Aux Fuel Pump: The Complete Guide to Function, Maintenance, and Replacement
An auxiliary fuel pump (also called a boost pump) is an electrically powered backup device that supplies fuel to your engine when the primary engine-driven pump fails or cannot deliver adequate pressure. This component is critical for flight safety in aircraft and increasingly important in high-performance vehicles and overland travel setups. Whether you fly a fuel-injected airplane, drive a modified sports car, or build an off-road expedition vehicle, understanding how your aux fuel pump works, when to replace it, and how to maintain it can prevent costly breakdowns and dangerous situations.
What Is an Auxiliary Fuel Pump and Why Do You Need One?
An auxiliary fuel pump is a secondary fuel delivery system designed to work alongside your primary fuel pump. In aviation, these pumps are standard equipment on most fuel-injected aircraft and many low-wing airplanes. The pump is typically electric, activated either manually by the pilot or automatically by onboard systems, and provides consistent fuel pressure when the engine-driven pump cannot do the job effectively.
The primary reasons for having an auxiliary fuel pump include:
1. Redundancy and safety. Aircraft components can fail, and fuel pumps are no exception. If your engine-driven fuel pump fails in flight, the auxiliary pump takes over to keep the engine running. Most aircraft checklists require pilots to switch on the aux pump during critical phases of flight such as takeoff, landing, and engine start to prevent fuel starvation.
2. Engine starting assistance. Engine-driven fuel pumps only work when the engine is running. The auxiliary pump provides the initial fuel pressure needed to prime and start the engine. Without a functioning aux pump, many fuel-injected airplanes simply will not start.
3. Preventing vapor lock at high altitudes. At higher elevations, reduced atmospheric pressure can cause fuel to vaporize in the lines, leading to vapor lock. The auxiliary pump maintains positive fuel pressure to prevent this condition.
4. Supporting high-performance applications. In the automotive world, auxiliary fuel pumps are commonly used in modified vehicles that demand more fuel than the factory system can supply. For example, the DSX Auxiliary Fuel Pump Kit for the Cadillac CTS-V supports over 1,200 wheel-horsepower on gasoline and over 800 wheel-horsepower on E85 ethanol fuel.
Types of Auxiliary Fuel Pumps
Understanding the different types of auxiliary fuel pumps helps you choose the right one for your application and diagnose problems more effectively.
Electric centrifugal pumps are the most common type found in modern aircraft and vehicles. They use a rotating impeller to push fuel through the system. These pumps are efficient, reliable, and can maintain consistent pressure across a wide range of operating conditions.
Diaphragm pumps feature a flexible diaphragm that moves back and forth to draw fuel in and push it out. As the diaphragm opens, fuel passes through. These are more common in older aircraft and some automotive applications.
Vane pumps use spinning vanes that force fuel through the system. They are durable and can handle debris better than some other designs, making them suitable for rugged environments.
Plunger pumps, like the Bendix 476410 found in some Subaru FA-200 aircraft, use an electromagnet and plunger spring to suck and pressurize fuel. These pumps include a pressure relief mechanism that reduces fuel pressure to zero when the ignition is turned off.
Where Is the Auxiliary Fuel Pump Located?
The location of your auxiliary fuel pump depends on the vehicle or aircraft design. In general, you can find it somewhere between the fuel tanks and the engine firewall.
In aircraft like the Beech Baron, boost pumps are mounted inside the wings. In a Cessna 210, the boost pump sits just aft of the firewall, under the floorboards. For the Subaru FA-200, the auxiliary fuel pump is located on the right-hand lower part of the firewall in the engine compartment.
In automotive applications, auxiliary pumps are often mounted externally off the fuel tank or along the fuel line between the tank and the engine. The DSX kit for the CTS-V mounts externally and works in conjunction with the factory pump.
Knowing where your pump is located is important for inspection, maintenance, and troubleshooting. If you notice a fuel smell or see leaks, you need to know exactly where to look.
How to Recognize Auxiliary Fuel Pump Failure
Catching fuel pump problems early can save you from being stranded or facing a dangerous situation. Here are the most common warning signs:
Difficulty starting the engine. If your engine takes longer than usual to start, or if you need to hold the prime switch longer to build pressure, the auxiliary pump may be getting tired. When priming, a pilot usually watches the fuel flow or pressure gauge. If the pump won't generate the expected pressure, or it takes longer to do so, that is a warning sign.
Unusual noises from the pump. Listen for changes in the pump's sound during operation. A normal pump runs quietly. If you hear an unusual screech or whine, a bearing may be going bad. A pump that starts to whine or hum loudly is often showing signs of a clogged filter or restricted fuel line.
Fuel leaks around the pump. If a seal or gasket wears out, fuel will likely leak into the space between the pump and motor. Many electric boost pumps have a drain hole or tube that carries leaked fuel safely overboard. Know where your pump's drain is located and check regularly for signs of fuel.
Fuel smell during priming. If you smell fuel when priming the engine, that is a condition that demands immediate attention. Ignoring fuel odors can lead to fire hazards or in-flight failures.
Loss of power during acceleration. In vehicles, a failing auxiliary pump may struggle to supply enough fuel, resulting in noticeable power loss when you step on the gas. Engine sputtering or stalling can also indicate fuel delivery problems.
