YJ: Poly gas tank repair - say no to crack

[6spdYJ]

I had my gas tank crack 3" vertically. It's very strange since my skidplate hadn't had any impacts since Holcomb a few years ago. After seeing a cheap replacement was $400, I suddenly had the inspiration to learn plastic welding. The trick with welding plastic is that some plastics mix perfectly and some do not- sort of like welding aluminum or steel or cast. Some can't weld at all and some need a particular filler rod that works (but is not necessarily the same material). Long story short I looked high and low to find that YJ tanks are polypropylene. Nobody would say HDPE or LDPE. What worked perfectly was using the LDPE cap from an oil bottle.
Of course the disclaimer; This is not an instruction. Any work around gasoline and especially vapor should only be performed by experienced individuals who have applied necessary precautions.
That being said, here's what I did...
I used the HF 80W plastic welding kit. It's basically a cheap soldering iron with a spoon blade on the end. I used the blade to melt a groove almost to the full depth of the crack. I tried some HDPE from the oil bottle. I used a bright color to watch the melt to see if it would emulsify. While it did melt in, it would not blend the colors and darken the orange plastic to match the tank. Out with the orange. Seeing that HDPE did not work, only LDPE was left. With my plastics background, I remembered that list are commonly LDPE, so I tried it. I cut the cap in a coil, following the threads down to the top of the cap. I just stopped there. I laid the tip of the coil in the groove and set the blade on it. It was instant. The stuff flowed in and spread like butter. I filled the groove with slight pressure - relying on the heat to get the molecules to link up. As the weld filled I started moving the heat into the surrounding area 1" on the sides and ends to the point where I could see the side of the tank give easily under slight pressure. I used the remainder of my filler to build up a bead of material about 1/4" thick over the original crack.
The outer surface ended up a smooth goose egg over the crack and from the inside, the crack now has a smooth transition from the tank wall to where the groove pushed material inward. It looks like a surgical scar, it has a "ropey" look to it but is smooth to the touch and has no defineable corners that could be or become new cracks.
I'll report after the first fill-up.

[RAT]

If you have the factory tank I have my stock one that are welcome to have. Just let me know.

[Elusive]

I think the Cherokee tanks are the same too... which are easy to find in junkyards,
but this tech is really cool. Im inspired to try this

[6spdYJ]

Thanks for the offer Kurt. Looks like I probably won't need it.

Well, first fill-up today. Not a trace of leaking or gas smell.

The sending unit/fuel pump gasket was rotted out and sketchy at best. Of course the stealership had a two day lead time to order one in and all the local retailers were proud to offer a fuel pump gasket for a carbureted 258. I wound up using some rubber/cork Fel-Pro gasket material with some Permatex Ultra Grey gasket sealer. It's gasoline resistant so I should be fine.

I hate dropping the tank on this thing only second to doing the heater core. I figured I want this to be the last time I drop this tank so I did the floor cut-out while I had the tank out. Starting at the rear seatbelt bolt, I measured one inch toward the tailgate and drilled a small hole centered on the bolt. I measured 3 3/4" to each side and drilled another hole - one to mark each corner. From those I measured back 7 1/2" and drilled the corner holes for the back edge. Then I just cut out the panel marked by the corner holes. Going back this far meant I had to cut half-way into the rear channel that stiffens the floor. This is noted in many other write-ups, but it allows the easiest removal of the sending unit.

What I did a bit different is I measured the diagonal distance across the opening and cut a 9" piece of 1/2" EMT. It's aluminized steel tubing, so it has strength and corrosion resistance. I slid the EMT into the opening of the channel in the floor on the left side and then pulled it into the right side a few inches. From there I pulled it forward all the way. It fit the channel height perfectly, somewhat restoring the floor rigidity. I saddled the opening of the channel that was left on the panel back over the EMT exposed in the opening. It forms a hinge to swivel the panel and is plenty strong. Then I added some 14 gauge strips to the sides of the panel to fill the gaps and let the panel just rest shut.