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Project Scope Creep (alfred): Lets Play with the Gas Tank


NAXJA Member
For as long as I have had my Jeep I have had an occasional whiff of gas vapors at the back. I have even had it pointed out by some jerk on the trail. Having dealt with the fuel pump before (that done with the tank still in place) I was pretty confident I had no problems on the pressurized side of the equation and so figured I had a problem with the vents up on top where nothing can be accessed without dropping the tank. Doing more and more welding at the back I finally got motivated enough to drop the tank and have a look see.

Sure enough, there is a problem with the vents, but not the problem I have been smelling. One of the lines was used as cushioning for the tank.


But notice those dark patches. There are a couple in the center of the picture and another at the top left. Those are special. Here is what they are all about:




I am going to guess that a previous owner decided to mount some sort of speaker system in the cargo area and discovered the gas tank in the process. Given the quality of workmanship in this repair, and elsewhere, I am going to also guess it was only repaired in order to make the Check Idiot light go away. To that extent the repair has worked. But Bondo isn’t really rated for gasoline. There is a better way. Repair kits are sold where auto parts are sold. Or you can put together pretty much the same thing if you have some JB Weld and fiberglass mat handy:


But I was curious to learn a bit more about what can be done with the gas tank capacity in a ’96. There are options for larger tanks for everything except a ’96. There are aftermarket stock replacement tanks that supposedly hold more fuel than the OEM tanks. And there are vent modifications that will increase the capacity of an OE tank. I decided to see what I could gain with an aftermarket stock replacement tank and a vent modification. This is the JP2B tank you can get for $75 on Amazon, although I got this one from RockAuto. It says it is for ’91-’95, but it seems to work just fine for a ’96 too. Here is what I have managed to learn in the process of modifying and installing one.

The first obstacle I encountered was the inability to get my hand through the hole for the pump so that I could modify the vent tube. If you have big hands you may need to enlist the help of someone else. Fortunately for me I have an assortment of different sized Varmints and was able to compel one of them to handle the interior aspect of this job (He is actually pretty eager to help with this sort of thing). I will also note at this point a little detail about these cheap aftermarket tanks: the tubes do not run more than a few inches into the interior of the tank. On the OEM tanks the needed end of the vent tube appears to be almost straight in from the opening. On this aftermarket tank the needed end is all the way in the far corner. There will be a bit more detail on this later.

The second obstacle involves the diameter of the vent tube. The vent modification on an OEM tank involves an up-turn on a 5/8” O.D. tube. Both ½” copper plumbing fittings and ½” EMT conduit lend themselves to this modification. Well, the aftermarket tank may be a nominal 5/8”, but it is evidently a metric conversion, and the chicoms don’t hold their tolerances close enough to get away with the usual fare. My calipers say that vent tube O.D. is 41/64”. Not a lot bigger, but enough so that neither the ½” copper fittings nor the EMT will slip over. I did manage to scrounge up a reamer that would get me where I needed to go (and a bit extra) though, so I applied that to some EMT and then bent it as tight as I could with a proper conduit bender. I added a couple of slots with a dremel and a cut-off wheel so that I could clamp it down nicely. After a few fit tests this is where I ended up:


That was then installed with a conservative application of JB Weld (I applied some to the interior of the EMT and the Varmint applied some to the exterior of the vent tube inside the tank) and the hose clamp was slid into position and tightened down by the Varmint. This is the final install of the vent tube extension:


Notice those bumps that are circled in red? That is where the support bracket is supposed to be for the vent tube and fill tube both. You can see them on the outside here:


Lamentably I do not have a picture of the interior of the OEM tank. At any rate, this is one of the areas in which the cheap replacement tank shows its cheapness. I don’t know if those dimples are there because they would be required if the bracket were to be installed and the stamping dies simply already have them, or if the dimples have been added to create the illusion that there is a bracket right there.



One of the questions I wanted to figure out was how the aftermarket tanks could be holding more fuel than the OEM tanks. From the pictures it should be pretty clear that they have retained the same shape. It is not as if they left out a depression or some such thing. However, if you take a tape measure to the tanks you can find a difference:




Note that the tape measure used for this is a flat back tape so that it can follow contours consistently. I have simply measured from the ground plane on one side, up and over the tank and back down to the ground plane on the other side. I did this on both ends of both tanks and consistently got a ½” difference between the two. In short, the aftermarket tank is ¼” deeper than the OEM tank. That is probably part of the extra capacity. (If someone needs to see more pictures of how I measured, and the comparable measurements for the other end of the tank, go ahead and ask, but they are pretty boring and redundant to what is already here.)
Re: Project Scope Creep: Lets Play with the Gas Tank

Anticipating that the seals for the vents could be a problem I had already ordered new ones. It appears Crown is the only source for these anymore:


Here it is, just waiting for the pump assembly to transfer over. I used Gorilla tape to make sure I didn’t repeat the errors of the previous owner and to cushion the points of contact for the mounting of the tank:


Transferring the pump showed another cheap detail. The flange on the new tank is flat:


Whereas on the OEM tank it is radiused to better support the O-ring for the seal:


I put it all together and then added 4 gallons of gasoline (measured by filling a 2 gallon Rotopax to the brim, balanced on the opposite, bottom corner, twice) and then drove 1 mile to the gas station where I then put in another 22 gallons:


That is the most I have ever put in.

Call it a 26 gallon tank. It is no 31 gallon Genright, but it is probably as good as one can do with a ’96. And it doesn’t cost a grand.

I intend to finish my repair on the OEM tank, modify the vent and then reinstall that one to see what I get in comparison. Hopefully I will get around to that in late spring. I think the OEM tank is a better quality unit. If I can get within a gallon or so of this tank then I will stick with the OEM unit.

One aside detail I thought I would mention here has to do with the J-bolts for the tanks. One of mine had been cut short and was posing a challenge at reinstall time. For the time being I have solved it with a coupling nut, a couple of jam nuts and some all-thread. Hopefully the junkyard will yield me a proper replacement. New ones cost $25! However, I did find that this may be a viable alternative:


Hopefully that provides some useful information for someone else.
Re: Project Scope Creep: Lets Play with the Gas Tank

Great write up. An extra 4 or 5 gallons would be very useful.
Wonder if raising the vent is possible the 97+ tanks?
Re: Project Scope Creep: Lets Play with the Gas Tank

Not sure if you can extend it on a late model tank but I know you can swap in a ZJ tank for a few extra gallons over the 97+ XJ. Bolt in swap
Re: Project Scope Creep: Lets Play with the Gas Tank

Nice write up! I did the same thing (same tank etc) except i used copper pipe and jb weld for the extended vent. Mine holds ~27ish gal so about the same. I could fit my hand in there except i cut the crap out of my wrist and my hand was all red the next day from lubing it up and shoving it in the tank (hole)....wow that didn't come out right. Anyways....