in the pump circuit before making any conclusions. A digital multimeter (DMM) can be used to check the circuit for anything unusual.

With both continuous and returnless systems, you’ll also want to check the integrity of the wiring in the pump circuit before jumping to conclusions. A digital multimeter (DMM) can be used to check the circuit for continuity, power and ground, as well as any unwanted excessive resistance.

Part 3: Advanced tests

There are a couple of advanced tests available for those who have the interest – and the money – to try them out.

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Author

Mia Bevacqua

Chief Mechanic at CarParts.com

Mia Bevacqua is an automotive expert with over 15 years of industry experience. She holds ASE Master, L1, L2, and L3 Advanced Level Specialist certification, as well as a bachelor's degree in Advanced Automotive Systems.

Throughout her career, Mia has applied her skills toward automotive failure analysis inspections, consulting, diagnostic software development, and of course, freelance writing. Today, she writes for companies around the world, with many well-known clients showcasing her work.

Mia has a passion for math, science, and technology that motivates her to stay on top of the latest industry trends, such as electric vehicles and autonomous systems. At the same time, she has a weakness for fixer-upper oddballs, such as her 1987 Chevy Cavalier Z-24 and 1998 Chevy Astro Van AWD.

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