That was a common problem with the early JKs. Ethanol in the fuel caused the check valve in the tank to swell so it would not close when the tank became full.
At first, the whole tank with an updated valve was replaced. Later on, they developed a check valve that could be inserted in the filler tube, about half way between the gas cap and the tank.
Look at your filler tube from under the vehicle. If it's a simple tube all the way to the tank, your JK does not have the new larger diameter section of tube with the added check valve.
To get the new check valve look up this part number: CNNNX321ac
You will remove a section of the original tube, and insert the new, larger diameter tube with the check valve in it.
This is assuming a swollen OEM check valve in your tank is the problem.
2008 Unlimited Rubicon Automatic
Last edited by ronjenx; 02-20-2019 at 07:32 PM.