Expert Tips to Troubleshoot a Fuel Pump

How to Troubleshoot a Fuel Pump Like a Pro

When a fuel pump starts acting up, you don’t have time to guess. If you ignore the warning signs, you could find yourself stranded on the side of the road. I write this article to help DIYers use a professional approach to diagnose fuel pump issues. Whether you’re dealing with a no-start condition, low power, or sudden stalling, troubleshooting a fuel pump correctly can save you time, money, and frustration.

The Four Most Common Fuel Pump Failure Modes

A bad fuel pump can fail in different ways, and knowing these failure modes helps with troubleshooting:

• No fuel pressure – The pump isn’t supplying any pressure to the fuel rail.
• Low fuel pressure – The pump is running but not delivering enough pressure.
• Low fuel volume – The pump produces the correct pressure but doesn’t move enough fuel under high load.
• Pressure leak-down when the engine is off – Fuel pressure drops too quickly after shutting off the engine.

Common Symptoms of a Failing Fuel Pump

• Engine Sputtering at High Speeds or Loss of Power Under High Load— The fuel pump struggles to deliver a consistent fuel volume to the engine.
• Difficulty Starting the Engine After a Long Cold Soak— If the engine cranks much longer than usual on the first cold start of the day, that’s a sign the fuel pressure has leaked down after sitting or the pump can’t output the proper pressure during cranking.
• Engine Stalling—If your engine stalls unexpectedly, particularly during idling or at low speeds, it could be a sign of intermittent fuel pump failure.
• Whining Noise from the Fuel Tank— A loud whining noise from the fuel tank can indicate that the fuel pump is failing. This noise is often caused by a worn-out pump struggling to function.
• Check Engine Light— In some cases, a failing fuel pump can trigger the check engine light on your dashboard. An OBD-II scanner can help retrieve specific trouble codes related to the fuel system.

The Most Common Causes Of Fuel Pump Failure

• Driving frequently when the tank is near empty—A brushed electric fuel pump runs at about 100°F, even on a cold day. So, carmakers rely on the fuel to dissipate the heat. When you drive frequently with the tank near empty, the pump can run hot. Once that happens, the carbon brushes degrade and deposit on the copper portions of the armature, causing additional electrical resistance. Higher electrical resistance means lower current flow, which reduces pumping ability, causing pressure and volume loss.

• Fuel Tank Contamination— Dirt and rust in the tank can clog the fuel strainer, increasing suction resistance and causing the pump to run hotter than normal. If the particulate matter gets sucked through the strainer, it can wear out the pump impeller, resulting in a loss of pressure and volume capability.

Fuel tank contamination can also damage the seal in the fuel pump check valve. The check valve maintains fuel rail pressure when the engine is shut down. When the check valve fails, fuel rail pressure bleeds down, resulting in long crank times on the next cold startup.

• Electrical Resistance/Voltage Drop— Corrosion on the power feed or ground side of the fuel pump circuit can cause the armature coils to overheat and fail early.  In addition, high resistance in the electrical connectors can cause them to overheat, warp, and melt, further increasing the electrical resistance or stopping the flow of power altogether.

Troubleshoot a fuel pump step-by-step

Determine if the vehicle is equipped with a fuel pump driver module. These systems operate differently than the standard fuse/relay systems. Refer to the shop manual for the vehicle to determine if it uses a fuel pump driver module. To learn more about fuel pump driver module systems, see this article.

1) Check the fuse to the pump— A fuse and a fuel pump relay power all-electric in-tank fuel pumps. So, eliminating a blown fuse is always the first step on your checklist to troubleshoot a fuel pump that doesn’t provide any pressure.

2) Check the fuel pump relay— Tap the fuel pump relay with the handle of a screwdriver, and then try starting the engine. If that works, the confirms a faulty relay. If it doesn’t help, try swapping in a compatible relay.

3) Check for power and ground at the electrical connector on top of the fuel tank— The connections for an electric fuel pump are located on top of the fuel tank, where they’re exposed to the elements and can corrode. Troubleshoot the fuel pump connection by disconnecting the connector and checking for power and ground in the wiring harness connector terminals. If your meter shows power and ground, move on to the tips below. However, if you don’t see power at the fuel pump connector, and you’ve already checked the fuse and relay, look for an open in the wiring harness or a bad fuel pump ground connection on the body.

Smack the fuel tank to jump-start the fuel pump

If the fuel pump has developed a dead spot on the motor armature, one troubleshooting method is to smack the tank with a soft blow hammer or the heel of your shoe. Sometimes, the vibration from the heel of your shoe is enough to get you going. If this trick works, don’t think you’ve avoided the costly repair. It won’t work for long. Get the pump replaced as soon as possible.

Troubleshoot a fuel pump pressure leak down issue

A “cranks but won’t start” condition first thing in the morning or after the vehicle sits for a long time may be caused by a leaking check valve in the fuel pump. The check valve is designed to prevent all the fuel in the line from leaking back down into the tank when the engine is shut off.

