Mercedes Mullins
Testing a fuel pump outside of a car is possible and can be done in several ways. The purpose of a fuel pump is to transfer fuel from the fuel tank to the engine. When it fails, the vehicle will no longer run, but it may show some subtle signs before it completely fails, such as a sputtering engine, losing power when driving uphill, or a car that won't start. Before testing, it's important to know if your vehicle has a serviceable fuel filter, as many modern vehicles have filters that are only changed when the pump is replaced. Some common methods to test a fuel pump include checking the voltage at the pump, using a fuel pressure gauge, and performing an amperage draw test.
| Characteristics | Values |
|---|---|
| Fuel pump failure signs | Struggling to accelerate, car not starting |
| Fuel pump function | Transfer fuel from fuel tank to engine |
| Fuel pump types | Mechanical, Electric motor |
| Electric fuel pump location | Inside the fuel tank |
| Fuel pump failure causes | Age, Lack of maintenance, Fuel contamination, Running vehicle low on fuel |
| Fuel filter replacement | Every 20,000 miles or per manufacturer's specifications |
| Fuel pump test | Check voltage, Use a fuel pressure gauge |
| Fuel pump test steps | Hook up fuel pressure gauge, Rev engine, Observe needle movement |
| Amperage draw test | Place amp clamp over positive wire, Record amperage reading |
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What You'll Learn
Check for signs of failure
The fuel pump is one of the most important components in a car's fuel system, delivering fuel to the engine and ensuring smooth and efficient operation. However, fuel pumps can encounter issues that disrupt your vehicle's performance over time. Here are some signs that indicate potential fuel pump failure:
Engine Misfires
Your engine may sputter, misfire, or threaten to stall, especially during acceleration or when driving at higher speeds. This could be because the pump cannot produce a constant stream of gas at higher speeds, causing the engine to not get enough gas. However, if the engine is getting some gas from the pump, even a lesser amount than needed, it won't likely stall.
Difficulty Starting the Engine
You may experience difficulty in starting your car, requiring extended cranking before the engine kicks in. In some cases, the car may not start at all, especially in cold weather. This could be due to a failure of the fuel pump to create a vacuum, a clogged filter, or a kinked/blocked fuel line.
Stalling or Surging
The engine may stall or surge unexpectedly while driving, which could be due to the pump pushing too much fuel into the engine. This can lead to power surges, resulting in speed spikes and drops.
Decreased Fuel Efficiency
You may notice that you're making more trips to the gas station and experiencing a significant drop in your car's mileage or fuel efficiency. This could be a sign that the fuel pump is pushing too much fuel into the engine, causing you to burn more gas than normal.
Fuel Smell
You may detect the smell of gasoline inside or around your car, which could indicate a potential issue with the fuel pump.
It is important to identify and troubleshoot these problems early on to maintain optimal functionality and prevent unexpected breakdowns. While some issues may be resolved through essential maintenance, others might require professional assistance.
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Test with a fuel pressure gauge
Testing a fuel pump with a fuel pressure gauge is a straightforward process. First, you need to locate the fuel pump test point, which is usually near the fuel injectors, and the point at which the pump hooks up with the filter injector rail. There should be a separation joint or a test port, where the pressure gauge attaches. Consult your owner's manual for specific instructions as the location of the fuel pump varies from vehicle to vehicle.
Before attaching the pressure gauge, be sure to relieve the fuel system pressure as outlined in your repair manual. Put on your safety glasses and have a fire extinguisher nearby.
Next, hook up the pressure gauge to the test port. Different gauges may have slightly different instructions, so refer to your manual. Start the car and let it idle. Run the pump and note the pressure reading. Then, have a helper rev the engine while you check the gauge. Check the pressure at idle speed and at the rated speed listed in your pump specifications. If you don't know the rated speed, simply rev the engine and observe how the pressure reacts. If the needle doesn't move or doesn't move much, the fuel pump is faulty and needs to be replaced.
In many cases, you can also use a fuel pressure gauge to measure volume. 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.
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Check the voltage
Checking the voltage of a fuel pump is an important test to ensure the pump is effective. The fuel pump is located at the back of the vehicle, next to or in the fuel tank, and it pumps fuel from the tank to the engine.
