Jeep Wrangler JK Gas Tank Overflow Fix


Staff Member
Sep 28, 2015
Salem, Oregon
I am posting this guide to show how to fix the JK gas tank overflow issues. I know a lot of people have this problem, and for $40 and 15 minutes of work, why not fix it? The replacement procedure is straight forward and doing it yourself will save you some money in labor and spilled gas.

First the cause of the issue: It seems that the in-tank check (one-way) valve fails overtime due to O-ring erosion caused by the Ethanol in the gas. The result of this is when you are at the pump, backflow from the tank comes back out of the filler neck after the pump shuts off. The gas spills all over the side of the jeep, fender and bumper. This problem has been persistent in my recently purchased 2012 for a long time judging by the splotchy fading of the plastic on the fender and bumper right below the fill door. Chrysler’s fix was to introduce a flapper style check-valve into the fuel fill tube above the rear axle.

The check valve assembly part number is Mopar CNNNX321AC. I paid about $40 for it on Amazon.


Replacement procedure (ensure gas tank is 3/4 or less full):

1. Clean around the hose connection points on the filler pipe that runs above the rear axle so as not to introduce any debris into the fuel system when making the swap. I used hot water and a rag to remove the sand, salt and dirt that had collected over the winter.

Dirty Gas Tank Side:


2. Use a pair of wire cutters to clip the two zip ties that hold the small gas tank vent line to the larger gas fill line.


3. Using a 7mm nut driver or flat head screwdriver, loosen the hose clamp closest to the gas tank and the hose clamp on the rubber elbow that connects the upper gas filler tube to the lower.

4. Insert a bone tool (or similar) between the rubber elbow and rigid line on the filler side of the assembly to be replaced. Work the tool around the joint to free the rubber elbow from the rigid line. Mine was stuck pretty good after 4 years of being on the road. Be careful not to tear or puncture the elbow.


5. Work the hose off of the plastic nipple on the gas tank.


6. Twist back and forth as you pull the rigid pipe free from the existing rubber elbow on the driver's side.


7. Once the old assembly is removed, wipe around the plastic nipple on the gas tank and wipe out any debris collected in the existing elbow on the drivers side.


8. Install the new check valve assembly, driver’s side first. Then tighten the hose clamps.


9. Use 2 zip ties to secure the small vent line to the new check valve assembly.

Next time you fill up, it shouldn't spill any gas!
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