Turbine-style fuel pump showing the turbine, motor, and check valve

When a check valve fails, the fuel lines are empty and can take up to 10 seconds of cranking to refill the lines. Here’s the procedure to test for a failed check valve

Turn the key to RUN, but do not crank the engine. Leave it in the RUN position for 2-3 seconds. Then turn to OFF. Turn the key back to the RUN position for another 2-3 seconds, but do not crank the engine. Repeat this procedure 2 more times. After four “prime” attempts, try starting the engine. If it starts right up, that proves the check valve is bad.

Troubleshoot a fuel pump low-pressure problem

Low fuel pump pressure can cause a “cranks but won’t fire up” condition. That’s because a fuel injector doesn’t really “inject” fuel. It’s just a valve that opens and closes. It relies on fuel pump pressure to atomize the fuel once the injector opens. However, fuel injector operation isn’t just a fully open or fully closed operation. The ECM pulses the fuel injector to regulate on/off time, which meters how much fuel enters the cylinder.

When the fuel pump pressure is too low, insufficient fuel enters the combustion chamber, so the engine can’t fire up. One way to trick the system is to press the gas pedal about halfway during cranking. That overrides the computer’s factory programming and tells the computer to add more fuel. So, the computer will tell the fuel injectors to remain open longer, which feeds more fuel to the cylinders. If that trick works to get you started, don’t rely on it forever. The pump is still failing, and it must be replaced.

The gold standard test is to install a fuel pressure gauge and test “dead-head” pressure.

Troubleshoot a fuel pump low-volume issue

The symptom of a low-volume fuel pump issue is a lack of acceleration, especially at higher speeds. In other words, the pump can’t output enough fuel to satisfy the engine’s requirements. This can be a confusing symptom because a clogged fuel filter can also cause this condition.

Test for cranking/running fuel pressure

Connect a fuel pressure gauge to the fuel rail using the correct fittings. Note the fuel pressure during cranking, when idle, and about five minutes later. Don’t get suckered into thinking the pump is good just because it outputs the correct pressure during cranking. A worn fuel pump can sometimes generate the proper pressure during cranking or at initial startup but fail to provide enough pressure or volume during extended idle or once the fuel pump heats up. In other words, testing fuel pressure takes longer than just a crank, note, and shut off. That approach will bite you in the butt.

Next, perform a fuel volume test

Connect your gauge to the fuel line from the pump to get a direct measure of fuel volume in a 30-second crank. Compare to the specifications. If the volume is low, finish testing the tests below before condemning the pump.

Move on to check the condition of the fuel pump’s electrical connectors and perform voltage drop tests

Check the condition of the electrical connector— High resistance in the terminals can cause enough heat to warp and melt the electrical connector. Warping, scorch marks, or melting are immediate signs of high resistance. Disassemble the connector and check for corrosion. Remove all corrosion and replace a melted/warped connector before moving on to voltage drop tests. Once you’re sure the connections are good, perform these voltage drop tests:

Voltage Drop Test On The Fuel Pump Ground Connection— Test for excessive voltage drop on the fuel pump ground.

Voltage Drop Test On The Fuel Pump Power Connection— Test for excessive voltage drop on the fuel pump ground.

Check the fuel filter for clogging that can reduce flow and pressure

If the vehicle has an external fuel filter, remove it and tap the filter on your workbench. If you see crud come out, replace it.

Check the mechanical pressure regulator

Next, consider the pressure regulator as a potential cause of low fuel pump pressure. The method for testing the regulator depends on the type of system. If the regulator is vacuum-style and mounted on the fuel rail, use a handheld vacuum pump to alter the regulator’s vacuum while watching the pressure gauge.

If You Replace The Fuel Pump

You must clean the tank. Not cleaning the tank is the #1 cause of repeat fuel pump failures. Follow this guide to clean the tank (From Fuel Pump Manufacturer Carter)

Safety Tips For Cleaning a Gas Tank

Wear appropriate clothing and protective gear such as safety goggles
Place a Class B or C fire extinguisher nearby
Store fuel in an approved container or tank
Keep all containers tightly closed and handle them gently to avoid spills
Set materials at least 50 feet away from ignition sources
Dispose of fuel per federal, state, and local guidelines

STEP-BY-STEP INSTRUCTIONS FOR PROPER FUEL TANK CLEANING:
1 Drain all fuel from the tank with an approved container or a fuel caddy
2 Drop the tank (see your service manual for further instructions)
3 Clean the area where the pump is being removed with a brush
4 Remove the fuel pump from the tank (avoid letting residual debris fall into the tank)
5 Insert a garden hose into fuel tank and begin pouring a stream of clean, hot water into the tank.
6 While the water is filling, spray a mild detergent in the tank
7 If the fuel pump opening allows, use a brush to loosen debris from the sides of the tank
8 Stop filling with water.
9 Swish the water around to ensure all debris has been captured
10 Dump the dirty, contaminated water out into a pan, being sure to capture any liquids
11 Further wipe the tank out with a lint-free washcloth
12 Dry the fuel tank completely with compressed air
13 Inspect the tank for rust or physical damage; if present, the tank must be repaired or replaced
14 Add fuel line antifreeze/water remover to help absorb any residual moisture
15 After the tank has been dried, let it sit for 30 minutes

©, 2019 Rick Muscoplat