mechanic diagnosing bad fuel pump

How to Test a Fuel Pump

Diagnose a Bad Fuel Pump

Mechanic showing how to test a fuel pump to see if it's bad

If your engine won’t start, you could have a bad fuel pump. When you switch on your vehicle, the fuel pump activates and sends pressurized fuel from the gas tank to the engine. This can be heard as a subtle hum or whine immediately after the key is turned on. However, if you have a bad fuel pump, you may hear a fast cranking sound and experience difficulty in starting your car. Here we’ll show you how to test a fuel pump.

There are plenty of car problems that mimic a failing fuel pump. One Such problem is a dead car battery. It’s therefore, always important to first carry out a thorough diagnosis to confirm that the pump is the problem. In today’s post, we’ll take a look at the step-by-step process of how you can easily diagnose a bad fuel pump. 

Steps to Diagnose a Bad Fuel Pump

Step 1: Ensure the battery is fully charged

Mechanic testing the battery for charge

The fuel pump receives power from the battery. As such, it is important to have the battery fully charged to help with diagnosing a failing fuel pump. This will help determine whether the problem is with the pump or the connection that leads to it.

A dead battery can also cause the engine to crank slow. You want to ensure that the battery is working fine and that it’s not the cause of the cranking sound emanating from the engine when you start the car. 

Step 2: Use a test light to check the fuel pump fuse

Testing to see if the fuse is bad

Start by testing the test light to ensure it works fine. Connect the lead end to the negative battery terminal and the test light tip to the positive terminal. Once done, open your fuse box and test each fuse. You’ll want to find out if you have a blown fuel pump fuse

A bad fuse is normally a symptom of a bigger problem. Your fuel pump could be sucking too much amperage or it could have a ground connection that’s shorted. Replacing the bad fuse is a temporary solution that could bring things back to normal, but you’ll want to check the connection and fuel pump as well. 

Step 3: Check the fuel pump pressure

Testing fuel pump pressure

Most modern cars that run on an internal combustion engine use electric fuel pumps that supply the high fuel pressure demanded by the vehicle’s system. The normal pressure ranges between 50 psi and 60 psi. You’ll need to check your owner’s manual or do a bit of research to check what your car’s fuel pressure should be. 

To test fuel pressure, look for a service cap located on your fuel rail. You should find it near the fuel injectors. Removing the cap will reveal a Schrader valve to which you can connect a fuel pressure gauge. Turn the ignition key to the on position and check the reading on the gauge. It should jump to between 45 psi and 60 psi. 

Step 4: Locate the fuel pump

fuel pump located under back seat

If your fuel pressure gauge shows there’s no pressure, you could have a problem with the fuel pump. You’ll want to first locate where it is. Most cars have it mounted inside the fuel tank. You can either access it through the back seat by removing the seat base cushions or by first taking out the fuel tank and accessing the fuel pump on top of it. 

Step 5: Test for power and ground

testing voltage to the fuel pump with a test light

Once you locate your fuel pump, you’ll find a connector that sends power to it. Start by slightly wiggling it before taking it out. This helps to ensure it’s not loosely connected. Disconnect it once you’ve confirmed that it’s okay. 

Like most vehicles, the connector should have two thick wires and two thin wires connected to it. The thick wires are for power and ground while the thin wires are the float or level sensors that send signals to the gauge cluster. 

Pick a pair of jumper wires and connect one to the power wire and the other to the ground. Connect the other ends of the jumper wires to the test light to form a complete circuit. Connect the ground wire to the lead end of the test light and the power wire to the tip of the test light. Have someone turn the ignition key on as you hold the connection. 

If the test light lights up, you have good power and ground going to the fuel pump. This confirms that you have a bad fuel pump that’s not pressurizing the fuel. It will need to be replaced. If the test light didn’t light up, you’ll need to check for lost power or lost ground. 

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Step 6: Check for lost power or ground

testing the connectors on a fuel pump

To check if you have no power going to the fuel pump, connect the lead end of your test light to a good ground and its tip to the power wire. A good ground connection could be a bolt located on the car’s door. Have someone turn the key on and off. If you have good power, the test light should light up. 

To check for lost ground, connect the lead end of the test light to the positive terminal of your car’s battery and the test light tip to the ground wire. Have someone turn the ignition key to its on position. If the test light powers up, you have a good ground. If it doesn’t, you may have a problem in the circuit. 

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Summary
How to Test a Fuel Pump
Article Name
How to Test a Fuel Pump
Description
If your engine won’t start, you could have a bad fuel pump. When you switch on your vehicle, the fuel pump activates and sends pressurized fuel from the gas tank to the engine.
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Publisher Name
1A Auto
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