Mercedes Mullins
Dual fuel tanks are a useful way to extend the range of a vehicle. They can be plumbed in a variety of ways, from using a simple manual valve to switch between tanks, to more complex systems that automatically siphon fuel from one tank to another. In this diagram, we will explore the different methods of plumbing dual fuel tanks, the advantages and disadvantages of each, and how to choose the best setup for your vehicle. We will also discuss some common issues and troubleshooting tips for dual fuel tank systems.
| Characteristics | Values |
|---|---|
| Fuel gauge indication | The dash gauge indicates the fuel level in the OEM tank, but it reflects the level of the total fuel supply when the tanks are plumbed together. |
| Plumbing design | The supply, return, and vent lines of the two tanks can be joined using Y connectors to maintain equilibrium and the same fuel level in both tanks. |
| Switching mechanism | A manual dual-port valve or a 3-way valve can be used to switch between the two tanks. |
| Fuel pump activation | In some systems, flipping a switch activates a module that turns on the fuel pump in the selected tank. |
| Fuel transfer | In some cases, fuel may be siphoned from one tank to another when the switch is flipped. |
| Fuel level discrepancy | The fuel gauge may indicate a higher level than the actual amount of fuel in the tank due to factors such as old gas, clogged filters, or faulty wiring. |
| Fuel pump issues | Problems with fuel pumps not working or activating may be related to power supply, fuel pump relay, or other connected components. |
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What You'll Learn
Fuel gauge accuracy
Fuel gauges in vehicles with dual fuel tanks can be inaccurate for several reasons. For instance, a faulty sending unit, faulty wiring, or a loose connection can cause the fuel gauge to malfunction. In some cases, the fuel gauge may read empty even when there is fuel in the tank, or it may not accurately reflect the amount of fuel in the tanks.
To improve fuel gauge accuracy, it is important to regularly check and maintain the fuel sending units and wiring. This includes cleaning the grounds and connectors, as well as ensuring that the connections are secure. Additionally, it is recommended to have multiple means of cross-checking the fuel level, such as using a scan tool or manually checking the fuel level to ensure accuracy and prevent fuel mismanagement.
In terms of calibration, fuel gauges should ideally be calibrated in terms of remaining usable fuel. Regulations, such as FAR 23.2430, require fuel systems to provide an accurate means of determining the total usable fuel available. This is especially important for safety, as errors in measurement or calculation can lead to dangerous situations.
For dual fuel tanks with different volumes, a simple divider circuit may not be sufficient to accurately measure the fuel level. Instead, a ratio divider network or other creative solutions, such as using the gauge sweep twice (once for each tank), may be required. Additionally, the use of resistors and voltage-based gauges can help determine the fuel level in each tank by measuring the voltage drop across the sender resistance relative to a set voltage.
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Fuel tank plumbing
Another consideration is the mounting location of the valve. Some prefer to mount it under the frame, making it easier to access the handle without getting under the truck. This location also provides passive anti-theft benefits due to the valve's off position. Additionally, dual saddle tanks can be utilized, with gravity-fed fuel systems and ball valves to drain the auxiliary tank into the main tank. The decision between these plumbing methods depends on factors such as vehicle specifications, personal preferences, and the desired level of complexity.
When installing an auxiliary fuel tank, one option is to keep the tanks separate and use a transfer pump and momentary switch to connect them. This approach can be simpler than plumbing the tanks together. However, it is important to consider potential issues with cross-feeding on hills. Alternatively, a dual-circuit ball valve can be used to ensure that both the supply and return lines are switched simultaneously. This method can help prevent overfilling the main tank.
In summary, fuel tank plumbing for dual fuel tanks involves decisions about connecting the supply and return lines, vent lines, and mounting locations for valves. The chosen method depends on factors such as vehicle specifications, convenience, and personal preferences. It is important to carefully consider the advantages and potential challenges of each plumbing approach to make an informed decision.
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Manual dual port valves
A manual dual port valve typically has three ports: two inlets and one outlet. It allows you to draw fuel from only one tank at a time. The valve is usually located between the fuel pump and the fuel tank.
One example of a manual dual port valve is the Groco FV-6-5038 bronze 6-port marine valve. This valve can be used to switch between the two tanks and also switch the return lines. It is designed to be mounted under the cab, with the lever accessible through a rubber grommet in the cab floor.
