Engine Swap Old 4.2 Liter with a Newer 4.0 Liter

Scramlr

Scrambler Rookie
LIFETIME
CJ-8.com Member
City
Arlington
State
TX
#1
Has anyone ever did an engine swap from the old 4.2 liter (1983) to a newer 4.0 liter engine?
If so will it bolt right up to my existing bell housing and T-5 tranny?
 

Kane

CJ-8 Member
CJ-8.com Member
City
Berlin
State
ct
#3
It will bolt up but you need to run the 4.0 flywheel, starter and cut a hole in the bellhousing for the position sensor.
 

Scramlr

Scrambler Rookie
LIFETIME
CJ-8.com Member
City
Arlington
State
TX
#5
Any issues with motor mounts, computer?
Will gauges work?
Any other obstacles?
 

bigwalton

Picture cravin' AK Postal nut
Staff member
SOA Member
City
Dexter
State
MI
#6
Thousands of people have. I have (not using a T5) and there's an immense amount of information out there (including on here).

I'm probably going to come off sounding like an #^$, but it's the voice of experience: You need to do a crap ton of research and reading.

Posting this kind of thread is only really going to let you find out that people have done it successfully. In order to even try to get started on your own swap, you'll need to do many, many hours of searching and reading. Everything is out there for you to find, though, that's the good news.

This is a very involved swap to do "right" and there's a *lot* that is going to depend on the year/model of your donor.

So I'm not trying to be difficult, but the kinds of questions you're asking don't have simple "social media" kinds of answers and you'll need to do the legwork yourself to figure it out more than a "yes" or "no".

There are multiple 4.0L swap threads on here (mine included) that would be a good place to start. The issue is that the details you need will depend on where your donor motor comes from. These swaps are ALL ABOUT the little details that it takes to actually make everything work once you mount the motor in the Jeep. The physical swap of getting a 4.0L into your Jeep is simple, getting everything sorted out from there is the PITA.

It can all be done, it's been done countless times before, but getting yourself educated on what is needed by searching and reading is the only way you'll get enough knowledge to make it work if you're going DIY. Start there and then we're here to help with specific questions.
 

tower210

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Olathe
State
KS
#7
I did this last winter. Big Walton is right; either do your research and then buy the engine or buy the engine and do the research.

Good news is that every part on a 4.0 is dirt cheap and available at every blue, green or red auto parts store. Pretty sure I replaced every sensor on the engine and did a new distributor, plugs and wires or less than $200.

The harness is key. I bought HESCO, which was awesome (but pricey); but glad I did.

LOVE it now that its done. Cold starts are easy.

Do note, that it makes torque much higher in the RPM band; so it might make you want to swap gears.....
 

bigwalton

Picture cravin' AK Postal nut
Staff member
SOA Member
City
Dexter
State
MI
#8
The harness tower mentions is a perfect example. I used the donor harness and thinned it down the way I wanted to keep the power distribution center for powering other things in the Jeep which was basically free in that it came with the donor setup. Or you go the standalone HESCO or similar route to do a “swap harness” and spend $$$. Both work (assuming you DIY the wiring properly) but are vastly different approaches.

DIY wiring requires digging into the FSM electrical diagrams for the specific years of donor and CJ that takes research.
 

gr8dain

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Ashburn
State
VA
#13
I am about to do this. My old and tired 4.2 is coming out and I purchased a 4.0 from a 1997 XJ that comes with an aw4 and a transfer case that I won't be using. I am going to use the dana 300 with the 4 to 1 ratio and the twin stick that I installed right before I had issues with the motor.

Now to go find Big Walton's build thread. I need to start finding parts like the auto trans control module, the euc and the harness.
 

cdvcj8

It's Not My Scrambler
LIFETIME
CJ-8.com Member
City
West Friendship
State
MD
#14
I’ve been looking at swaps/upgrades myself. Couldn’t you use the 4.2 intake manifold on the 4.0 motor and keep the Holley?? Shouldn’t have many wiring headaches.
 

gr8dain

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Ashburn
State
VA
#15
I’ve been looking at swaps/upgrades myself. Couldn’t you use the 4.2 intake manifold on the 4.0 motor and keep the Holley?? Shouldn’t have many wiring headaches.
The 4.0 is already fuel injected. So honestly I don’t know what would be needed to use the old offy intake on it. I will discuss that with the smarter folks helping me out.
 
City
White Rock
State
NM
#16
I recently replaced my 258 with a 4.0 in front of a 904/999 auto using the HESCO EFI kit on my 258. Worked out fine using the HESCO CPS mount on the front of the engine but IDK if you need that for the AW4 or not. Ive been pretty disappointed with my results, the pinging is gone but I had vapor lock issues when it was hot and its got less zip than the 258 hybrid driving around, at least here above 6000 ft. I wish I spent the extra money on a v8 swap for my situation. If you were closer Id make you a good deal on the whole set up.
 

bigwalton

Picture cravin' AK Postal nut
Staff member
SOA Member
City
Dexter
State
MI
#17
I recently replaced my 258 with a 4.0 in front of a 904/999 auto using the HESCO EFI kit on my 258. Worked out fine using the HESCO CPS mount on the front of the engine but IDK if you need that for the AW4 or not. Ive been pretty disappointed with my results, the pinging is gone but I had vapor lock issues when it was hot and its got less zip than the 258 hybrid driving around, at least here above 6000 ft. I wish I spent the extra money on a v8 swap for my situation. If you were closer Id make you a good deal on the whole set up.
But the vapor lock isn’t a 4.0 issue but some sort of install problem, right? The 4.0 doesn’t typically have vapor lock. Are you positive you have proper fuel pressure?

While you may need to be on the gas more than before because of the difference in torque and power bands, the idea that it has less zip seems like something isn’t right. I can’t imagine that with my before/after experience.

I could understand wishing for even more power with a v8 swap, but something doesn’t seem right to me here?
 

ag4ever

Average Nut
LIFETIME
CJ-8.com Member
City
Richmond
State
TX
#18
City
White Rock
State
NM
#19
True its not unique to the 4.0 but it was much more sensitive. Yes fuel pressure is in spec I've been down this road with the hybrid just not as frequently. Biggest complaint is the relative lack of power even relative to the hybrid. And maybe that's the jist of it going from a 258 4.0 hybrid to a stock 4.0. Maybe my hybrid had more than I gave it credit for. It is nice to run regular gas and still not ping. The hybrid pinged on premium
 
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