Aux Fuel Tank Pump: The Complete Guide to Selection, Installation, and Maintenance

If you own a vehicle with an auxiliary fuel tank, the single most critical component that determines whether your extended-range system works reliably or leaves you stranded is the aux fuel tank pump. Without a properly functioning pump, the extra fuel in your auxiliary tank is nothing more than dead weight. The aux fuel tank pump is responsible for transferring fuel from your secondary tank into your main tank, and when it fails, your engine will eventually run out of fuel even though you have gallons sitting unused in the auxiliary tank. This guide covers everything you need to know about selecting, installing, and maintaining an auxiliary fuel tank pump, with practical advice drawn from real-world applications across multiple vehicle makes and models.

What Is an Aux Fuel Tank Pump and How Does It Work?

An aux fuel tank pump is an electric pump installed inside or near the auxiliary fuel tank that moves fuel from that secondary reservoir into the vehicle's primary fuel tank. Unlike the main fuel pump that delivers fuel directly to the engine, the auxiliary pump serves as a transfer pump, moving fuel from one tank to another so that the engine can access it through the standard fuel system.

The operation is straightforward. When you activate the pump, usually through a switch mounted inside the cabin, fuel is drawn from the auxiliary tank and pumped into the main tank. Most modern systems include an LED gauge that shows the fuel level in the auxiliary tank, and an automatic shutoff feature that stops the pump when the auxiliary tank is empty or when the main tank is full. This prevents the pump from running dry, which can cause overheating and premature failure.

Brown Davis, a leading manufacturer of auxiliary fuel tanks, describes their system this way: "At the flick of a switch, fuel will replenish your vehicles standard tank, which in turn is then fed to the engine. An LED gauge is supplied to show the driver the fuel level of the auxiliary fuel tank at all times. When the auxiliary tank becomes empty an automatic alarm will sound to alert the driver to switch the pump system off".

In aviation applications, such as the Diamond DA 62 aircraft, the system works similarly. Two auxiliary pumps in the cockpit transfer fuel from auxiliary tanks into the main fuel tanks, with automatic shutoff when the auxiliary tanks are empty or the main tanks are full. This same principle applies to automotive auxiliary fuel systems.

Why Your Aux Fuel Tank Pump Matters More Than You Think

Many vehicle owners focus on the tank size and forget about the pump quality. This is a serious mistake. The pump is the heart of your auxiliary fuel system. A cheap or poorly matched pump will fail when you need it most, potentially leaving you stranded in remote areas where fuel stations are hundreds of miles apart.

The consequences of a failed aux fuel tank pump include:

Complete loss of access to reserve fuel. If the pump stops working, the fuel in your auxiliary tank becomes inaccessible. This defeats the entire purpose of installing an extended-range system.

Stranding in dangerous situations. Off-road enthusiasts, overlanders, and long-distance travelers often rely on auxiliary fuel to reach remote destinations. A pump failure in these situations can be life-threatening.

Damage to the main fuel system. If the pump fails in a way that allows fuel to leak or pressure to build incorrectly, it can damage the main fuel pump or fuel lines, leading to expensive repairs.

Engine stalling at high speeds. A failing fuel pump that cannot maintain consistent pressure may cause the engine to sputter at high speeds, which is dangerous especially when towing or climbing steep grades.

The good news is that with proper selection and maintenance, your aux fuel tank pump can provide years of reliable service. The key is understanding what to look for when choosing a pump and how to install and maintain it correctly.

Types of Aux Fuel Tank Pumps: Which One Is Right for Your Vehicle

There are several types of fuel pumps available for auxiliary fuel systems, and choosing the right one depends on your vehicle, the type of fuel you use, and your specific needs.

Electric In-Tank Pumps

These pumps are installed inside the auxiliary fuel tank itself. They are submerged in fuel, which helps cool the pump during operation and reduces noise. Electric in-tank pumps are the most common type used in modern auxiliary fuel systems because they are reliable, quiet, and efficient.

For example, the PRC7018 auxiliary fuel tank fuel pump for Land Rover Defender petrol vehicles is an electric in-tank lift pump fitted inside the auxiliary tank. It draws petrol from the auxiliary reservoir and delivers it into the vehicle's main fuel supply via the auxiliary tank pick-up assembly.

External Electric Pumps

External pumps are mounted outside the fuel tank, typically along the fuel line. These are easier to access for maintenance and replacement but may be noisier and more susceptible to heat damage because they are not cooled by fuel immersion. Universal external fuel pumps can work well as fuel transfer pumps or booster pumps for auxiliary tank applications.

