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Wrangler 15g to 20g Conversion
 It's believed that late model YJs used the same physical gas tank, regardless of whether you have a 15 gallon or 20 gallon tank. What seems to be a difference in capacity is actually just an extended filler vent tube which prevents you from filling the tank as far if you only have the 15 gallon version. By shortening that filler vent tube, you can reclaim the extra capacity.

[Disclaimer, for those who need such things: Gas is flammable. Really. Gas vapors are even more dangerous. If you can't work safely in these conditions, don't attempt this.]

Here are the steps I took to increase the capacity of my tank:

First, remove the plastic splash shield that covers the underside of the driver's side rear corner. It's partially held in place with a few of those annoying push-in plastic fasteners

With the shield off, you can see the two large diameter hoses that go into the driver's side rear upper portion of the tank. The one we're concerned with is the rear hose, which is the filler vent hose. You need to remove that rear hose from the tank by loosening the hose clamp, and then pulling the hose off.

How easy or difficult this job is from this point depends on how sadistic the assembly line worker was that installed the hoses. In my case, the bolts for the hose clamps were impossible to reach without dropping the gas tank about an inch or so. If your clamps can be loosened without dropping the tank, it's even easier. But if you need to drop your tank (like I did), it's pretty simple. There are 3 bolts that hold the front of the tank to the crossmember, and 4 that hold the rear of the tank to the rear frame crossmember. Note that on the rear, in addition to the four nuts you need to remove, there are 2 additional fasteners with long bolts on them. You don't need to (or want to) remove the nuts on those two extra fasteners.

Once you've got that rear hose removed, you can now see the smaller plastic hose inside the vent fitting (on the tank). You've got to remove that smaller inner plastic hose.

I found that needle-nosed vice grips worked best at getting that tube out. It's a tight fit, and takes some effort to remove.

Here's the hose removed. It was just over 9.5" long.

I cut mine down to 2.75" long. This was enough so that when it was re-inserted into the tank's vent fitting, it wouldn't extend much beyond the fitting into the tank. I was thinking about not re-installing this hose at all, but I heard (but can't confirm) that this hose adds some rigidity to the connection on the tank, so that the hose clamp is less likely to hurt that fitting.

After cutting down to size, I put the tube back inside the vent fitting on the tank. Since it's a tight fit, I tapped it in with a rubber faced dead blow hammer (a rubber mallet would have been fine too).

Now, just reverse all your steps for re-assembly, and enjoy the extra capacity.

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