To check the voltage, you will need a digital multimeter or a voltmeter. First, connect the multimeter to the positive and negative terminals of the fuel pump. Ensure there are no broken circuits between the fuel pump and the relay. Insert the key into the ignition and turn it on without starting the engine. You should hear a soft whirl, indicating that the pump is ready to supply fuel to the engine.
If the pump is functioning, the next step is to check if it is receiving enough electrical power. A deficiency in power supply may cause the multimeter to indicate a false voltage reading. If the voltage drop is greater than the recommended reading of 0.1 volts, this confirms that the wiring or harness connectors are faulty.
If there is difficulty removing the pump from the vehicle, take the battery to the back of the vehicle. Connect the jumper wires to the battery and the fuel pump, and test for voltage, being careful not to cause sparking.
For a more detailed test, you can perform a voltage drop test. Connect the positive lead of the voltmeter to the battery and the negative lead to the positive wire at the fuel pump connector. Turn the key in the ignition and observe the voltage. If the voltage drop is greater than 0.5 volts, there is an issue with the wires. Repeat the test for the negative side.
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Test with a scan tool
Scan tools have become the most important tool when diagnosing a fuel supply problem. They can be used to verify if the modules controlling the fuel pump are receiving the correct data, such as oil pressure, crank position, and security information.
To begin the testing process, pull the codes and make sure the modules are communicating on their communication buses. With the codes pulled, you can develop diagnostic strategies and further tests to resolve the no-start condition. Service information is just as critical as a pressure gauge. Every fuel system has a set of parameters that must be set for the pump to be energised. For some systems, this may include a crank sensor signal, oil pressure, and a check of the vehicle theft deterrent module.
If the vehicle has any "loss of communication" codes that start with a "U", resolve those problems first before diagnosing or replacing the fuel pump. While these codes may seem unrelated to the fuel pump, a dead module or short in the serial bus can often result in a no-start condition.
Some high-end scan tools allow remote fuel pump testing. By pushing a few buttons on the tool, you can activate the pump to see if it runs. If the fuel pump is completely dead, the engine won't run, and your vehicle won't start.
Factory and enhanced scan tools can monitor pressure transducers on the high and low sides of the system. This information can be used to diagnose the health of the low-side and high-side pumps. These tools will have the PID parameters for these components as part of the Mode 6 data. These parameters can tell you what the pressures should be during the different modes of operation.
Additionally, if this data is used in conjunction with the waveforms of the injector pulses, it's possible to perform cylinder balance and other diagnostic tests.
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Check the filter
It is important to know what type of fuel system your vehicle has before beginning any tests or procedures. Low fuel pressure generally indicates a failed fuel pump. However, it can also point to a restricted line, restricted filter, or faulty pressure regulator.
If you suspect a fuel system issue and do not know when your filter was last replaced, you may want to replace it first. If your car is struggling to accelerate, especially on the highway, or you notice other signs that your car might not be getting enough gas, the problem could be caused by a partially blocked or clogged fuel line, fuel filter, pump, or injector. If your car has started sputtering when you accelerate quickly, this could be a symptom of a clogged fuel filter or a bad fuel pump.
To check the filter, remove it from the vehicle and drain the excess fuel from the filter. Use a short piece of rubber hose on the filter inlet. Blow through the filter inlet, paying attention to the resistance, which should be minimal. Inspect the screen for debris and replace the filter if necessary by blowing through the outlet side of the filter into a white rag or towel.
If you are unsure of what type of fuel system you have, or are not comfortable testing your fuel pump, then you should enlist the assistance of a certified mechanic.
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Frequently asked questions
You can test a fuel pump out of the car by locating the fuel pump, hooking up a fuel pressure gauge to the fuel pump test point, and having someone else rev the engine. If the needle doesn't move, the fuel pump is faulty.
A faulty fuel pump can cause the engine to sputter or the car to suddenly die while driving. You may also hear strange whining sounds from the fuel tank. Your vehicle may lose power when driving up an incline or carrying heavy cargo.
If you suspect a fuel pump issue, you should first check if your vehicle has a serviceable fuel filter. If the filter is clogged, you can remove and drain it, then blow through the filter inlet to check for resistance. If the filter is fine, you can perform an amperage draw test to see how hard the pump is working. Compare the reading to the manufacturer's specifications to determine if the pump needs to be replaced.