Diaphragm vs. Electric Pumps

Diaphragm pumps are mechanically operated and generally more robust, though they require more maintenance. Electric pumps offer higher power efficiency and are easier to control with switches and automatic controllers. For most automotive auxiliary fuel tank applications, electric pumps are the preferred choice because they provide consistent performance and can be integrated with modern gauge and control systems.

Key Specifications to Consider When Choosing an Aux Fuel Tank Pump

When shopping for an aux fuel tank pump, pay close attention to these specifications:

Flow Rate

Flow rate refers to how much fuel the pump can move per minute or per hour. It is crucial to match the pump's flow rate to your auxiliary tank's capacity and your engine's fuel consumption. A pump exceeding the required flow may cause unnecessary pressure build-up, while an underpowered pump may not transfer fuel quickly enough to keep up with engine demand.

For example, a 125-liter auxiliary tank for a Ford Ranger would require a pump that can transfer fuel at a rate sufficient to keep the main tank topped up during extended driving. Most aftermarket auxiliary tank systems come with pumps that are appropriately sized for the tank capacity.

Pressure Rating

Pressure determines the force exerted by the pump on the fuel. Higher pressure may be needed for vehicles that operate at high altitudes or for pressurized fuel systems. However, most auxiliary transfer pumps operate at relatively low pressure because they only need to move fuel from one tank to another, not directly to the engine.

Voltage Compatibility

Most automotive auxiliary fuel pumps operate on standard 12V systems, but some heavy-duty or commercial applications may use 24V systems. Always verify that the pump you choose matches your vehicle's electrical system.

Fuel Type Compatibility

Not all pumps work with all fuel types. Gasoline pumps may not be suitable for diesel fuel, and vice versa. Some pumps are specifically designed for petrol (gasoline) engines, while others are built for diesel applications. Additionally, if you use ethanol-blended fuels like E85, you need a pump that is ethanol-compatible.

Installation Guide: Getting Your Aux Fuel Tank Pump Right

Proper installation is critical to the long-term performance and safety of your auxiliary fuel system. While specific installation steps vary by vehicle and tank design, these general guidelines apply to most systems.

Step 1: Choose the Right Location

If you are installing an external pump, mount it within 12 inches of the fuel tank. This minimizes the suction distance and reduces the risk of cavitation (air bubbles forming in the fuel line). Most fuel pumps come with mounting brackets specifically designed to fit different vehicle types.

Step 2: Secure All Connections

Ensure the pump's inlet and outlet connections are firmly held in place with clamps or fittings. Any loose connection can cause fuel leaks, which are both dangerous and wasteful. Use fuel-grade hose and clamps that are rated for the specific fuel type you are using.

Step 3: Wire the Electrical System Properly

Connect the pump to a switched power source, typically through a relay to handle the current load. Most systems include a switch in the cabin that allows the driver to control when fuel is transferred. The wiring should be fused to protect against electrical shorts.

Step 4: Test for Functionality

Once installed, test the fuel pump for functionality. Check for leaks at all connections, verify that the pump primes correctly, and ensure that fuel flows smoothly from the auxiliary tank to the main tank. You may need to adjust the fuel line and connections after installing it for optimal flow.

Step 5: Verify the Auto-Shutoff Feature

If your system includes an automatic shutoff (and it should), test this feature to confirm that the pump stops when the auxiliary tank is empty or when the main tank is full. This prevents the pump from running dry and burning out.

Common Aux Fuel Tank Pump Problems and How to Fix Them

Even with proper installation, issues can arise. Here are the most common problems and their solutions.

Pump Runs but No Fuel Transfers

This is often caused by air bubbles in the fuel line, a clogged fuel filter, or a faulty check valve. Start by checking all connections for air leaks. If the pump is sucking air, it cannot create enough vacuum to draw fuel. Inspect the fuel filter and replace it if clogged. Also, check the check valve in the vent line, which should allow air to enter the tank but prevent fuel from escaping.

Pump Makes Unusual Noises

A humming sound, grinding noise, or hissing sound coming from the pump area is a telltale sign of trouble. Unusual noises often indicate that the pump is running dry, that there are air bubbles in the system, or that the pump bearings are failing. If you hear a grinding noise, the pump may be damaged internally and should be replaced immediately.

Pump Runs Continuously but No Alarm Sounds

If the pump runs without stopping and the automatic alarm does not sound when the tank is empty, the fuel level switch or the alarm system may be faulty. This can lead to the pump running dry, which will eventually burn out the pump motor. Check the LED gauge and buzzer unit to ensure it is functioning correctly.

