Understanding Fuel Pump Pressure: The Basics To Know

Fuel pump pressure is critical to the performance of liquid-fuelled engines. A fuel pump transfers fuel from the tank to the engine, where it is mixed with intake air. The pressure of the pump can affect the engine's performance, and inadequate pressure can cause issues such as stalling, hesitation, and misfiring. To test fuel pump pressure, one must attach a fuel pressure gauge to the engine's fuel rail or fuel pressure feed line. If the pressure is inadequate, there may be an issue with the pump, fuel filter, or fuel pressure regulator.

Testing a fuel pump

Prepare for the Test:

Before beginning the test, ensure that you have the necessary tools, such as a screwdriver, fuel pressure gauge, ratchets, and sockets. It is also important to prioritise safety. Wear safety gear, such as glasses and gloves, work in a well-ventilated area, and avoid smoking or any potential sources of sparks.

Locate the Fuel Pressure Test Port:

Park your vehicle on flat ground, apply the parking brake, and turn off the engine to allow it to cool down. Locate the fuel pressure test port, which is usually found near the fuel injectors or the fuel tank. Place a rag under the test port to catch any fuel that may be released during the test.

Install the Pressure Tester:

Install the pressure tester to the test port. Refer to the manufacturer's instructions or a repair manual for specific details on the installation process, as it may vary depending on your vehicle's make and model.

Check the Fuel Pressure:

Start the engine and record the pressure reading. Compare the reading to the manufacturer's specifications, which can typically be found in the vehicle's manual. A typical port-injected vehicle requires fuel pressure between 30 and 80 PSI. If the pressure is lower than specified, it could indicate a problem with the fuel pump.

Perform a Fuel Volume Test:

If the fuel pump is supplying sufficient pressure, perform a fuel volume test to ensure that the proper amount of fuel is being delivered to the fuel injectors. This can be done by collecting a fuel sample for a specified duration (e.g., five seconds) while the pump is running and comparing it to the manufacturer's specifications.

Additional Checks:

If the fuel pressure and volume tests indicate a problem, you may need to perform additional checks, such as verifying the fuel pump fuse, checking the voltage at the pump, or performing a drop test using a voltmeter, as described in the "Performing an Electrical Test" section of one of the sources.

Seek Professional Help:

If you are unsure about any aspect of the testing process or if the results indicate a problem with your fuel pump, it is recommended to consult a qualified mechanic for further diagnosis and repair.

Checking fuel pressure

Method 1: Throttle Linkage Test

Remove the air cleaner and look into the throat of the carburetor. Pump the throttle linkage a few times. If the fuel pump is working properly, you should see fuel squirting into the carburetor. If there is no fuel or a weak stream, it could be due to a faulty pump, a blocked fuel line or filter, or an empty fuel tank.

Method 2: Visual Inspection

Inspect the fuel pump for any signs of fuel dripping. A leaky diaphragm is a sure sign that the pump needs to be replaced. Be cautious, as leaking fuel can be dangerous and may ignite, causing a fire.

Method 3: Carburetor Test

Disconnect the fuel line at the carburetor and place the end of the line into a container. Crank the engine to observe if the pump is pushing fuel through the line. Strong, steady spurts of fuel indicate a functioning pump. No fuel or a weak stream suggests a faulty pump, a clogged fuel filter, a blocked fuel line, or an empty fuel tank. Exercise caution during this test to avoid any potential fire hazards.

Method 4: Fuel Pressure Gauge Test

Connect a fuel pressure gauge to the pump outlet or tee it into the fuel line at the carburetor. Crank the engine and check the pressure reading. If there is no pressure or the pressure is below specifications, the pump requires replacement. This method provides an accurate measurement of the fuel pump's performance.

Additional Tips:

• Always exercise caution when working with fuel. Avoid smoking or creating sparks near the carburetor or open fuel lines.
• Ensure the engine is cool before performing any work on the fuel system.
• Avoid skin contact with gasoline and do not inhale the vapors.
• If you need to replace the fuel pump, opt for a quality name-brand pump with a warranty.

Low fuel pressure issues

Low fuel pressure can cause a range of issues with your vehicle's performance and reliability. Fuel pressure refers to the force that delivers fuel from the tank to the engine. When fuel pressure is insufficient, the engine may not receive enough fuel, leading to a lean fuel mixture and a range of problems.

Symptoms of Low Fuel Pressure:

• Difficulty Starting the Engine: Low fuel pressure may cause difficulty in starting the engine, especially after it has been sitting idle for a while. The engine struggles to ignite due to inadequate fuel supply.
• Poor Acceleration: Insufficient fuel pressure can result in hesitation or stumbling during acceleration. The engine doesn't receive the necessary fuel to increase power output quickly.
• Engine Stalling: Low fuel pressure can cause the engine to suddenly stall, especially at low speeds or when idling.
• Misfires and Rough Idling: A rough or shaky engine, or one that feels like it's "skipping," could be a sign of low fuel pressure.
• Check Engine Light: The check engine light can indicate various issues, and low fuel pressure is one of them.
• Reduced Fuel Efficiency: A sudden drop in fuel efficiency could be a sign of low fuel pressure. The engine compensates by burning more fuel, leading to increased consumption.
• Excessive Exhaust Emissions: Incomplete combustion due to low fuel pressure can result in higher emissions from the exhaust, potentially causing issues with emissions tests.

