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School me on 4 cyl to 6 cyl swap

OkieXJ

NAXJA Forum User
NAXJA Member
Location
Muskogee, OK
Housekeeping first:
Vehicle: 87 shortbed MJ 2wd, 2.5L throttlebody FI, 4 speed manual (AX4?).
Drivetrain I'm think about swapping in: '90 4.0L, 5 speed manual (AX5?), NP 231 t-case.

Basically, what I want to know is what do I need to watch out for because I've done my fair share of wrenching but never an engine swap. This thread is also supposed to help me decide whether or not to buy this drivetrain.

-I assume I'll need the computer with the engine and all the sensors.
-Will a 6 cylinder bolt in or does the body need to be modified?
-Will the transfer case bold on to the current cross member or will that have to be replaced or modified?
-Obviously I'll need a new driveshaft do to longer drivetrain, hopefully I can find one at a JY.

I did a google search, and NAXJA search but didn't come up with anything even though I'm sure this has been covered a hundred times. So, feel free to point me to a relevant thread. Any help is appreciated.
 
With the 87, there should be no big issues with the hood and firewall. You will need a radiator, computer, wiring harness, with dash harness, and a matching tranny. If you have those things, it is pretty much a plug and play deal. I did it on an 85 and did all the sheetmetal. If you can get the dash harness along with the engine harness, it really simplifies life.
 
I didn't know about the dash harness and I forgot about the radiator. The radiator shouldn't be too big a deal though. Besides gauges, why is a new dash harness required?

The main thing I am concerned about is having to modify the body or frame. If I have to do very much of that, I'll just go find a replacement 4cyl.
 
The connection through the firewall into the fuse box is different between engines. The easy way to do it is to grab the dash harness instead of trying to figure out the wires and splice the harnes. It is easy to pull the dash, then simply swap over the harness. That part only took me about an hour, but I have done it a few times.
 
So just to confirm, the engine mounts are the same for a 4cyl and 6cyl?

The dash harness makes sense. Thanks for filling me in.
 
might be easier to do a V8 swap, especially with the brackets and such that novak sells now. If you hunt aroud a little you could get a newer GM truck motor and tranny for not much.

Just a thought

edit: I had an '89 MJ, shortbed, 2wd about a year ago. I was looking pretty heavily into a LS1 or other GM LSx V8 swap. It was not going to be much more than an inline 6 swap.
 
The mounting brackets for a GM V8 from Novak are $210.
http://www.novak-adapt.com/catalog/engine_mounts/mmxj1.htm
According to craigslist, everyone here in OK wants at least $500 for a running V8.

I have a line on a '90 YJ drivetrain for $350 that includes engine, tranny and TC. It would be a lot cheaper to swap in a 4.0 drivetrain if the mounts are the same.

Though that does bring up another thought. Are '90 YJ 6 cylinders 4.0L or 4.2L? And what is the downside of swapping in a 4.2L?
 
I think the drivetrain might be an H.O. 4.0 afterall. The seller says the build date on the Jeep is 9 of 90, so it could be a '91 model and therefore an H.O. I'll have to figure out how to tell for sure. Are there any other obvious signs besides Renix having a closed coolant system and H.O. having an open system?
 
I think the drivetrain might be an H.O. 4.0 afterall. The seller says the build date on the Jeep is 9 of 90, so it could be a '91 model and therefore an H.O. I'll have to figure out how to tell for sure. Are there any other obvious signs besides Renix having a closed coolant system and H.O. having an open system?

just google what the renix throttle body looks like and the HO ones. pretty different and easy to see.
 
Did this swap on a friend of mines MJ.

What we started with was a 1990 MJ 2.5 2wd 4-speed manual trans and we ended up with a 1990 MJ 4.0 4wd AW4 with a 242 t-case.

We had the donor vehicle on sight so it went really smooth. I don't recall changing the dash harness but its been 4-5 years since the swap was done. I only recall having one connector that didn't plug into the other and it was a 3 wire connector under the dash and was part of the engine harness. I think it was the 3 wires that hook to the engine module from the interior. We did have to tap new cross member holes for the longer trans, the holes were already there but they had no threads.

Total time was about 3 weeks of random weeknights and weekends. It was on the road for 4 days before he made a 2 hour drive in it and wheeled it. Its like a child we all raised, he would have to check with all that built it before he could get rid of it LOL.

I don't think it would be so simple if we didn't have the donor on sight that really made it easy.
 
Having a complete donor will make life much simpler. Especially if its an HO, you will definitely need the dash harness. You may also have to change the radiator core support, as the radiators are much different, That would involve some welding and drilling of spot welds to swap the correct support over. Everything else should be bolt up and plug and play.
 
Having a complete donor will make life much simpler. Especially if its an HO, you will definitely need the dash harness. You may also have to change the radiator core support, as the radiators are much different, That would involve some welding and drilling of spot welds to swap the correct support over. Everything else should be bolt up and plug and play.

When we swapped the core supports there was no welding or drilling involved...all bolt in?
 
When we swapped the core supports there was no welding or drilling involved...all bolt in?

I have a complete upper core suport laying in my driveway. I was bolted in.

You may not have the nutserts in the body to move the trans crossmember where it needs to be, so look up how to put those in without the special tool. It's been covered here before.
 
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