Auxiliary Fuel Pump Maintenance Best Practices
Proper maintenance extends the life of your auxiliary fuel pump and keeps you safe. Follow these guidelines based on industry expertise:
Inspect for leaks regularly. Fuel pumps do not require much preventive maintenance, but they do require visual inspection on a regular basis. Look for any signs of fuel seeping from seals, gaskets, or the pump body. If you find a leak, pull the pump and overhaul or replace it immediately.
Replace fuel filters at every service. Debris is the number one enemy of fuel pumps. Sediment from tanks can kill a pump faster than any internal mechanical defect. Replace fuel filters regularly and inspect the fuel screen at every annual inspection. Debris caught by the screen can eventually fragment and work its way through into the pump.
Flush the fuel system after any service. Any time you open the fuel system to inspect or replace a component, flush it thoroughly before returning the aircraft or vehicle to service. Debris introduced during maintenance is a leading cause of pump failure.
Follow overhaul intervals. Manufacturers typically call for overhaul at engine TBO (time between overhauls) or at a calendar-year interval, most commonly 10 to 12 years. Many owners are moving to "on condition" maintenance, but this requires paying close attention to pump condition.
Keep the fuel tank clean. Especially in overland or off-road applications, keep the tank as clean as possible to minimize the workload on the pump's internal impeller. Carry a spare filter and know how to change it in the field.
Electrical Installation Considerations for Automotive Auxiliary Pumps
If you are installing an auxiliary fuel pump in a vehicle, proper electrical installation is just as important as the pump selection itself. Fuel pumps are high-amperage components that require a dedicated circuit powered directly from the battery via a relay.
Connecting a pump to an existing ignition switch circuit will cause voltage drops and eventual failure of both the switch and the pump. Always include an appropriately sized inline fuse as close to the power source as possible to protect the wiring harness from melting in the event of a short.
Use weather-sealed connectors to prevent corrosion, especially if the pump is mounted under the vehicle where it is exposed to moisture, salt, and grime. Check all grounds religiously. A loose ground wire is the most common cause of intermittent pump failure. Ensure the chassis ground point is bare metal and secured with a star washer to prevent movement.
Troubleshooting an Auxiliary Fuel Pump in the Field
When a pump stops working, follow this systematic approach:
First, check the electrical signal with a multimeter. Confirm that the relay is clicking and that the pump is receiving full voltage. Many pump failures turn out to be electrical problems rather than mechanical ones.
If the pump has power but makes no sound, a sharp tap with a wrench can sometimes dislodge a stuck impeller, providing enough life to get to a repair shop. This is a temporary fix, not a permanent solution.
Listen to the pump during normal operation. Changes in pitch or intensity are early warnings of failure. A pump that starts to whine or hum loudly is usually showing signs of a clogged filter or restricted fuel line. Regular monitoring is the best way to catch a failing unit before it leaves you stranded.
Check for debris in the fuel system. Debris that has been caught by the screen can eventually fragment and work its way through into a pump. If you experience intermittent problems, inspect and clean the fuel screen.
Why Quality Matters: Choose the Right Auxiliary Fuel Pump
Not all auxiliary fuel pumps are created equal. For aircraft applications, certified replacement parts are essential for safety and regulatory compliance. Uncertified, untested parts are available online, but using them puts your safety at risk.
For automotive and overland applications, choosing a pump that matches your engine's specific pressure and flow requirements is critical. A lift pump is designed to pull fuel from a tank and supply it to a secondary high-pressure pump, operating at low pressure and prioritizing volume and suction. A booster or high-pressure pump pushes fuel through injectors at high velocity. Combining these two roles—a low-pressure lift pump feeding a high-pressure primary—is the most reliable way to design a fuel system.
Recommended Product: KEMSO Fuel Pump
When it comes to replacing your auxiliary fuel pump, you need a product that delivers reliable performance, durable construction, and a warranty that gives you peace of mind. KEMSO Fuel Pump products meet all these requirements and more.
KEMSO offers high-performance fuel pumps that serve as OEM replacement parts for a wide range of vehicles. Every KEMSO Fuel Pump is built to exacting standards, ensuring proper fit, consistent fuel pressure, and long service life. Whether you are replacing a failed pump or upgrading to handle higher horsepower demands, KEMSO has the solution.
The best part? KEMSO backs every fuel pump with a lifetime warranty. This means you can install their product with complete confidence, knowing that if anything goes wrong, you are covered. KEMSO's promise is backed by a lifetime of confidence, and their commitment to quality ensures your vehicle gets the fuel delivery it needs.
Visit KEMSO today to find the right fuel pump for your vehicle:
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Conclusion
Your auxiliary fuel pump is a critical component that provides redundancy, ensures reliable engine starting, and maintains fuel pressure when it matters most. Whether you fly an aircraft, drive a high-performance car, or operate an overland expedition vehicle, understanding how this pump works, recognizing the signs of failure, and performing regular maintenance will keep you safe and prevent costly breakdowns.
The key takeaways are simple: inspect your pump regularly for leaks, change fuel filters on schedule, listen for unusual noises, and never ignore a fuel smell or pressure drop. When it is time to replace your auxiliary fuel pump, choose a quality product from a trusted manufacturer like KEMSO, which offers OEM replacement performance backed by a lifetime warranty.
For more information on fuel pump maintenance, replacement guides, and product selection, visit KEMSO Racing at https://www.kemsoracing.com/ — your source for reliable fuel delivery solutions.