So I bought a 99 XJ a couple weeks ago. I crawled completely under the truck when buying it and this was the only rust on the underside of the body I could find at all. Only other signs of rust I found were a small, quarter sized spot at the top driver corner of windsheild, small spot near bottom of passenger door and small traces of rust at the bottom corner of the passenger side rear most window. Everywhere else the body looks rust free, even everywhere under the body.

I used a flat screwdriver and scrapped a little flakes off this spot and checked that I could stick the screwdriver up into the unibody frame rail, just in the hole that is visible in the pics. It doesn't seem to be bad, just in this one spot.

I pulled the heat shield for the gas tank down a bit and the frame looks nearly new under it. Nowhere else that I've found on the frame or anywhere has rust or even looks like it might.

I am only going to use this XJ as a "farm truck", to haul my family back into the woods around our land. I will of course drive it some around town and here and there, but I didn't buy it with intentions of depending on it for anything other than a "farm truck". I'd still like to know what you guys who are far more knowledgeable on the XJ would recommend I approach fixing this. Also, is it in a location that will lead to structural issues or more body flex when we are off roading back on our land and flexing the suspension out?







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I would have that cut out and weld in new metal. that rot is all the way through and is weakening your truck considerably.

 

The rails are the worst place to have rust. But, It can be repaired. You will have to get the rust out of there or it will continue to spread. Plus you will need to reinforce the rails.

 

So, it will probably be a bit before I get to get into it, but I will at least try and get all the rust from the area ground away and coat it with something until I can start the cut out.

You all think the frame stiffeners would work for this or just plate it? Or is it too hard to tell until I really get in there and grind away and see how far it is?

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That's pretty rough man. If the rest of the XJ isn't in very good shape it may just be easier to get another. If you want to/need to keep this one: cut all that rust out, rebuild the frame rail. Stiffeners would definitely be a big help. It is hard to tell how bad it is, but where that big hole is, you'll be cutting away quite a bit. Gently stab around with a screwdriver, it should give you an idea of how solid things are.

 

Yea, the rest of the jeep looks nice, just this one spot looks like that.

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Given this, I'd just drive it and ignore the rust. If it were a DD that you had to depend on, lots of good advice above.

 

Go around the underbody with a hammer, it may look like swiss cheese when you are done.

 

So my OME lift came in yesterday and after getting the rear axle dropped down and springs out of the way I was removing what was left of the factory bumpstops and all the bolts on them just twisted off (broke). Anyway, I started pecking around the area with a small ball peen hammer and yea, it's a little worse than I thought it may have been. Looks like that rail may be bad about a foot or so long.

We are going to be moving to our new place this summer and I've got a nice shop over there that I am going to install a 2-post lift in. I'm just going to run it like it is for now and once moved over there where I can get it up on the lift and get good access to everything I'll cut it all out and plate and cover with the frame stiffeners. While I've seen much worse rust posted here and fixed, it still sucks because even though I bought this for a farm truck, I like it and can see wanting to use it for a part time DD or just running around in. Maybe I'll end up getting the bug worse (I can feel it starting to set in now) and pick up another one for that

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mine is like that too, not as bad though.... i plan on grinding and cutting the rust out. putting some primer on it and putting reinforcement plates over it.