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2.5l to 4.0l swap


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i have a 86 mj with 2.5l and want to put a inline six in it. i no the tranny mounts will be moved back. How different are the engine mounts? and for the wiring isnt there companies that you can send in your stock wiring harness and they will convert it to the inline six? any help would be great

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  • 2 weeks later...
thanks for the help shows what this forum helps. good luck with ur mjs on thirty ones .

 

Was that sarcasm?

 

Let me apologize for everyone on this board who didn't want to explain the entire process again, because at least 3 threads asking this very question seem to come up every month. Many, many people have done this swap. There are probably dozens of threads on it. You need to do some searching. Check the DIY forum. Peruse through the tech forum. Going from a 2.5 is the same as going from a 2.8, or from renix 4.0 to HO 4.0.

 

Tons of people on here have 31s on their trucks. It's not a problem.

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Whoops. :oops: Not sure exactly how you fell though the cracks. I assure you it wasn't intentional. But you have to remember that there's always the possibility that only a handful of people know the answer to your question and it's your responsibility to bump it if they miss it. Please be patient, answers will come in due time. :thumbsup:

 

This might give you some insight on the wire harness:

viewtopic.php?f=2&t=16946

I've never heard of a company that can convert the 2.5 harness to a 4.0 harness. Try to get a complete donor XJ for this project. You'll need to restructure the front end to accommodate the 4.0 radiator and hood.

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Someone should write a DIY about putting a 4.0 into a 2.5/2.8. Not necessarily a step-by-step write up, but atleast cover all the problems you would run into, what parts you need and some possible solutions to the most difficult problems. I have thought about writing one myself a couple of times but it would take too long time for me. My english just isnt that good (see my "location") Best way is to jump into it, get a complete donor truck and start from scratch, by the time you are finished you will know your truck inside out :thumbsup:

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I've got a fairly extensive writeup/post in the projects forum on my 2.5/4.0 swap, but in this case most of it didn't apply to the OP, so I didn't mention it. Mine's an 87, so there were no firewall and/or front clip issues, and I had a 2.5 not a 2.8 so all the wiring on mine was just plug and play. I won't hazard a guess on the 86 wiring schematics because I simply don't know the differences in the harnesses between the years and different engines. But I did extensive research on my swap, including tracing every circuit from both the bulkhead connector to the engine, and from the engine to the ECU.....everything is the same between a 2.5 and 4.0 so long as you stay within the Renix era. Wire colors changed a bit, but the circuits are all the same (I'm running an 89 MJ underhood harness plugged into my stock 87 2.5 bulkhead/interior harness)

 

Same thing goes with swapping an HO engine into a Renix or visa versa.......I don't comment on those swap questions simply because I don't know the differences.

 

Jeff

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I've got a fairly extensive writeup/post in the projects forum on my 2.5/4.0 swap, but in this case most of it didn't apply to the OP, so I didn't mention it. Mine's an 87, so there were no firewall and/or front clip issues, and I had a 2.5 not a 2.8 so all the wiring on mine was just plug and play. I won't hazard a guess on the 86 wiring schematics because I simply don't know the differences in the harnesses between the years and different engines. But I did extensive research on my swap, including tracing every circuit from both the bulkhead connector to the engine, and from the engine to the ECU.....everything is the same between a 2.5 and 4.0 so long as you stay within the Renix era. Wire colors changed a bit, but the circuits are all the same (I'm running an 89 MJ underhood harness plugged into my stock 87 2.5 bulkhead/interior harness)

 

Same thing goes with swapping an HO engine into a Renix or visa versa.......I don't comment on those swap questions simply because I don't know the differences.

 

Jeff

 

 

OP says he has a 2.5 so, in theory, most of his truck's wiring "should" be the same as in 87. Can't say for absolute sure though. He'll still have to deal with the 86 front clip.

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I've got a pinout of the 84-86 bulkhead connector, and it's quite different circuitry from the 87-90 one. I would comfortably say you'd have to swap full harnesses (underhood and interior) for "everything" to function correctly. If you would be looking to just getting the engine to fire and run, you could probably splice a few wires together from the existing harness.

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I dunno :dunno:

 

all of my research was done on 87+

 

circuit-wise, there's only a few that pass thru the BH connector to the fuse panel and interior harness that really affect drivetrain management (starter circuit, fuel pump, etc). Most everything else is accessory wiring. Most all of the engine management is handled by the tail end of the underhood harness (which connects to the front half of the harness with multiple weatherpack connectors right around the relay center area on the passenger side fenderwell), which connects the various sensors to the ECM.

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I dunno :dunno:

 

all of my research was done on 87+

 

circuit-wise, there's only a few that pass thru the BH connector to the fuse panel and interior harness that really affect drivetrain management (starter circuit, fuel pump, etc). Most everything else is accessory wiring. Most all of the engine management is handled by the tail end of the underhood harness (which connects to the front half of the harness with multiple weatherpack connectors right around the relay center area on the passenger side fenderwell), which connects the various sensors to the ECM.

 

do you have a project thread or any write up on the swap? I am considering it.

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  • 4 weeks later...

How is it not running well? What is it doing/ acting like? Could be something as simple as an O2 sensor. When I got my 87 2.5, it ran like crap too. I put through I don't know how many cans of fuel system cleaner, a tune up, etc, with only minor gains. I finnally decided to slap an O2 sensor in it just for kicks, and Oh what a difference! Its been running great since. Let us know whats going on and maybe we can help you not have the expense and time of doing the swap...unless you really want to!

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Someone should write a DIY about putting a 4.0 into a 2.5/2.8. quote]

 

I've done it before, but I'll do it again for you.

 

1. Buy a 4.0L MJ

2. Swap any desirable parts from the 2.5/2.8 onto your new MJ

3. Sell the 2.5/2.8

 

If you do it right, you could actually make money by doing that! You'll be done in a weekend too.

 

MJ's are to cheap to consider a 4.0 swap. If you do your research on the time, money, and complexity of the swap, you'll see my 3-step plan is the way to go! :thumbsup:

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