Measuring Electric Fuel Pump Pressure: A Step-By-Step Guide

A fuel pump is a critical component of a car's powertrain system, transferring petrol or diesel from the fuel tank to the engine. A malfunctioning fuel pump can cause a lack of power output from the engine and even cause the vehicle to break down. Therefore, it is important to be able to identify the warning signs of a failing fuel pump and know how to check its pressure. This can be done by performing an electrical test and a fuel pressure test.

Using a fuel pressure gauge

First, park your vehicle and apply the parking brake. Allow the engine to cool down. Locate the fuel pressure test port—most car models have one on the fuel injector rail. Place a rag under the test port to catch any fuel that may be released, then install the pressure tester to the port.

Now, start the engine and record the pressure reading. You can find the required pressure specification in the manufacturer's manual. If your fuel pump is not meeting the required pressure, it may be failing to pump fuel to the engine. A typical port-injected vehicle requires fuel pressure between 30 and 80 PSI.

If you don't have a fuel pressure tester, you can try a few things first. Firstly, check that there is actually fuel in the tank. If there is, and the car still won't start, move on to the next step: verifying that the fuel pump works. To do this, ask an assistant to turn the ignition switch to "On" and listen for a two-second hum or a series of rapid clicks as the fuel pump pressurizes the fuel line. If you don't hear this, the pump may not be getting power or it may have failed. Check the fuel pump fuse and relay, and if those are fine, check the wiring to the pump.

Once you've established that the pump has power and is delivering fuel, it's time to use the fuel pressure tester. Pop the hood and find the Schrader valve fitting on the fuel rail. Remove the Schrader valve cap and attach the appropriate fuel pressure tester fitting, ensuring a leakproof fit. Turn the ignition to "On" and check the psi reading. If the pressure drops, this indicates a leak in the system. If the pressure holds, start the engine and let it idle. You should see steady fuel pressure, within a few psi of the recommended pressure. Once the engine is warmed up, rev it slowly and check that the pressure rises with the RPMs. If the pressure holds steady and rises with engine speed, your engine problem is likely not fuel-related.

Checking the fuse

Locate the Fuse Box:

Start by locating the fuse box. Modern cars have multiple fuse boxes, usually well-hidden and requiring access to panels like the glove box, trunk carpeting, or kick panel removal. Refer to your vehicle's owner's manual to identify the correct fuse box and the specific fuel pump fuse. The manual should provide a diagram with the fuse box location, fuse number, rating, and colour.

Remove the Fuel Pump Fuse:

Once you've located the correct fuse box, remove the fuse using a fuse puller tool attached to the fuse box cover or a small pointy nose plier. It's important to grip the fuse firmly and pull it straight up to avoid damaging the terminals.

Inspect the Fuse:

Hold the removed fuse up to the light and inspect the metallic strip inside. A blown fuse will have a broken or burnt strip. If the strip is intact, the fuse is likely functioning correctly. You can also use a test light or a voltmeter to check the fuse without removing it, although removing it is generally recommended due to potential corrosion.

Replace the Fuse if Necessary:

If the fuse is blown, replace it with a new one of the same rating. Using a fuse with a lower rating may cause it to blow again, while a higher-rated fuse can lead to damage to other components and increase the risk of fire. Refer to the owner's manual or fuse cover diagram to determine the correct fuse rating.

Test After Replacing the Fuse:

After replacing the fuse, attempt to start the vehicle. If the new fuse blows, it indicates a direct short circuit, and further diagnosis is required. Take your vehicle to a mechanic or perform additional tests to identify the specific issue.

Checking the voltage

Locate the Fuel Pump Fuse: Check your owner's manual to find the location of the fuse box. Identify the fuse that corresponds to the fuel pump and carefully remove it. Inspect the fuse for any signs of failure, such as a break or burn marks. If the fuse appears damaged, replace it with a new one of the proper amp rating.

Check Voltage at the Pump: Even if the fuse looks intact, it's crucial to ensure that power is reaching the fuel pump. Refer to your vehicle's service manual to locate the correct testing point and follow the recommended procedure for checking the voltage. This step confirms that the circuit is supplying power to the pump.

Perform a Drop Test with a Voltmeter: To perform a drop test, you'll need a voltmeter. Connect it to the power wire and ground wire of the fuel pump. Check for the full voltage on the power wire and ensure that the ground wire is properly grounded. If you observe a voltage drop of more than one volt, it indicates an issue with corroded wires or problems in the positive or negative circuit.

Test for Source Voltage: If the voltage at the pump seems fine, the next step is to determine whether the good charge leaving the fuse is reaching the pump. Perform a source voltage test to confirm. If there is no power reaching the fuel pump, move on to the next step.

