Dual Gas Tank Ford F150 – Troubleshooting And Maintenance Guide
A dual gas tank ford f150 setup offers incredible range for towing and off-roading, but it comes with unique mechanical quirks. This guide explores how to diagnose selector valve failures, fix fuel transfer issues, and keep your classic Ford running reliably.
If you own a classic “Old Body Style” (OBS) Ford, you know the convenience of having extra fuel on tap. However, you also likely know the frustration when the dual gas tank ford f150 system starts acting up.
Whether your fuel gauge is lying to you or your truck is “making” gas in one tank while the other empties, these systems require specific knowledge to maintain. We are going to break down how these systems work and how you can fix them in your own garage.
From the mechanical selector valves of the 70s to the high-pressure fuel delivery modules of the 90s, we will cover the essentials to keep your rig on the road. Let’s get under the chassis and see what makes these dual gas tank ford f150 systems tick.
Understanding the Dual Gas Tank Ford F150 System
Ford utilized a twin-tank configuration for decades, primarily to give work trucks and long-haulers the range they needed before gas stations were on every corner. Typically, you have a midship tank located inside the frame rail and a rear tank located behind the rear axle.
The heart of this system is the switching mechanism. In older carbureted models, this was a simple manual or electric solenoid valve. In fuel-injected models, particularly from 1987 to 1996, the system became significantly more complex.
Fuel-injected trucks use a Dual-Function Reservoir (DFR) or a series of Check Valves to manage flow. These components ensure that fuel returns to the same tank it was drawn from, preventing the dreaded “cross-filling” issue that plagues many owners.
The Evolution of Ford Fuel Switching
In the late 70s and early 80s, the switch on your dash simply moved a motorized valve. This valve diverted the suction line and the return line between the front and rear reservoirs. It was a mechanical solution to a simple problem.
By the late 80s, Ford introduced high-pressure EFI systems. These required a more robust way to handle fuel pressure. They moved to a system with low-pressure lift pumps in each tank and a single high-pressure pump on the frame rail.
The final evolution in the mid-90s saw high-pressure pumps inside each tank. In this setup, the “switching” is purely electrical. When you flip the dash switch, you are simply choosing which fuel pump receives power.
Common Problems with a Dual Gas Tank Ford F150
The most notorious issue with a dual gas tank ford f150 is fuel transfer. This happens when fuel is drawn from one tank but returned to the other. You might notice your rear tank overflowing even though you haven’t filled it in weeks.
This is usually caused by a failing internal check valve or a stuck Dual-Function Reservoir. When the valve fails to seal, the return line pressure pushes fuel into the dormant tank. This is not just annoying; it can be a fire hazard if the tank vents fuel onto the pavement.
Another frequent headache is the fuel gauge. Does your needle peg to “Full” or drop to “Empty” the moment you flip the switch? This often points to a sending unit failure or a bad ground wire near the rear harness.
Identifying Selector Valve Failure
If your truck runs fine on the front tank but stumbles and dies on the rear, the selector valve is the prime suspect. These valves can become clogged with sediment or the internal diaphragm can rupture over time.
You can often test this by bypassing the valve with temporary fuel lines, though this is only for diagnostic purposes. If the truck runs perfectly with a direct line, you know the valve has reached the end of its life.
Listen closely when you flip the dash switch with the key on. In older models, you should hear a distinct “clunk” from the frame rail. No sound usually means the solenoid has failed or the switch in the cab is dead.
Troubleshooting the Fuel Selector Switch and Wiring
Before you drop a heavy fuel tank, always check the electrical circuit. The dash switch is a common failure point because it handles the current for the pumps and the sending units. Over time, the contacts inside the switch can corrode or melt.
Use a multimeter to check for 12V at the switch connector. If you have power going in but nothing coming out when toggled, the switch is toasted. This is a 10-minute fix that saves you hours of labor under the truck.
Check the wiring harness along the driver-side frame rail. This area is exposed to road salt, mud, and debris. A frayed wire here can cause the fuel pump to lose power intermittently, mimicking a mechanical failure.
Testing the Fuel Sending Units
If your dual gas tank ford f150 has a working pump but a dead gauge, the sending unit is likely the culprit. The sending unit uses a float arm and a resistor to tell the gauge how much fuel is left.
You can test the gauge by grounding the signal wire at the tank connector. If the gauge sweeps to “Full,” the wiring and gauge are fine. This confirms the rheostat inside the tank is worn out and needs replacement.
Keep in mind that on 1987-1991 models, the sending unit is often integrated with the low-pressure lift pump. Replacing one usually means replacing the entire fuel hanger assembly.
Step-by-Step Maintenance for the Fuel System
Preventative maintenance is the best way to avoid being stranded. Start by replacing your fuel filter every 15,000 to 20,000 miles. A clogged filter puts immense strain on the pumps and the selector valve.
