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DIY Tech: Test And Seal A Fuel Tank

Tech & How-to

DIY Tech: Test And Seal A Fuel Tank


Whether it’s a used stock fuel tank or a brand-new custom one, before sending it out for paint you should pressure-check the tank for leaks. Even a small seepage of gasoline will destroy a tank’s paintwork. If there’s any rust inside the tank or if it’s been dented near a seam or weld, definitely check for leaks. Any internal rust should also be removed and a sealant that’s impervious to ethanol-based fuels applied. There are various sealing kits available, like POR-15 or the kit we used from Bill Hirsch, that has all the solutions needed to clean, etch, and seal the tank. A bit of preventative work now will eliminate future headaches like fuel leaks or a rust-clogged fuel filter. If you have a new tank that didn’t come internally treated with a sealant, definitely do this before painting to prevent future rust issues. Personally, I also pressure-test a new tank, just to make sure I won’t have a problem after it’s painted.

The Leak Test
When doing a leak test, it doesn’t matter if your tank is covered with a paint job you want to keep or it’s naked steel. To do a proper leak check, you must pressurize the tank with air and either fully submerge it in water or use soapy water at possible leak spots. When a tank is submerged in water, a leak will produce a stream of air bubbles coming from the hole to show you exactly where it is. It’s the same as putting an inflated inner tube (remember those?) under water to find the damaged spot. As for the soapy water method, make a fifty-fifty solution of dish washing soap and water. As you brush the soapy solution onto a seam, weld, or suspected area, bubbles will immediately form if there’s an air leak.

The best way to pressurize a small tank is with a hand pump, like one for bicycle tires, and an old tire valve. A hand pump allows you to keep the tank at the required pressure without running the risk of applying too much pressure and popping a seam or causing the tank’s walls to bulge. I used the hand pump from a radiator test kit I own. Since the kit’s radiator cap adapters don’t fit the tank’s gas cap opening, I removed the fitting from the end of the pump’s hose so I could connect the pump’s hose directly to the gas tank’s vent tube. Resist the urge to use a shop air compressor and regulator setup since you’re only going to pressurize the tank to about 3 psi. These regulators are normally set at 100 psi or more and don’t regulate very low pressure settings well. However, you can (if you’re careful) fill a small air compressor tank to 3 psi and then attach the air tank, without the compressor running, to your gas tank. Whichever method you use, keep in mind that you’ll need to replenish any air that leaks past the gas cap, etc. to keep the pressure at
3 psi during the test.

The next step is to seal all the tank’s openings, except for the one you plan on using as the air source. Use an old unvented gas cap to seal the tank’s filler hole and put some bearing grease where the cap’s seal will contact the tank to reduce air leakage. If your cap is vented, you may be able to seal the vent hole with duct tape. Or you can just duct-tape the tank’s filler hole closed. If you do use tape to seal a hole, you’ll have to cover the tape with your hand to keep it sealed tight once the tank is pressurized.

To seal the petcock/fuel port bung, you can use an old petcock or the proper size pipe plug (the better option) with the threads wrapped with some Teflon tape. If your tank doesn’t have a vent tube, pressurize the tank via the petcock/fuel port using an old petcock. Or, as we did with a custom tank (the better way), use a hose fitting with the proper pipe thread size on one end. Another tank we tested is a stock H-D saddle tank, which has a vent tube on top and a fuel crossover tube on the bottom. Using one of these tubes is the safest and easiest way to add air to the tank. We used the fuel crossover tube as our air source, and the other tube was sealed with a rubber hose, two hose clamps, and the proper diameter bolt with a shank that has a smooth section (no threads) at least 3/4″ long.

To do the submersion test, fill a sink or bucket that’s deep enough to completely cover the tank with water with 5″ to spare. Then have a buddy add 3 psi of air pressure as you hold the tank under water (it’ll want to float) and cover any sealing tape with your hand. If you see air bubbles around the gas cap, it’s not a problem as long as the bubbles are coming from the cap and not where the filler neck joins the tank. The same goes for your petcock/fuel port. As long as the leak is not from where the petcock’s threaded bung joins the tank, you’re good. Bubbles coming from where the petcock/fuel port threads into its bung is not a problem. If you see bubbles coming from a seam, etc., put your finger over the hole to stop the leak. Then, while keeping your finger over the hole to mark the spot, pull the tank from the water and mark where the hole is with a black marker. If you don’t see air bubbles coming from places they shouldn’t, your tank is good to go.

Clean & Seal
Once you know your tank is tight, it’s time to clean out any rust and then coat the inside with sealer. Though a leak test will not hurt a paint job, the chemicals you’ll be using to clean and seal the tank will definitely trash any paint they get onto. If your tank has a paint job you want to keep, completely cover the entire tank with several layers of tape to protect the paint from the cleaner, etcher, and sealant. Use blue painting tape instead of duct tape on the actual paint because blue tape has weak glue, so it’s easy to remove afterwards. However, since duct tape will do a better job of protecting the paint, I put blue tape on the paint job and then cover the blue tape with duct tape. That said, even with the tank covered in tape avoid letting any chemicals get onto the tape. If anything does get onto the tape, immediately clean it off.

Put extra layers of tape around the filler neck and petcock/vent tube areas. These spots are where you’ll be adding and draining the chemicals. Since you can’t have layers of tape on the filler cap’s threads and still get the cap on, put one or two layers of duct tape over the threads to prevent fluids from getting under the edge of the outer layers of tape. If your tank has a vent or crossover pipe, using one of these tubes is the safest way to drain all the chemicals from the tank. The filler neck has a high inner edge, so it’s hard to get all the chemical treatment out. The petcock bung has a smaller inner edge than the filler neck, but it takes a while to get the chemical over it and out. However, the vent or crossover tube usually has the smaller inner ridge.

With the tank’s openings plugged and following the sealant kit’s instructions regarding cleaning inside the tank, I use a mop bucket to hold the tank as required so the cleaner can do its job. I also use a mop bucket after coating the inside of the tank to position the tank as per the kit’s instructions. Once the cleaning and etching processes are complete, and before pouring in the sealant, some kits require that the inside of the tank be absolutely dry. Blowing warm air into the tank with a hair dryer helps speed this process.