Ameer Randolph
The 1969 Mustang is a classic car, but like any older vehicle, it may need some work to keep it running smoothly. One issue that owners have faced is how to run a return fuel line to prevent vapour lock and percolation. While some suggest that a return system is unnecessary unless the car has a power adder such as a supercharger or turbo, others have opted to install one to prevent fuel from heating up and to keep it circulating. There are several ways to do this, including using the drain plug, drilling a hole in the filler pipe, or removing the sending unit and drilling and soldering a pipe into it.
| Characteristics | Values |
|---|---|
| Fuel line routing options | 1. Run new aluminum fuel lines on the passenger side similar to the factory fuel line on the driver side. 2. Run new aluminum fuel lines on the driver side, copying the current factory line. 3. Run one aluminum fuel line on the passenger side, copying the factory fuel line, and use the factory fuel line as the return line. 4. One of the above scenarios with a PTFE fuel line. 5. Run aluminum fuel lines along the transmission tunnel. 6. Run new aluminum fuel lines along the inside passenger side and out through the firewall. |
| Fuel line routing concerns | Drilling holes in the torque box or widening existing holes. Running fuel lines inside the trunk. Using existing grommets for the catch can and breather setup. Running lines near the firewall. |
| Fuel line materials | Aluminum, steel, PTFE |
| Fuel system components | Fuel pump, fuel lines, fuel tank, vent valve, fuel filter, fuel pressure regulator, cool can, AN fittings, fire sleeve, foam insulation, fuel pressure gauge |
| Fuel system considerations | Vapor lock, percolation, heat, pressure, pump suction, fuel circulation, fuel temperature, fuel volume, fuel return, fuel line diameter, fuel line routing, fuel line material |
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What You'll Learn
Running a return fuel line in a carbureted Mustang
Firstly, it is important to determine the benefits of installing a return fuel system in a carbureted Mustang. A return fuel system can help keep the fuel circulating and cool, which can improve the performance of the vehicle. However, some sources suggest that a return system may not be necessary unless you are using a power adder such as a supercharger or turbocharger. It is also mentioned that a return system can turn your fuel system into a hydronic heating system, which may not be desirable.
Secondly, it is crucial to select the appropriate fuel lines for your Mustang. Some options include using aluminum fuel lines or steel tubing. According to one source, aluminum fuel lines may not be recommended by some EFI manuals due to their softness, which can lead to cracking or pinhole development from impacts with rocks and debris. Steel tubing, on the other hand, is more durable and can be assembled using compression fittings.
Once you have determined the benefits and selected the appropriate fuel lines, you can begin planning the fuel line routing. There are several options for running the fuel lines in a 69 Mustang:
- Run the new aluminum fuel lines on the passenger side, similar to the factory fuel line on the driver's side. This option requires drilling holes through the torque box.
- Run the new aluminum fuel lines on the driver's side, following the current factory line. This option may require widening the hole in the torque box.
- Run one aluminum fuel line on the passenger side, copying the factory fuel line, and use the factory fuel line as the return line.
- Run aluminum fuel lines along the transmission tunnel, but this option may have space constraints and potential heat issues from the exhaust.
- Run the new aluminum fuel lines along the inside passenger side and out through the firewall.
- Use PTFE fuel lines instead of aluminum for any of the above scenarios.
When installing the return fuel line, it is important to consider the venting and pressure regulation. One method is to drill a hole in the fuel tank and weld a nut with the correct threads before filling it with fuel. This vent can be connected to a rollover vent valve, and a hose can be run from the valve to a drop-off in the quarter panel. Additionally, a bypass return fuel pressure regulator may be required.
It is also important to consider the potential for vapor lock, which is a common issue in fuel systems. To address this, it is recommended to install a regulator/return as close to the carburetor as possible and use a pump that generates sufficient volume and pressure to prevent vapor lock.
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Using aluminum fuel lines
While there are several ways to run a return fuel line on a 69 Mustang, using aluminium fuel lines is a more complex process. Aluminium fuel lines are softer than steel and tend to work-harden when hit by rocks and debris underneath the car or in the wheel wells. This can lead to cracking, splitting, or the development of pinholes.
However, some people opt to use them anyway. To do so, you can run the new aluminium fuel lines on the passenger side, similar to the factory fuel line on the driver's side. This will require drilling holes through the torque box like the factory routing. Another option is to run the new aluminium fuel lines on the driver's side, copying the current factory line. This may require widening the hole in the torque box. A third option is to run one aluminium fuel line on the passenger side, copying the factory fuel line, and use the factory fuel line as the return line.
It is important to note that some EFI manuals specifically advise against using aluminium fuel lines, without providing a reason. However, one user on a Mustang forum noted that their motor blew up, but the aluminium lines were not an issue. As long as rubber-insulated clamps are used to secure the aluminium lines, there shouldn't be any problems.
If you are still unsure about using aluminium fuel lines, you might consider using steel tubing, which is not too difficult to work with. Fragola Performance Parts offers a line of fittings and PTFE smooth bore hose that can be used with steel tubing.
