Symptoms of a Bad Fuel Pump

The fuel pump is one of the most important components of a car. If the fuel pump is faulty or broken, the engine won’t start or may start but run rough. Let’s look at common symptoms of a faulty fuel pump and how to determine whether yours has stopped working.

Symptoms

The most common symptom of a bad fuel pump is that the car won’t start, or it may start but run rough. Typically, if the fuel pump is failing, you will experience difficulty starting the engine, loss of power, low gas mileage, an erratic idle, the engine dying intermittently, or sputtering. Other common symptoms include:

Poor Gas MileageA bad fuel pump may not always significantly affect engine performance.  A failing fuel pump may not generate the required fuel pressure or may only intermittently work, causing the engine to stall sporadically.

Loss of Power / Slow AccelerationIf your vehicle has a failing fuel pump, it may not accelerate like it used to.  If your fuel pump is faulty, the ECU will detect insufficient fuel and may adjust engine timing and spark to reduce power output and reduce the risk of engine damage. Luxury vehicles such as BMW, Porsche, and Mercedes-Benz are known to go into limp mode if the fuel pump malfunctions.

Difficulty StartingThe most common symptom of a bad fuel pump is that the engine will crank but won’t start. The pump must build up fuel pressure before starting the engine.  A faulty pump takes longer to prime the engine and, subsequently, much longer to get the engine running.

Engine MisfiresA bad fuel pump will not deliver enough fuel, leading to a misfire, check engine light, and fault codes ranging from  P0300 to P0312. If that happens while running, the air/fuel mixture may be lean; in other words, too much air and insufficient fuel can cause the engine to misfire.

Poor IdleA malfunctioning fuel pump can cause the vehicle to stutter while idling and run incorrectly. If your engine RPMs fluctuate at idle or the engine idles too low and almost dies, the problem could be the fuel pump.

Troubleshooting Tips

It is not uncommon for the fuel pump to stop working before reaching 100,000 miles, but most fuel pumps typically last much longer.The fuel pump on most cars is inside the fuel tank, located behind or below the rear seat, as shown in this picture.  The good news is that you can find out if the fuel pump is not running without removing it.  There are multiple ways to check if you have a bad fuel pump.

Let’s start with the simplest tests.

Listening TestTurn the key to the accessory position just before you turn the engine on. As you turn on the ignition, have a friend listen carefully to the rear seat. The fuel tank and fuel pump are right behind the rear seat. In most cars, you can hear the fuel pump run for a few seconds when the ignition is turned on.

Check FusesCheck the fuse for the fuel pump.  Every car has a fuse box on either the left or right side of the engine bay.

On the underside of the lid for that box is a diagram showing which fuse is for what function.  Find and remove the fuse for the fuel pump.  If the fuse is broken or burned, the pump has blown a fuse and needs to be replaced.  That should solve the issue.

Check Fuel PressureCheck the fuel pressure using a fuel pressure gauge. They are relatively inexpensive at most auto parts stores.  Most cars have a connection for a fuel gauge on top of the engine, at the end of the fuel rail, just before the fuel line goes to the fuel rails and injectors.

Hook the pressure gauge to the valve, turn the ignition on, and the indicator should jump from around 50 psi to 60 psi. Turn the engine on and verify the idle pressure.  Most cars have approximately 35 psi to 65 psi of pressure when idling.  Then, rev the engine to about 3000 rpm and look at the gauge.  If it falls sharply instead of rising, you have a problem.Another thing to check is the fuel filter.  This device is designed to keep contaminants out of the engine.  It occasionally gets clogged and needs to be replaced.  Some cars have the filter built into the pump, but most have the filter in the lines just after the gas tank or in the engine bay before the fuel rails. If you have a cartridge fuel filter, remove the cartridge and inspect it.  If it is nearly black, it is dirty and needs to be replaced.

The symptoms we listed above can be due to a worn-out fuel pump, but they may also be due to:

• Faulty Engine Control Unit (ECU)
• Damaged wire harness
• Corrosion contacts
• Damaged fuel lines

These are all issues that can cause fuel pump failure. If you replace the fuel pump and still don’t see fuel pressure or have fuel delivery issues, start looking at these possibilities.The next test is to use a digital multimeter to measure the voltage at the fuel pump. Disconnect the electrical connector from the fuel pump, then use the DC voltage setting to verify a signal. If your car has any problems, take it to a mechanic and have it checked out.  It may not be the fuel pump, but you may find a problem that needs immediate attention.  It’s better to fix the problem now than later.

Frequently Asked Questions 

We hope you find the Symptoms of a Bad Fuel Pump guide helpful. Check these troubleshooting and repair guides for more help on your vehicle.

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