Causes of Low Fuel Pressure:

• Faulty Fuel Pump: A weak or malfunctioning fuel pump may not generate enough pressure to deliver fuel to the engine. Symptoms include whining noises from the fuel tank, difficulty starting the engine, and poor acceleration.
• Clogged Fuel Filter: Over time, the fuel filter can become clogged with debris, restricting fuel flow and leading to low fuel pressure. This can cause engine misfires, stalling, and reduced fuel efficiency.
• Malfunctioning Fuel Pressure Regulator: If the fuel pressure regulator malfunctions, it can cause a drop in pressure, resulting in poor acceleration and engine stalling.
• Leaking Fuel Injectors: Leaking or malfunctioning fuel injectors can lead to a drop in fuel pressure, creating a lean fuel mixture and causing misfires, rough idling, and poor fuel efficiency.
• Electrical Issues: Problems with the electrical supply to sensors, wiring, or the fuel pump relay can cause the fuel pump to underperform or fail, resulting in low fuel pressure.

High-pressure fuel pumps

There are several types of high-pressure mechanical pumps, including common rail radial piston, common rail two-piston radial, inline, port and helix, and metering unit configurations. Injection pumps are fuel-lubricated, which is essential to prevent oil contamination in the fuel.

Port and Helix pumps are commonly used in marine diesel engines due to their simplicity, scalability, and reliability. They are similar to radial piston-type pumps but use a machined plunger without seals. The plunger's movement controls the amount of fuel delivered to the injector, with the fuel forced through a check valve into the injector nozzle.

Another type of pump is the plunger-type pump, which is often used in diesel engines. These pumps contain a chamber with a moving plunger that increases or decreases the volume, along with check valves to control the fuel flow. Plunger-type pumps typically operate at fuel pressures between 3,600 and 26,100 psi (250-1,800 bar).

Maintaining proper fuel pressure is critical for engine performance. Even a small change in fuel pressure, as little as 1 to 3 PSI, can cause noticeable driveability issues. Therefore, it is important to regularly check the fuel pressure and ensure it meets the specifications for your vehicle.

Fuel pump replacement

Step 1: Park Your Vehicle

Park your vehicle on a firm, level surface and engage the parking brake. This ensures your car doesn't move during the fuel pump replacement process.

Step 2: Locate the Fuel Pump

Most vehicles have the fuel pump located inside the fuel tank. Open the fuel cap and turn the key to the "ON" position. Listen at the filler opening; a working fuel pump should produce a hum for two to three seconds.

Step 3: Check the Fuel Pump Fuse and Relay

If the fuel pump is not functioning, check the fuse and relay. If the fuse is blown, replace it with one of the same amperage.

Step 4: Check for Power and Ground

If the fuse and relay are functional, check for power and ground at the fuel pump. This may require removing the fuel tank or the back seat. If there is power and ground at the pump, it is faulty and needs replacement.

Step 5: Prepare for Fuel Pump Removal

Relieve the fuel system pressure by disconnecting the negative battery cable. Siphon or drain fuel from the tank to minimize the risk of spillage. Disconnect the filler tube hose and electrical connection to the pump.

Step 6: Remove the Fuel Tank

Support the fuel tank with a jack and a block of wood. Remove any retaining straps or bolts holding the tank to the frame and carefully lower it.

Step 7: Disconnect the Fuel Lines and Remove the Old Fuel Pump

Disconnect the fuel lines from the pump and remove it from the tank. Compare the old pump to the new one to ensure you have the correct replacement.

Step 8: Install the New Fuel Pump

Connect the fuel lines to the new pump, lift the fuel tank, and secure it with the retaining strap. Reconnect the filler tube hose and electrical connector.

Step 9: Reattach the Battery and Refuel

Reconnect the negative battery cable. Refill the fuel tank and conduct a road test to ensure the new fuel pump is functioning properly.

Additional Tips:

• Work in a well-ventilated area to minimize the risk of fire and inhalation of fumes.
• Always wear safety gear, including glasses, gloves, and appropriate clothing to protect against fuel spills.
• Clean around the fuel pump to prevent debris from falling into the tank.
• Consider replacing related components such as the fuel filter and fuel pump strainer.

Cost Considerations:

The cost of a new fuel pump can vary from $15 to over $2,000, depending on the vehicle's specific requirements. If you're hiring a mechanic, labor costs will also apply.

Symptoms of a Faulty Fuel Pump:

• Difficulty starting the engine
• Loss of power while driving
• Engine stalling or rough running
• High-pitched whining noises from the fuel tank area

Frequently asked questions