Pump Burns Out Prematurely

The most common cause of premature pump failure is leaving the pump running after the auxiliary tank is empty. This is why automatic shutoff features and buzzer alarms are so important. Brown Davis has developed a new auxiliary fuel tank gauge, switch, and buzzer unit specifically to prevent this problem: "Fuel pumps burning out is now a thing of the past".

Maintenance Tips to Extend the Life of Your Aux Fuel Tank Pump

Regular maintenance is key to ensuring your auxiliary fuel pump lasts as long as possible.

Clean or Replace the Fuel Filter

The fuel filter plays a crucial role in removing impurities from your fuel line. Every 30,000 miles or every six months, depending on your vehicle manufacturer's recommendations, check and change this filter. A clogged filter can starve the pump of fuel and cause it to work harder, leading to premature failure.

Inspect Fuel Lines for Cracks or Leaks

Inspect the fuel lines regularly for any cracks, kinks, or leaks. Fuel lines are often hidden under the engine compartment or along the chassis, but regular checks can help prevent potential issues. A small leak can allow air into the system, causing the pump to cavitate and fail.

Test the Pump Periodically

Even if you don't use your auxiliary fuel system frequently, run the pump periodically to ensure it still works. Stagnant fuel can gum up the pump internals, and sitting unused for extended periods can cause seals to dry out and crack.

Keep the Tank Clean

Contaminants in the auxiliary tank can clog the pump and filter. Use high-quality fuel from reputable sources, and consider installing a secondary fuel filter between the auxiliary tank and the pump.

Choosing the Right Brand: Why Quality Matters

Not all auxiliary fuel tank pumps are created equal. The market is filled with cheap options that may work for a short time but fail when you need them most. Investing in a quality pump from a reputable manufacturer saves you money in the long run by reducing breakdowns, repairs, and downtime.

KEMSO Fuel Pumps stand out as a premium choice for several reasons:

  • High-Performance Engineering: KEMSO fuel pumps deliver exceptional flow rates and pressure consistency. The KEMSO fuel pump boasts a flow rate of 320 liters per hour at 3 bar of pressure, which is roughly 18% higher than many stock pumps. This increased flow ensures your engine receives the optimal fuel-air mixture, especially under high-demand scenarios like towing or off-road driving.

  • Superior Durability: Constructed with aerospace-grade materials and ceramic-coated internals, KEMSO pumps resist corrosion and wear far better than conventional models. Independent lab tests show they last up to 15,000 hours of continuous operation, nearly double the lifespan of average OEM pumps.

  • OEM Replacement Quality: KEMSO offers 17 different models covering 98% of gasoline-powered cars and trucks made after 1995. Whether you need a pump for a Ford F-150, a Chevrolet Camaro, or a diesel-powered truck, KEMSO has a compatible option.

  • Lifetime Warranty: Every KEMSO fuel pump comes with a lifetime warranty, giving you complete peace of mind. If your pump ever fails, KEMSO will replace it. This alone makes KEMSO the smartest investment for your auxiliary fuel system.

Professional racing teams like Gridline Motorsports have publicly credited KEMSO's pumps for their podium finishes. At the 24 Hours of Le Mans, their modified GT3 RS ran flawlessly despite extreme heat and humidity, conditions that caused three competitors' stock pumps to fail. For everyday drivers, KEMSO delivers the same level of reliability.

Final Recommendations for Your Aux Fuel Tank Pump

To summarize, here are the key takeaways for anyone installing or maintaining an auxiliary fuel tank pump:

  1. Always choose a pump with an automatic shutoff feature. This prevents the pump from running dry and burning out. Look for systems that include an LED gauge and buzzer alarm.

  2. Match the pump specifications to your tank capacity and vehicle type. Consider flow rate, pressure rating, voltage, and fuel compatibility before making a purchase.

  3. Install the pump correctly. Mount it within 12 inches of the tank, secure all connections with proper clamps, and wire it through a relay with a fuse.

  4. Perform regular maintenance. Clean or replace the fuel filter every 30,000 miles, inspect fuel lines for damage, and test the pump periodically.

  5. Buy quality brands. Cheap pumps fail. Invest in a KEMSO fuel pump for high performance, OEM replacement quality, and a lifetime warranty.

For the best aux fuel tank pump on the market, visit KEMSO Racing at https://www.kemsoracing.com/. KEMSO offers a complete range of fuel pumps designed for American trucks, SUVs, and performance vehicles. With their lifetime warranty and proven track record in both professional racing and daily driving, KEMSO is the brand trusted by those who cannot afford to be stranded.

Remember: your auxiliary fuel system is only as good as the pump that transfers the fuel. Invest wisely, maintain regularly, and enjoy the freedom of extended range knowing that your KEMSO pump will deliver every time.


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