What do I need for a 4-cylinder to 6-cylinder engine swap?


The rifleman

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Eolia, Ky
Hi folks. I'm new to the Jeep ownership club and I was fortunate enough to buy an '87 Wrangler YJ that has a bad 2.5l engine. Yay! I was wondering if anyone could tell me if I can install a 4.0 or a 4.2 in it, and if so, what am I going to need to know ahead of time. It has a 5 spd. Tranny and l hope to be able to use it, but I'm not sure if it will bolt up to the 6 cyl's. Or not. Please help. Thanks in advance.
 

jeepjoe43

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Black Canyon City, AZ 85324
You will need an engine harness for a 4.2, and probably it's ECM.
As far as the 4.0, I think it was a much late model (TJ) and I don't know if that harness is compatible.
The 2.4 (I have one) comes with the AX5 5 speed, the six came with the AX15 5 speed.
As far as I know, it will bolt up but the AX5 won't hold up behind a six, so you will need that too, unless you're going to do a swap.
Personally, I hate my AX5, and after axle upgrades am looking to put in a SM420 truck box.
None of the the 4bangers came with autos.
When you get the motor, get the tranny that's with it, which ever way you want to go, auto or manual.
 
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machoheadgames

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You can pretty much convert to either but you will need a lot of parts to do it. The 4.2 would probably be a mess due to the old emissions parts and lack of them. And you may not need the emissions compliance, probably don't, but it may be difficult to figure out a hodge podge of parts for the 4.2 that work.

I believe most of the tub holes and such are the same on early and late YJs. I believe they built the harnesses to still use the same holes even on early models and later fuel injected models. so I think if you buy basically everything (all wiring, etc) you can probably make a 4.0 swap work relatively plug and play.

You will absolutely need a new transmission, AX15 would be my choice if I were in your shoes.
 
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machoheadgames

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Unfortunately they are not plug and play. The motor mounts are positioned differently. The old ones must be cut off and new ones welded in.
Forgot to mention that. Was distracted by something else when I was typing and submitted it. Plug and play with exception of motor mounts, both due to location and the fact that the passenger 2.5 mount uses that funky piece sandwiched between mount and frame bracket.
 
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The rifleman

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Nov 6, 2020
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Eolia, Ky
You will need an engine harness for a 4.2, and probably it's ECM.
As far as the 4.0, I think it was a much late model (TJ) and I don't know if that harness is compatible.
The 2.4 (I have one) comes with the AX5 5 speed, the six came with the AX15 5 speed.
As far as I know, it will bolt up but the AX5 won't hold up behind a six, so you will need that too, unless you're going to do a swap.
Personally, I hate my AX5, and after axle upgrades am looking to put in a SM420 truck box.
None of the the 4bangers came with autos.
When you get the motor, get the tranny that's with it, which ever way you want to go, auto or manual.
Thank you for your help. Will the ax5 hold up under normal driving conditions rather than off roading?
 
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The rifleman

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Nov 6, 2020
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Eolia, Ky
You can pretty much convert to either but you will need a lot of parts to do it. The 4.2 would probably be a mess due to the old emissions parts and lack of them. And you may not need the emissions compliance, probably don't, but it may be difficult to figure out a hodge podge of parts for the 4.2 that work.

I believe most of the tub holes and such are the same on early and late YJs. I believe they built the harnesses to still use the same holes even on early models and later fuel injected models. so I think if you buy basically everything (all wiring, etc) you can probably make a 4.0 swap work relatively plug and play.

You will absolutely need a new transmission, AX15 would be my choice if I were in your shoes.
Will the ax5 tranny work as long as I am not off-road, just every day street driving?
 

smgager

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Dec 16, 2020
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Wilmington, NC
If you want to stay with the ax5 you will be limited to the 2.5L. That said you can get a rebuilt long block for around $2k. I have lots of fun in my 2.5 low output with a mild re-gear and 31x10.5s.
 
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The rifleman

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Eolia, Ky
If you want to stay with the ax5 you will be limited to the 2.5L. That said you can get a rebuilt long block for around $2k. I have lots of fun in my 2.5 low output with a mild re-gear and 31x10.5s.
Ok. Thank you so much for your help.
The AX5 won't bolt up to a 4.0 or 4.2 6 cylinder at all. Even if it could, it would be debatable as to how long it would last. Most would say not long.
Okay and thank you for the clarification. That's a big help.
 

YJ103

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Nov 7, 2020
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Boston
You will need an engine harness for a 4.2, and probably it's ECM.
As far as the 4.0, I think it was a much late model (TJ) and I don't know if that harness is compatible.
The 2.4 (I have one) comes with the AX5 5 speed, the six came with the AX15 5 speed.
As far as I know, it will bolt up but the AX5 won't hold up behind a six, so you will need that too, unless you're going to do a swap.
Personally, I hate my AX5, and after axle upgrades am looking to put in a SM420 truck box.
None of the the 4bangers came with autos.
When you get the motor, get the tranny that's with it, which ever way you want to go, auto or manual.
Not entirely true- As much as it pains me to say it, I have a 95 4 banger with an auto. They are not easy to find, but my wife will not drive a 5 spd. But it is fun for how we use it. On the bright side, the 2.5 auto I did find was in Miami and is absolutely rust free except for a little under the hood latches. A very good find for someone from New England. Also been watching to see whats involved in a swap. Sounds like a donor vehicle with the desired drivetrain is a good option.
 
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