Another option for a manual dual port valve is the FV1 / FV1T Fuel Selector Valve Kit. This valve is suitable for vehicles with one main and one auxiliary fuel tank. It comes with stainless steel hose clamps, wire, connectors, and a fuse. However, it is not suitable for vehicles with fuel return lines or marine use.
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Fuel tank switching
Manual Switching:
Some dual fuel tank setups use a manual switching mechanism. This typically involves a dual port valve that allows users to switch between the two tanks. The Groco FV-6-5038 bronze 6-port marine valve, for example, is a popular choice for this purpose. This valve can be mounted under the cab, with the lever accessible through a rubber grommet in the cab floor. This setup provides easy access for switching while also offering passive anti-theft due to the valve's "off" position.
Automatic Switching:
In some cases, dual fuel tank systems may be designed for automatic switching between tanks. This can be achieved by plumbing the two tanks together using a "Y" connector for the supply, return, and vent lines. This setup allows for equilibrium between the tanks, maintaining the same fuel level in both. As a result, the dash gauge indicates the combined fuel level, providing a seamless transition between tanks without manual intervention.
Selector Valves:
Dual tank switching valve selectors are commercially available and can be purchased for specific vehicle models. These selectors typically come with clamps, wires, connectors, and other necessary components for installation. They allow for easy switching between the main and auxiliary tanks, ensuring a consistent fuel supply to the engine.
Operational Considerations:
When switching between fuel tanks, it is important to consider the fuel level in each tank. Some users prefer to run one tank dry before switching, while others may adjust fuel levels to balance their vehicle, such as in boats. Additionally, it is crucial to ensure a secure connection when switching tanks to prevent any issues during operation.
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Fuel pump issues
Priming Issues
One common issue is that the fuel pumps are not priming. Priming refers to the short period when a fuel pump engages. To diagnose this issue, you can use a multimeter or voltage meter to check if the fuel pumps are receiving electrical power. Inspect the various electrical connections to determine if the required amount of electrical current is reaching each fuel pump.
Low-Pressure Fuel Pump Malfunction
The low-pressure fuel pump is responsible for drawing fuel from the tank into the Vapor Separator Tank (VST) or Fuel Supply Module (FSM). If there is no fuel present in the VST/FSM, the low-pressure fuel pump may be malfunctioning. Gain access to the VST/FSM and inspect if fuel is being deposited. Refer to your authorized service manual for proper removal procedures.
High-Pressure Fuel Pump Malfunction
If the low-pressure fuel pump is functioning correctly and there is an adequate amount of fuel available, the issue could lie with the high-pressure fuel pump. The high-pressure fuel pump pressurizes the high-pressure fuel rail, enabling the EFI system to function. Both the low and high-pressure fuel pumps have unique technical specifications and testing procedures, so be sure to consult the appropriate documentation for each pump.
Fuel Gauge and Selector Issues
In dual fuel tank systems, issues with the fuel gauge or selector switch can occur. The fuel gauge may provide inaccurate readings, indicating that one tank is empty while the other is full. Additionally, there may be problems with the fuel selector switch, preventing the flow of fuel from one tank to the engine. It is important to consult the vehicle's documentation and wiring diagrams to trace the fuel lines and identify potential issues with the selector switch or wiring.
Fuel Pump Balance Module
In dual fuel tank setups, the Fuel Pump Balance Module plays a crucial role in averaging the resistance for both fuel senders and providing an accurate fuel level reading. This module is located near the front (main) lift pump, and you can diagnose issues by checking the voltage and resistance at specific terminals with a digital volt/ohm meter.
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Frequently asked questions
A dual fuel tank system consists of two independent tanks that can be used to store and supply fuel.
The system has a low-pressure pump and sending unit in each tank, with a single high-pressure pump on the frame rail. A mechanically operated Dual Function Reservoir (DFR) is located between the two tanks and is operated by the low-pressure pump. When a tank is selected, the fuel pressure moves a valve to allow fuel flow from that tank and block the ports to and from the other tank.
Yes, you can switch between the two tanks manually. There is a switch on the dash that changes the gauge to the desired tank, and the actual switch for selecting the tank is manual and located on the floor behind the driver's foot.
Ensure that the crossover valve is open so that fuel is pulled equally from both tanks. Check the fuel line condition and look for any signs of rust in the fuel tank, clogged filters, or clogged carbs. Additionally, the old gas gauge, wiring, or sending units may be faulty, leading to inaccurate readings.
One common issue is fuel theft, especially when one tank appears to be losing fuel more rapidly than the other. Another potential problem is a stuck DFR, which can prevent access to one of the tanks.