- Relieve Fuel Pressure: Locate the Schrader valve on the fuel rail and depress it (with a rag) to bleed off pressure.
- Inspect Lines: Look for cracks in the nylon lines or dry rot in rubber sections.
- Clean Grounds: Find the ground strap for the fuel system (usually on the frame) and scrub it with a wire brush.
- Check the Reservoir: If you have a DFR, unscrew the bottom cup and check for debris or water.
Always use flare nut wrenches when working on fuel line fittings. These fittings are often made of soft metal and will round off easily if you use a standard open-end wrench. A little penetrating oil the night before goes a long way.
Dealing with Rusty Tanks
If your truck has been sitting, the interior of the steel tanks can rust. This rust flakes off and clogs the pump strainers and the selector valve. If you see “red dust” in your fuel filter, it is time for new tanks.
Modern replacement tanks are often galvanized or made of high-density plastic to prevent this. While you have the tanks down, it is the perfect time to replace the filler neck hoses, which often crack and cause leaks during fill-ups.
Don’t forget the rollover valves on top of the tanks. These small plastic valves prevent fuel from leaking if the truck flips. If they are cracked, you might smell gas fumes around the truck even if there are no visible leaks.
Upgrading Your Dual Tank Setup
Some owners choose to simplify their dual gas tank ford f150 by converting to a single, larger tank. Companies like Bronco Graveyard or LMC Truck offer 38-gallon rear tanks that replace the factory 18-gallon unit.
This eliminates the need for a selector valve and reduces the number of potential failure points. However, it does require some modification to the spare tire carrier, as the larger tank occupies that space.
If you stick with the dual setup, consider upgrading to high-quality pumps like Walbro or Bosch. Cheap “no-name” pumps found online often fail within a year, especially in the demanding environment of a dual-tank system.
Off-Road Considerations
For the off-roaders, the midship tank is generally better protected than the rear tank. If you are rock crawling, the rear tank is susceptible to impacts. Installing a heavy-duty skid plate is a smart move for any dual-tank rig.
Ensure your fuel lines are tucked high within the frame rails. Use cushioned P-clamps to secure any loose lines. Vibrations from rough trails can cause lines to rub against the frame, leading to a pinhole leak miles from civilization.
Carry a spare fuel pump relay in your glovebox. These small plastic cubes are known to fail, and having a spare can be the difference between driving home and calling a tow truck.
Safety First: Working with Fuel
Working on a dual gas tank ford f150 involves handling highly flammable liquids. Never work on the fuel system in an enclosed garage with a pilot light (like a water heater). Always have a fire extinguisher nearby.
Disconnect the negative battery terminal before starting any work. This prevents any accidental sparks from the fuel pump wiring. If you are dropping a tank, try to get it as empty as possible; a full 19-gallon tank weighs over 120 pounds.
Use fuel-rated O-rings and gaskets only. Standard rubber will degrade when exposed to modern ethanol-blended gasoline. Always double-check your connections by cycling the key a few times to prime the system before starting the engine.
Frequently Asked Questions About Dual Gas Tank Ford F150
How do I know which tank I am using?
The dash switch is usually labeled “Front” and “Rear.” On most models, the fuel gauge will automatically switch to show the level of the tank currently selected by the dual gas tank ford f150 system.
Why does my truck die when I switch tanks?
This is usually due to a failed selector valve that isn’t opening, or a dead fuel pump in the selected tank. It could also be an air leak in the suction line, which causes the pump to lose prime.
Can I run the truck with one tank removed?
Yes, but you must cap off the open fuel lines to prevent leaks and keep debris out of the system. On EFI models, you may need to bypass the selector valve entirely to maintain a closed loop for the return line.
What is the total fuel capacity?
Most F-150s from this era have a 19-gallon front tank and an 18-gallon rear tank, giving you a total capacity of roughly 37 gallons. Long-bed models sometimes featured slightly larger capacities.
Is there a recall for the fuel transfer issue?
Ford issued several technical service bulletins and recalls (like 93T05) for fuel cross-filling in the early 90s. While most trucks have been serviced, some may still have the original, faulty check valves.
Final Thoughts on the Twin-Tank Setup
Owning a dual gas tank ford f150 is a badge of honor for many truck enthusiasts. It represents an era of utility and endurance. While the system can be finicky, understanding the mechanics behind the selector valve and pumps makes it manageable.
Keep your filters clean, listen for your pumps, and address any “making gas” issues immediately. With a little DIY effort, you can enjoy the massive range and reliability that Ford intended for these legendary pickups.
Stay safe, keep your lines tight, and enjoy the open road with your classic F-Series! Happy wrenching!
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