I have a pinhole leak on the very bottom of my plastic fuel tank in my car and I'm trying to repair it. If the tank was empty, the hole could be plugged with some epoxy, JB weld, or some other adhesive. But the leaking fuel (diesel, in my case) interferes with the patch before it can cure and creates a new leak.

Draining the tank is no simple feat, either. I would need to remove the tank cover and siphon out all the fuel into some container, undoubtedly spilling some on the carpet and stinking up my car. Running the engine until the fuel level is low enough isn't an option because: a) the hole is on the very bottom, so the tank needs to be completely empty, and b) running it dry can damage the fuel pump (which relies on the diesel fuel for lubrication).

Is there any way I can temporarily plug up the pinhole for long enough time for epoxy or adhesive to cure?

2 Answers 2


Bar soap. Rub it into it the pinhole and the fat in the soap will plug it. High-fat content soap such as Ivory works better. Obviously this will only work on pinholes, and I've only tested this on plastic tanks, though it will probably work on metal ones too.

Then, roughen up the surface with some 40 grit sandpaper, clean the tank with some degreaser and patch the hole with your material of choice. I suggest a more permanent solution such as JB Weld (which has held up fine for me).

If you don't care about the tiny amount of fuel that leaks out and you just need to satisfy the mechanic so your car passes the safety test, just using soap and cleaning the tank well so you can't tell there's a leak might work. However, it takes all of three minutes to mix up some JB Weld and permanently fix it, so you might as well do it right.

Note: Pinhole leaks in steel tanks often indicate that the tank is rusting from the inside out and you will continue to have leaks. In which case you will need to empty and remove the tank to fix it properly, or replace it all together. This doesn't apply to plastic, since plastic doesn't rust.


Insert and break off a toothpick or similar sliver in the hole?

  • I thought of something like that, but when I say pinhole, I really mean something much smaller than that. For example, while it was up on the vehicle hoist, I'd say it would drip maybe once every two hours. The only reason I fixed it was because the mechanic refused to write a safety certificate for it until it was fixed. (Note that this is diesel fuel, not gasoline/petrol. Diesel is actually quite difficult to ignite, unlike gas, and it gives off very little smell, again unlike gas. So from a practical perspective, there was no point in fixing it.)
    – user17389
    Sep 7, 2016 at 23:50
  • A rose thorn is quite fine and hard Sep 8, 2016 at 0:33

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