Regan Brown
Plumbing a fuel return line can be a daunting task, but with some planning, it can be a straightforward process. It is important to understand the type of fuel system in your vehicle, as this will determine the placement of the fuel pump and the routing of the fuel lines. The return-style fuel system, for example, is commonly used in high-performance applications and requires careful plumbing to avoid introducing air into the system. When plumbing a fuel return line, it is crucial to consider the type of fittings and hoses used, ensuring they are compatible with the fuel and provide adequate puncture resistance. The placement of the return line inside the fuel tank is also essential, as it can impact fuel pressure and the performance of the pump.
How to Plumb a Fuel Return Line
| Characteristics | Values |
|---|---|
| Fuel System Type | Return-style |
| Fuel Pump Type | Electronic or mechanical |
| Fuel Return Placement | Inside the fuel tank |
| Return Line Material | Steel, copper, braided stainless hose, Ni-Copp hard lines, AN (Army/Navy) fittings, aluminium |
| Return Line Size | 3/8-inch, 8mm |
| Return Line Location | Away from the pickup to eliminate aeration |
| Bulkhead Fitting | Required for return line |
| Regulator | May be required in the return line to pressurise the fuel system |
| Fuel Sampling Kit | Required for cars used in rallying |
| Hose Support | Required for push-on hoses to keep them in place |
| Ethanol Content | May require R9 specification fuel hose |
| Hose Ends | Swivel-seal to prevent twisting and collapsing |
| Return Line Angle | Angled fitting may be needed to maintain flow |
| Return Line Placement | Top of the tank to reduce pump pressure |
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What You'll Learn
Return line placement inside the fuel tank
The placement of the return line inside the fuel tank is a critical aspect of plumbing a fuel return line. The return line can be placed at the top or bottom of the tank, and each option has its own set of advantages and disadvantages.
Placing the return line at the top of the tank, in the free air space, is a common practice. This approach has been used by many without any aeration issues. The large opening at the top allows the fuel to enter at a low velocity, minimising the entrainment of air. Additionally, with this setup, any air bubbles have enough time to disperse. However, returning the fuel at the top can cause foaming and air bubbles if the return line delivers fuel above the surface level of the tank. This, in turn, can lead to spikes in fuel pressure and increased load on the pump.
On the other hand, placing the return line at the bottom of the tank has its own set of considerations. Returning fuel to the bottom of the tank or below the fuel surface helps prevent aeration. This method is similar to having a hose submerged under the water level in a bucket, which reduces air entrainment. However, returning fuel at the bottom can cause issues with aerated fuel getting sucked back into the pump inlet, potentially affecting fuel pressure. Additionally, if the return line is placed too close to the pickup, it can result in the pump sucking in air bubbles.
To mitigate these issues, it is recommended to position the return line as far away from the pickup as possible. This helps to eliminate aeration by ensuring that the returning fuel does not interact with the fuel being drawn into the pump. Additionally, when dealing with a small cell and a fuel pressure-sensitive setup, variations in return pressure may be observed as the fuel level changes. In such cases, ensuring that the lines are large enough and the cell is kept near full can help address these variations.
The decision between returning fuel at the top or bottom depends on various factors, including the size of the tank, the presence of a fuel pressure-sensitive setup, and the potential for aeration. It is important to carefully consider these factors and make adjustments as needed to ensure optimal performance and minimise issues such as foaming, air bubbles, and variations in fuel pressure.
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Return line materials
The choice of materials for your fuel return line depends on your budget and performance requirements. Earl's Performance Plumbing, for instance, offers a range of fittings and hoses to suit different systems' needs.
If you're looking for a cost-effective solution, push-on hoses are a good option. However, while they won't leak, they do allow vapour and fumes to escape, so they're not suitable for use inside the car. Another downside is that they require a lot of supporting clamps to keep them in place. If you're looking for something more flexible, R9 specification fuel hose is a good option, as it can handle pump fuel with ethanol content without perishing, swelling, or cracking.
If your car will be on the street a lot, running hard lines for the fuel is recommended. Braided lines can collapse over time, but a properly routed steel line will last almost forever. Ano-Tuff hard-anodized fittings are a good choice for resisting corrosion and wear. For fuel and transmission lines, Pro-Lite 350 hose is a good option for its lightweight and flexibility. Standard stainless steel AutoFlex hose and Econo-Fit clamps can be used for heater hoses. To prevent the hose from twisting during assembly, use Swivel-Seal hose ends for most of the fittings.
When it comes to the return line inside the fuel tank, you can use a steel or copper line. However, be careful not to submerge a braided-covered rubber hose in fuel. If you're using a hard line inside the fuel cell, consider using a stainless hard line. To eliminate aeration, ensure that the return line is located away from the pickup. If you're using a small cell, keep it near full, and for larger cells, foam can help reduce aeration.
