Steel automotive fuel tanks can leak at any age or condition. Rust, impact and road debris cause most of the obvious leaks. Warning signs of leaking gas tanks will be puddles on the ground and the obvious smell of raw gas. Some leaks can be more serious than others, especially when the fuel tank mounts under the seat of your vehicle, as in the case of many Jeep designs. Catching the leak early can eliminate costly repairs and hazardous situations. Most vehicle owners can repair small leaks using some specific putty products and a few repair steps.
Set the vehicle in park and engage the emergency brake. Use the floor jack to raise the rear of the vehicle chassis. Place two jack stands under the rear frame near the wheels. Use the floor jack to support the weight of the gas tank, placing it directly underneath and snug up against it. Use the appropriate socket and wrench to remove the bolts to the gas tank holding straps. Lower the tank carefully with the floor jack and wheel it out from under the vehicle.
Empty the fuel tank of all gasoline in an outside area, allowing it to drain with the filler neck down into a container. Be sure to dispose of excess gas in the proper manner, according to regulations. Allow the tank to drain and air out for a day or so.
Don gloves and a particle mask and move the gas tank to a well-ventilated outside area. Pour 1/2 gallon of muriatic acid into the tank at the filler neck. Stuff the filler neck closed with a knotted rag. Move the tank around by hand, swishing the acid back and forth within the tank. Make sure you turn the tank in all directions so the acid makes contact with all interior surfaces. Remove the filler neck rag and drain the contents into an approved container for disposal.
Pour one more 1/2 gallon of muriatic acid into the tank and plug the filler neck with a rag. Swish the acid around inside the tank as you did before. Remove the rag and let the acid drain from the tank. Use a high-pressure water hose to completely flush the inside of the tank. Turn the tank several times, flushing all sides of the interior. Let it drain and air out for a day.
Locate the exterior leak on the tank's metal surface. If the leak appears pinhole in size, use an ice pick to poke the hole, opening it up into a slightly larger hole. Use the ice pick to probe around the leak area for any other weak spots, like a rusty area. Open up any suspected corrosion areas by lightly poking and scratching with the ice pick. Once the leak holes have been located, draw a chalk circle around them.
Use the drill motor and sanding disk to sand the leak area, removing as much corrosion or rust as possible. Do not apply too much pressure, since the metal surface might be thin. Wipe away all sanding residue with a small amount of muriatic acid and let it dry.
Use a very coarse sandpaper (220-grit) to sand the leak area. Do not use extreme pressure. Sand in one direction first, then switch the sanding direction 90 degrees, making a cross-hatch pattern on the metal. Wipe all sanding residue away with an acid-damp rag. Let the surface dry for several minutes.
Apply a generous portion of putty, either an epoxy with hardener or the JB-Weld product, on the leak area. Use a putty knife to spread the mixture over the sanded areas. Use a firm pressure to shove the putty down into the holes that you probed. Don't worry about putty penetrating down through the hole. This will help it adhere to the tank surface. Let the putty mixture cure and dry, according to the directions. More than a few hours should be sufficient.
Sand the area of the putty patch if you need to decrease its thickness for clearance reasons (when fitting the tank back into its mount). However, you should let the putty remain as thick as possible, when applicable.
Install the tank in the reverse order that you removed it.
Working with muriatic acid is dangerous. Make sure you use protection and prepare the tank in a well-ventilated area.