Mercedes Mullins
Testing a fuel pump out of the tank can be done using a variety of methods. The first step is to verify that the problem is fuel-related, and then additional testing can be done to determine if the pump is faulty. This can include checking the voltage at the pump, performing a drop test with a voltmeter, or using a fuel pressure gauge to check fuel pressure and volume. If the pressure bleeds off too quickly, there may be a fuel-related problem such as a faulty pump check valve or regulator. Running the pump backward can help clear the filter, and it is recommended to use diesel or kerosene for safety reasons.
| Characteristics | Values |
|---|---|
| Step 1 | Verify the problem is fuel-related. |
| Step 2 | Listen for the fuel pump by putting your ear near the fuel tank and turning the ignition key to the "on" position. |
| Step 3 | If the fuel pump doesn't make an audible noise, check if the engine controller is awake by connecting a scan tool. |
| Step 4 | Determine if the vehicle has a fuel pump control module between the fuel pump relay and the pump. |
| Step 5 | If the relay directly powers the pump, try swapping out the relay. |
| Step 6 | Locate the fuel pump relay, the related fuse, and the fuel pump fuse. |
| Step 7 | Check the voltage at the pump itself using a voltmeter. |
| Step 8 | Perform a drop test to ensure the power wire shows full voltage and the grounding wire is properly grounded. |
| Step 9 | Hook a fuel pressure gauge to the fuel pump test fitting, which is usually near the fuel injectors. |
| Step 10 | Rev the engine and check the pressure at idle speed and the rated speed listed in your pump specifications. |
| Step 11 | If the pressure is below specifications, the fuel pump needs to be replaced. |
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What You'll Learn
Check for fuel-related issues
To check for fuel-related issues, you must first verify that the problem is indeed fuel-related. Place your ear near the fuel tank and ask an assistant to turn the ignition key to the "on" position. If you don't hear the fuel pump, you'll need to ensure that the engine controller is awake. Connect your scan tool and check if the PCM communicates with it. If it doesn't, find out why.
Next, locate the fuel pump relay and the related fuse. Check the fuel pump fuse, and if it's not blown, feel the relay while someone switches the ignition on and off. If you feel a click, proceed to the next step. If not, you may have an issue with the fuel pump circuit.
Now, with the key on and the test light clip connected to the negative battery terminal, look for two hot terminals. Mark these terminals, as one is the coil feed, and the other powers the relay delivering power to the pump. Move the test light clip to the positive post. If one of the other two terminals is grounded, the ground is coming through the pump windings, indicating a complete pump circuit. If neither shows a ground, the fuel pump windings are open, or there's an open circuit between the relay and the fuel pump. Keep the test light in place and ask someone to kick the gas tank or hit it gently with a soft rubber hammer. If the light illuminates, the fuel pump requires replacement due to worn-out brushes.
Additionally, you can perform a drop test using a voltmeter to ensure the power wire shows full voltage and the grounding wire is correctly grounded. If there's a difference of more than one volt, you likely have corroded wires or issues with the circuit on the positive or negative side.
To check fuel pressure, purchase a mechanical fuel pressure gauge, which is inexpensive and readily available. Relieve the fuel system pressure as outlined in your repair manual before attaching the gauge. Wear safety glasses and keep a fire extinguisher nearby. Most modern vehicles have a test port on the fuel rail, making it easy to connect the gauge. Check the pressure at idle speed and the rated speed listed in your pump specifications. If the needle doesn't move or drops below specifications, the fuel pump needs replacement.
You can also try the following: fill a container with gasoline, diesel, or kerosene/diesel, and place the pump in it. Apply 12 volts for the same duration it takes to prime when turning the key. Running the pump backward can help clear the filter.
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Test the fuel pump relay circuit
Testing the relay of a fuel pump is relatively simple, as the fuel pump relay circuit doesn't have many parts. The most common parts to burn out or break are the contact and the coil. When either of these begins to fail, the current to the electrical circuit will also fail, causing the fuel pump to cease to function properly.
To test the fuel pump relay circuit, start by turning off your vehicle and removing the key from the ignition. Open your vehicle's hood and find the fuse panel. The majority of vehicles have the fuel pump relay located near here, and it should look like a small plastic box with four to five electrical terminals. However, some fuel pump relays come in different shapes and sizes, so locate your manufacturer's diagram if you're having trouble finding it.
Once you've found the fuel pump relay, unplug it and place it on a work surface. Generally, there are four pins (terminals) on the fuel pump relay: one for input voltage, one for the ground pin, one load pin that goes to the fuel pump, and one that goes to the battery. To figure out which pin is which, there will usually be numbers on the bottom of the relay that indicate the pin positions, or you can locate a wiring diagram from your relay's specific manufacturer.
Now, set your multimeter to resistance mode and connect its probes to the relay. One probe should connect to the pin that goes to the fuel pump, and the other should connect to the pin that goes to the battery. Use a 12V battery or power supply to test the relay, and connect your ground connection on the power supply to the ground pin on the relay.
When the circuit inside the relay has power, the prongs should not have any resistance between them. Look at the readout on the multimeter's screen to see if it reads 0, meaning the circuit has no resistance between the points. If the reading is greater than 0, then the circuit inside the relay is faulty.
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Check the voltage at the pump
Checking the voltage at the pump is an important test to assess the effectiveness of the fuel pump. It is important to note that insufficient pressure and flow indicate that a fuel pump is underperforming, but this does not necessarily mean it is failing.
To check the voltage at the pump, you will need a digital multimeter. First, establish the location of the fuel pump relay on the dashboard or engine compartment. Remove the relay and have someone switch on the ignition without starting the engine. Now, check for voltage at the relay connector using the digital multimeter. Next, replace the relay and test the fuel pump connector at the back of the car for incoming voltage using the multimeter.