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Fuel system diagramming
When diagramming the fuel system, it is important to consider the type of vehicle and fuel system being used. For example, a return-style fuel system can use either an electronic or mechanical fuel pump to send fuel from the tank to a regulator and then to the carburetor or fuel injection system. Any excess fuel that is not used is returned to the tank via the fuel pressure regulator.
The placement of the return line is also critical. Some sources suggest returning the fuel to the top of the tank to keep the pump from having to work too hard. Others recommend returning it to the bottom of the tank or below the fuel surface to prevent aeration and the formation of air bubbles, which can cause spikes in fuel pressure.
When selecting the components for the fuel system, it is important to consider the budget and performance requirements. Different types of fittings and hoses are available, such as AN fittings, braided lines, or hard lines. The size and material of the lines and fittings will depend on the specific application and fuel system configuration.
Additionally, it is important to consider the safety and durability of the fuel system. This includes securing the fuel hose to prevent movement and protecting it from heat. The choice of hose material is also critical, as some materials may not be compatible with certain types of fuel, such as pump fuel with a high ethanol content.
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Fuel line sizing
If the fuel line diameter is too small, it may not be able to support your desired horsepower. This can hinder your engine's performance and prevent you from achieving your horsepower goals. On the other hand, if the diameter is too large, the lines will take longer to pressurize. If your pump cannot maintain the pressure, it may drop during periods of high demand, potentially leading to engine damage.
It is important to consult with fuel pump manufacturers or experts to determine the recommended fuel line diameter for your specific application. For example, some in-line fuel pump manufacturers suggest a fuel line diameter that works optimally with their pumps to prevent starving the pump. Additionally, the type of fuel used can impact the required diameter. Ethanol and methanol, for instance, demand more fuel volume than gasoline to generate equivalent horsepower.
When plumbing a fuel return line, it is essential to consider the placement of the return line inside the fuel tank. The return line should be positioned to release fuel below the fuel surface to prevent aeration and the formation of air bubbles. However, in larger tanks, the extra volume can act as a plenum, reducing turbulence and minimizing the impact of bubbles. Additionally, ensuring that the return line is located away from the pickup can help mitigate aeration issues.
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Fuel pump placement
When placing the fuel pump, it is important to ensure that the return line is positioned correctly. The return line should be placed inside the fuel tank, and its location relative to the fuel level is crucial. If the return line delivers fuel above the surface of the fuel, it can cause foaming and introduce air bubbles into the fuel. This, in turn, can lead to spikes in fuel pressure and increase the workload on the pump. Therefore, it is generally recommended to return the fuel below the fuel surface or at the bottom of the tank.
However, some factors should be considered when returning fuel to the bottom of the tank. As the fuel level in the tank decreases due to fuel consumption and normal sloshing, the pump's workload may vary, potentially affecting fuel pressure. Additionally, there may be concerns about aeration when returning fuel to the bottom of the tank. To mitigate these issues, ensure that the return line has a tube that reaches the bottom of the tank, especially if you have a small tank or a fuel pressure-sensitive setup.
The size of the tank also plays a role in fuel pump placement. Larger tanks tend to have fewer issues with aeration and fuel pressure variations. The extra volume of fuel acts as a plenum, smoothing out turbulence and reducing the impact of aeration. Therefore, if you have a large tank, you may have more flexibility in the placement of the return line.
When placing the fuel pump, it is essential to consider the type of vehicle and its specific requirements. Some vehicles, such as older, smaller-engined cars with carburettors, may require different hose sizes and fittings compared to larger-engined or fuel-injected cars. Ensure that you select the appropriate hose diameter and secure it properly with hose clips or swaging, which involves crimping a metal collar over the hose.
Additionally, the outlet type of the fuel pump should be considered. Whether it is push-on, threaded, or a banjo fitting, you will need to measure the relevant dimensions and identify the thread type to ensure proper connections and compatibility with other components in the fuel system.
In some cases, the fuel pump may need to be placed near the tank, especially if there is a regulator involved. This can impact the routing of the return line and the placement of the bulkhead fitting. It is recommended to consult a professional or refer to vehicle-specific forums for guidance on fuel pump placement and plumbing the fuel return line for your particular vehicle.
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Frequently asked questions
The fuel return line routes excess fuel from the engine back to the tank. Fuel leaks and vapor lock are two of the most common signs of a faulty fuel return line.
Fuel leaks can occur if the fuel return hose gets punctured or worn out. A leaking gasoline return pipe can cause fuel to accumulate under the car and create a strong fuel smell. A kink in the fuel return line can also cause the engine to produce strong fuel odours.
It is important to ensure that the return is placed inside your fuel tank. If the return delivers fuel above the surface of the fuel level in the tank, it will cause foaming and air bubbles to form. The fuel return line should be placed as far away from the pickup as possible to eliminate aeration.
It is recommended to draw a simple diagram of where you intend to mount the various components to visualise the setup and identify any potential issues. It is also important to consider the type of fitting required, with most production cars using push-on hose fittings.
