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Lets talk about engines and swaps for an 86 MJ

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As stated and in my sig as well, I have an 86 MJ with the 2.5, 5 speed etc etc.


I already know about the hood latch, and other obvious underhood differences in the 86 year. But I'm wondering what other issues there are with swappin a 4.0 into an 86 chassis.


I'm acquiring a complete 4.0 from an 89 XJ shortly, for one hell of a deal....free. Ran good, long story short he donated the vehicle to a fab company and they didnt want the engine...


I'm also about to score on a set of rods and crank from a 258 I6, SO theres potential stroker building in the future.


Obviously all of that change makes me nervous. I shyed away from building a stroker before, but did my research and have a fair knowledge of the build.


So I guess what I would like some opinions on, are installing a 4.0 into my MJ, what wiring I should gather, or buy a spare XJ and use its harness etc? Also other things you might know of that I might take into consideration being that its an 86.



Granted this is all down the road as I currently kinda like the little 2.5, and its running well. But in my spare time, it would just be fun to build a stroker as a back burner kinda project. I just don't wanna waste the time and money if you guys think its gonna be a giant headache....minor headache would be acceptable :dunce:

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Aside from the hood latch, there are other differences. The engine bay wasn't designed for the 4.0 until '87 & up. From what I've read, fitting one in an '86 requires changing out the front clip and massaging the firewall to clear the back of the block. The motor mounts are also in different spots. You'll also need the beefier transmission to go behind the 4.0.


IMO, finding a good +'87 donor MJ/XJ would be the way to go, so you'll have all the wiring, drivetrain, and body components you'll need.

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To go from an '86 2.5L to an '89 4.0L you will need the ENTIRE underhood wiring harness, plus the ECU (which is under the dash, above the driver's right foot). You will also need the transmission for the 4.0L, since the 2.5L tranny won't bolt onto a 4.0L block and likely wouldn't stand up to the torque anyway. If the 4.0L is an automatic, you'll also need the TCU and harness.


Personally, after studying the stroker for many years, I would not build a home-brew stroker. There are too many issues attaching to controlling the compression ratio and resultant detonation.

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Thanks for the replies.


I was definately planning an entire driveline swap at the same time. ax15 and 231tc. Also was factoring in the wiring swap as well, and I agree buying a parts vehicle with a 4.0 is probably the way to go, then just transfer everything over.


I'm not disagreeing about the 4.0 fitting, but I thought I read someones build on here that stuck a 4.0 in without any modifications? Maybe he or I was wrong about the MJ year ;)




As far as building the stroker, I would be heavily depending on the machine shop to help me alone with quench height and overall static and dynamic compression ratios. But I agree, that is definately what shyed me away from building one in the past...and part of the reason behind this post. I would hate to tear my MJ apart, redo the entire thing...to end up with a pinging 4._ stroker...


Now if I was to get a donor xj/mj, I'm guessing renix era is going to be the easiest to swap over (being the 4.0 I'm getting is an 89)?

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I know I'll get in trouble for saying this, but in my opinion I think a 4.3l Chevy would be just as easy, probably easier than, a 4.0 swap, especially in an 86.


The real question here is what is your budget? The cheapest and easiest way to do it, would be to sell your Jeep, then buy one with a 4.0. If you are smart and take your time, you might even come out of that having made money.

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Ill let you know more when I'm done btu I'm doing the swap right now

90 XJ I6 into my 86 MJ taken the whole dash apart now to switch the wirring harnest and more to come tommorow


anyway if you wanna follow this ill put pice as I go in my project post




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