Q: Every time I put gas in my 2001 Nissan Frontier, some fuel spills out onto the ground. It looks like the fuel is coming from underneath, but it's very hard to see what's going on down there. I don't want to remove the whole tank to find the problem. Any ideas?
A: Spilling from underneath and backing up out of the fuel filler neck are two different problems. Backing up out of the filler is usually caused by a failure in the refueling vapor recovery section of the evaporative emissions system. This subsystem cycles fuel vapor though the charcoal canister to prevent hydrocarbons from coming out the filler neck while refueling.
Spillage on the ground that's coming from below where the gas cap screws in is a simple liquid fuel leak between the filler hole and the tank. From my experience, it's most likely a rotted filler neck that requires replacement. The best means of confirming the source of the leak is with a smoke tester. This process involves injecting pressurized smoke into the fuel tank and visually spotting where it comes out. Without smoke, you may have to remove some plastic protective panels, and get in there with a flashlight and a mirror to inspect the filler neck for excessive rust, and possibly trail marks where the fuel has been coming out. It could be a hose, connection, or even the tank itself leaking, but most likely it's the filler neck.
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