Remove the fuel pump from the vehicle and place it on a flat surface. If there is difficulty in removing the pump, take your battery to the back of the vehicle. Connect jumper wires to the battery and the fuel pump and test for voltage, being careful not to cause sparking.
When using a digital multimeter, connect it to the positive and negative terminals of the fuel pump. Ensure there are no broken circuits between the fuel pump and relay. Insert the key into the ignition and turn it on without starting the engine. A soft whirl should be audible, indicating that the pump is ready to pump fuel into the engine. If the pump is functioning, check if it is receiving enough electrical power. A power deficiency may cause the multimeter to indicate a false voltage reading.
It is important to wear safety glasses and use the proper test probe when performing electrical tests. Additionally, some have suggested "backprobing" the connector by inserting the black (negative) probe from the multimeter alongside the black wire (ground) and inserting the red (positive) probe alongside the gray wire (power to the pump).
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Perform a drop test using a voltmeter
To test a fuel pump out of the tank using a voltmeter, you will need to perform a voltage drop test. This test will help you determine if there is a problem with the voltage supply to the pump, which could be due to damaged or corroded wiring or connectors. Here is a step-by-step guide on how to perform this test:
Firstly, locate the fuel pump and place it on a flat surface. The fuel pump is usually located at the back of the vehicle, next to or in the fuel tank. Disconnect the pump's electrical connector, which is often found near the fuel tank. Be careful not to damage any components during this process.
Now, set up your voltmeter. Ensure it is set to measure DC voltage, as you are looking for a reading between 12 to 14 volts. Identify the positive and ground wires in the connector, referring to your vehicle's manual if needed.
Next, connect the voltmeter to the fuel pump. Touch the positive probe of the voltmeter to the connector's positive terminal and the negative probe to the ground terminal. Ensure that the ignition key is turned to the "On" position, but do not start the engine.
If the voltmeter displays a correct voltage reading, it indicates that the pump's electrical supply is adequate. However, if the reading is lower than expected, there may be an issue with the voltage supply, and you should proceed to the next steps to further diagnose the problem.
To perform a voltage drop test, you will need to split the circuit into two parts: the ground phase and the positive phase. Use a jumper wire to power the fuel circuit and energize the pump. Connect the voltmeter to the negative terminals of both the battery and the pump. It is important to use a live circuit wire for this test.
If the voltmeter indicates a reading of more than 0.1 volts, it confirms a loss of voltage power. This could be due to damaged harness connectors or poor wiring. To pinpoint the issue, repeat the test on the positive side of the circuit. Connect the voltmeter to the positive terminals of the pump and the battery. If the voltage drop is higher than the recommended reading, it confirms a fault in the wiring or connectors.
By following these steps and performing a voltage drop test using a voltmeter, you can effectively diagnose issues with your fuel pump's voltage supply and identify potential problems with wiring or connectors. This information will help guide further troubleshooting and repairs.
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Check fuel pressure
Checking your vehicle's fuel pressure is important to ensure that enough fuel is reaching the engine under the right pressure. Low fuel pressure can cause the engine to not run properly and can indicate a failing fuel pump. Here are the steps to check your vehicle's fuel pressure:
Prepare the Vehicle:
Before testing, ensure your vehicle's fuel tank has at least two gallons of fuel. Confirm that the fuel gauge is operational, as you will need a functional fuel pump to get a proper reading. If the fuel gauge is not working, manually inspect the tank to confirm it is full.
Check the Fuel Pump:
To verify that the fuel pump is working, turn the ignition switch to "On" and listen for a two-second whir, hum, or series of rapid clicks. This indicates that the fuel pump is pressurizing the fuel line to the engine. If there is no noise, check the fuel pump fuse, relay, and wiring to the pump.
Use a Fuel Pressure Gauge:
To check the fuel pressure, you will need a fuel pressure gauge, also called a fuel system pressure tester. Work in a well-ventilated area as fuel fumes may escape. With the engine off, attach the fuel pressure gauge to the fuel pump test point, usually near the fuel injectors.
Test Fuel Pressure:
Have an assistant rev the engine while you observe the gauge. Refer to your vehicle's repair manual for fuel pressure specifications. Check the pressure at idle speed and at the rated speed listed in the pump specifications. If the pressure is too low or too high, it can affect the vehicle's performance.
Release Fuel Pressure:
Once you have taken the fuel pressure reading, shut off the engine and release the fuel pressure from the gauge into a container. Properly dispose of or reuse the released fuel. Disconnect the fuel pressure gauge and wipe up any spilled fuel.
Alternative Methods:
If you do not have access to a fuel pressure gauge, you can use an OBD diagnostic tool to check fuel pressure. Smartphone apps can also display real-time fuel pressure information by connecting to a third-party OBD-II wireless transmitter. Additionally, running the pump backward can help clear the filter.
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Frequently asked questions
To test a fuel pump out of the tank, you can put it in a container with gasoline and hit it with 12 volts for the same amount of time it primes when you hit the key.
First, verify that the problem is fuel-related. Listen for the fuel pump by putting your ear near the fuel tank and turning the ignition key to the "on" position. If you don't hear anything, check that the engine controller is awake. If the problem is fuel-related, you can do a drop test using a voltmeter to check the power wire for full voltage and ensure the grounding wire is properly grounded.
If you're struggling to accelerate or notice other signs that your car isn't getting enough gas, your fuel pump may be partially blocked or clogged. You can also check the fuel pressure and/or volume to determine if enough fuel is making it from the tank to the engine. If the pressure bleeds off too quickly, there is likely a fuel-related problem.