Check the Fuel Pump Relay Circuit: If the source voltage test indicates that power is not reaching the fuel pump, the issue may lie with the fuel pump relay circuit. A faulty relay could be the culprit. Check the relay and, if necessary, replace it to restore power to the fuel pump.

Consult a Professional: If you've completed all the above steps and are still experiencing fuel pump issues, it's time to seek professional assistance. Take your vehicle to a trusted mechanic or auto repair shop for further diagnostics and repairs. They will have the expertise and equipment to identify and resolve more complex issues.

Remember, working on a vehicle's electrical system can be dangerous, so always exercise caution and refer to reliable sources for detailed instructions specific to your vehicle's make and model.

Performing a drop test

Voltage Drop Test from the Power Side

First, ensure you are wearing safety glasses. Do not probe the vehicle electrical harness connector with DMM leads, as this can cause damage to the terminals in the harness. Instead, use the proper test probe to perform electrical tests. You will need a digital multimeter (DMM) and access to the fuel pump connector and battery. Set the multimeter to 20 volts D-C scale, or D-C if your multimeter has auto-ranging capabilities.

Connect the multimeter positive probe to the battery positive terminal. With the fuel pump wire harness connected to the fuel pump, connect the multimeter negative probe to the power feed wire at the fuel pump connector. Turn the ignition to the “on” position. The pump will only run for about two seconds while the relay is in prime position, or until it gets an RPM signal. The reading should be less than 0.2 volts. If the reading is greater, check for resistance in the power supply circuit for the fuel pump.

Voltage Drop Test from the Ground Side

The process is very similar for the ground side. Set the multimeter to 20 volts D-C scale, or D-C if your multimeter has auto-ranging capabilities. Connect the multimeter positive probe to the ground terminal of the fuel pump connector. Ensure the fuel pump wire harness is connected to the fuel pump. Then connect the DMM’s negative probe to the battery negative terminal. Turn the ignition to the “on” position. Again, the pump will only run for about two seconds. Your reading should be less than 0.2 volts D-C. If it’s greater than that, check for resistance in the ground supply circuit for the fuel pump.

Additional Checks

If you show more than one volt difference, this indicates a problem with corroded wires, or issues with the circuit on either the positive or negative side. Take your vehicle to a garage for further testing and advice.

You can also leave the pump plugged in, connect your black multimeter lead to a ground, and back probe the 12V with your read lead. It should read 12v while the pump is priming and 12V with the key in the on position. If the voltage drops while priming, the 12V wire you are back probed into has a problem. Follow that wire and look for the issue.

Testing the filter

• Remove the filter: Park your vehicle on a level surface and engage the parking brake. Ensure the engine is cool before starting work. Locate the fuel filter, which is usually found near the fuel tank or fuel lines. Place a container underneath the filter to catch any spilled fuel. Carefully disconnect the fuel lines leading to and from the filter, using appropriate tools, and remove the filter from the vehicle.
• Drain excess fuel: Once the filter is removed, allow excess fuel to drain into the container. Be cautious as the fuel may be flammable and handle it according to safety guidelines.
• Inspect for debris: Visually inspect the filter for any signs of debris, sediment, or clogging. A clogged filter may appear dark or discoloured. If the filter appears severely clogged, it is likely due for a replacement.
• Perform a blow test: Take a short piece of rubber hose and attach it to the filter inlet. Blow through the hose and filter, paying attention to the resistance. There should be minimal resistance if the filter is clear. If you experience significant resistance, it indicates that the filter is clogged and needs to be replaced.
• Check the screen: Remove the filter element or cut open the filter to inspect the screen or filter media. Look for any debris, gunk, or clogging on the screen. If the screen is heavily clogged, replace the filter.
• Backblow the filter: As an optional step, you can try backblowing the filter. Attach the hose to the outlet side of the filter and blow into the inlet side. Any debris or sediment trapped in the filter will be dislodged and blown out. However, this step may not be effective for heavily clogged filters.
• Reinstall the filter: If the filter appears to be in good condition, with minimal debris and no significant clogging, you can reinstall it. Ensure you follow the correct installation procedure and tighten all connections securely.
• Test the vehicle: After reinstalling the filter, start the engine and observe its performance. If the filter was the cause of the issue, you should notice improved acceleration and engine performance. If the problem persists, you may need to investigate other potential causes, such as a faulty fuel pump or a clogged fuel line.

Remember to refer to your vehicle's repair manual or seek advice from a professional mechanic if you are unsure about any steps or procedures. Working on fuel systems can be dangerous, so always prioritise your safety and have a fire extinguisher nearby.

Frequently asked questions