Additionally, if you are running a return fuel line, there are a few ways to do so. One option is to run the line back to the tank through the drain plug, if your tank has one. Another option is to drill a hole in the filler pipe and mount a fitting. A third option is to remove the sending unit and drill and solder a 5/16 pipe into it beside the outlet pipe.
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Running a return line to the tank
Before putting fuel in the tank, it is recommended to drill a hole and weld a nut with the correct threads to allow for venting. It is best to do this in one of the rearward corners so that it is out of the way. Then, screw in a rollover vent valve and run a hose from it to the drop off in the quarter panel. You will need a bypass return fuel pressure regulator.
There are a few different options for running the fuel lines themselves. One is to run the new aluminum fuel lines on the passenger side, similar to the factory fuel line on the driver's side. This will require drilling holes through the torque box. Another option is to run the new aluminum fuel lines on the driver's side, copying the current factory line. This may require widening the hole in the torque box. A third option is to run one aluminum fuel line on the passenger side, copying the factory fuel line, and using the factory fuel line as the return line.
It is worth noting that some sources advise against using aluminum fuel lines as they are softer than steel and can be damaged by rocks and debris hitting them, eventually cracking or developing pinholes. Steel tubing is not much more difficult to work with and can be assembled with compression fittings.
It is also important to consider the potential for vapor lock, which is a common problem with today's fuels. If you are having vapor lock issues, it is likely happening on the inlet side of the fuel pump, causing the pump to starve. To address this, you can install a regulator/return as close to the carb as possible and run a pump that generates enough volume and pressure in the line from the pump to the regulator to make vapor lock impossible.
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Preventing vapor lock
Vapor lock is caused by a combination of low pressure at the inlet and high temperature, which turns the liquid on the inlet side to vapour. To prevent vapor lock, you need to keep the fuel line cool.
One way to do this is to insulate the fuel line with foam insulation, like the type used for AC hoses or hot water pipes. You can also use a heat shield, which will allow air to circulate around the line. In the past, metal "butterfly" clips were used to act as heat sinks. Another method is to solder thin steel strips lengthwise on the tubing to dissipate the heat.
If your fuel pump is rated much higher than your regulator, it should be able to supply enough pressure to prevent vapor lock. Using a high-pressure tank-mounted electric fuel pump and a return-style regulator will also help, as pressurizing the fuel will stop the vapor lock.
You can also try to direct the heat away from the lines and carburettor. A phenolic carb spacer can help reduce heat, as can a glass filter. Routing the fuel lines further from the exhaust manifold and engine will also help.
Finally, using high-quality fuel and ensuring your engine is not overheating can help prevent vapor lock.
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Using a bypass return fuel pressure regulator
A return style regulator, also known as a bypass regulator, is a common choice for a performance fuel system. It is important to note that return style regulators cannot be used in some fuel systems, such as a nitrous system with a single pump and multiple regulators set at different pressures.
When using a bypass return fuel pressure regulator, fuel from the pump enters the inlet and flows past the "bypass valve," which is held closed by a spring. As the pressure increases, it pushes against the spring through a diaphragm, and when the pressure gets high enough, the bypass valve opens, redirecting some fuel back to the tank to reduce the pressure in the system. As the pressure drops, the spring closes the valve, allowing the pressure to rise again. This process maintains the set fuel pressure, and the pressure can be adjusted with the nut/bolt on top of the regulator.
One advantage of using a bypass return fuel pressure regulator is that it allows for the highest possible flow all the way to the carburetor. The closer the regulator is to the carburetor, the better the control of the pressures, which is why they are often mounted very close to each other.
When installing a bypass regulator, it is important to consider the entire fuel path configuration. The regulator can be installed in the rear, but it must be done correctly. The fuel line up front feeds the carburetor using a log-type line set-up, and then runs back to the rear tank before going into the bypass regulator and gauge and returning to the tank. Mounting a rear bypass regulator on the inlet side, shortly after the pump, can cause problems as it may bypass most of the fuel before it reaches the engine.
When deciding whether to use a bypass regulator, it is important to consider the potential benefits and drawbacks. For example, in a street/strip carbureted Mustang, using a return fuel line can help keep the fuel circulating and as cool as possible. However, it requires additional purchases such as a new regulator, return line, and proper fittings, and may involve welding work. Additionally, some sources suggest that a return system may not be necessary unless using a power adder such as a supercharger or turbo, as it can turn the fuel system into a hydronic heating system.
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Frequently asked questions
A return fuel system can help to keep the fuel circulating and as cool as possible.
A return system turns your fuel system into a hydronic heating system, allowing more heat into your fuel system.
There are three ways to run a return line back to the tank: through the drain plug, by drilling a hole in the filler pipe and mounting a fitting, or by removing the sending unit and drilling and soldering a pipe.
You can run new aluminium fuel lines on the passenger side or driver side, or use a PTFE fuel line.
You can use an external pump from an 88 Ford F150, or an in-line pump.
