Buying a YJ, what should I expect?


New Member
Sep 30, 2022
Santa Fe, NM
I'm buying a 1990 YJ, it's got the 4.2 L AMC 258 I6.

Guy says the engine doesn't work, but I'm confident me and my dad can fix most any problems, short of a cracked block.

I've never owned a Jeep Wrangler before however. So, not sure what I should be expecting.

Any advice?



Welcome to the Jeep owners club. Looks like an unmolested start for a build.
Fist off fill out your profile with info on your YJ. Specifics help us in diagnosing issues. What do you mean by it does not run? Basics are fuel, compression and spark. You need to find out witch of the 3 are not working. Fortunately the 4.2 is a simple engine to work on. Many here willing to help.
Tell us what's he is asking if you will. Next rust, rust and rust is the biggest killer of jeeps. Body mount, between the belly pan and frame, in the frame, Cracked frames from abuse and crappy repairs is a concern. Cobbled up wiring systems. I cant recall if that will have TB injection which is pretty limited on parts availability and weren't great compared to the later style. Don't run? like in engine is spent or just a bad fuel system? 258's are pretty basic engines but Id assume the worst. I've told myself more than once base of a sellers good word and found a pile of issues far beyond what was said to be the case. Depending on how long it's been sitting consider clutch hydraulics, brake system problems. You can't hear it run, nor can you drive it so it's open to every part of the running gear being compromised, remind the seller of that regardless of his promise it's a one owner little lady's jeep who only drove it to the market.
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I agree with Gilaguy...the mechanicals are the easy part, rust is a killer...check all of the body mounts and the rear frame in the area where the rear leaf spring aft mount bolts are located. Also pay attention to the area where the transmission skid plate mounts to the frame as it's prone to severe corrosion. I've seen rust so bad there that the frame has actually cracked. Just judging from the condition of the fuel tank skid plate I'd say there's a strong possibility of significant corrosion, especially if it's been parked over dirt. So, if you plan on a body lift be prepared for broken body mount bolts or stripped captive nuts, those are a bear to fix...The 4.2 is a great motor and the 1990 models were carburated which makes them pretty reliable and easy to work on. However, expect problems if the vacuum system has been gutted. A lot of folks do what's called a nutter bypass and end up with engines that don't run right if at all. You can do a Google search on that for a description...finally, buy with your head, not your heart so that you can make a sound decision. At the end of the day though, if you and your dad have the chops to do the work the time spent wrenching together is where the reward wishes with it...
Nice YJ ! You can expect to need a complete set of metric and SAE tools on hand and there's no rhyme or reason as to which one to use.
Mostly depends on what part you’re working on. Like on the 4.0 starter, you have one bolt that’s standard and one is metric. People would assume that’s just a random thing that happened, but really it’s because one bolt threads into the starter and one threads into the bellhousing. Starter was made by Denso for Mitsubishi who supplied them to Jeep. Asian part, so the bolt thread was metric. The other hole is threaded at the bellhousing which was an American made part, so the hole there is SAE.

Engine block, all standard except for the random knock sensor hole that they added for the 1987+ Renix which used a sensor that was a metric thread. They left that hole there on the 01+ XJ and YJs that didn’t have knock sensors (because they had Chrysler fuel injection that didn’t need it) so as far as I know, that’s the only SAE engine bolt.

Tub is pretty much all SAE except maybe the torx bolts for the dash? I don’t recall their thread. Hinges, seats to floor, etc are SAE.

Intake manifold I believe is all metric.

Axles are all standard themselves. The front axle unit bearings however are metric because those bearings are a foreign part by Timken, so they are that odd 12mm coarse thread with the 12-point 13mm head. Brake bolts and such are all SAE.

The manual transmissions and tcases are weird. I’m pretty sure the bellhousing bolts (to transmission) are standard but I’m not sure I used a 9/16 on them but maybe 14 would have been correct. Not sure of thread. Fill and drain plug (transmission) are 24mm hex, not sure of the thread. The shift tower bolts I believe are metric but because the tcase shifter hangs off two of them, they have a standard thread on top of the welded nut and use standard nuts to secure the tcase shifter. The tcase shifter bolts to the side of the transmission also with a metric bolt since it’s the transmission. Bellhousing to engine bolts are all SAE because engine block. Same for flywheel and clutch. Tcase is an oddball, there is a random 10mm 12 point long bolt, I’m pretty sure the others are all metric but it’s been a while. As far as I know the studs and nuts that hold the tcase to the transmission are SAE and 9/16, but maybe the thread is metric and it would have worked with a 14mm also if I tried.

Long story short, yeah ya need all the tools lol. Maybe that helps clear it up a teensy bit though (or maybe made it more confusing). After many years of working on them you kinda start to just remember what is what. Of course, all that means is you’ve spent a shitload of time working on the Jeep over the years. Hopefully that’s a good thing.

I’m glad that nowadays, pretty much everything is 100% metric. I’m fine with understanding either of them in my head (fractions vs 10s and mm’s) but it’s really nice to only reference one set of tools.