MJ project 2.5 -> 4.0 swap

After I SOA’d my MJ and went to 33’s and 4.56’s, the tired old 2.5L just wasn’t cutting it any more, so I decided to swap in a 4.0. Altogether, I spent about a good week of work on it, spread out over the course of a couple months of weekends between work and a few late nights after work. I don’t have a garage at my house so I ended up renting a 20X30 storage facility about a mile away from my house that worked out very well. I began preparing for the swap some time ago gathering parts and finally got going mid-August. Got it running on Saturday and drove it around a bit yesterday and the difference is amazing. Here’s somewhat of a recap with a bunch of pics of what all I had to do.

Got the facility, got the MJ in and began to tear down (most of this is in a thread I have going in the SEC now…..I wanted to get it running before posting it on the main forum):










pulled the underhood wiring harness (left the interior harness/fuseblock intact…..there was some debate over having to swap it out as well, but you don’t need to):






got a pile of leftover 2.5 parts at this point:

With the 2.5 driveline out and the engine bay empty, it was time to start building the replacement driveline…..

pulled a 4.0 MJ wiring harness out of the junkyard (and grabbed the interior harness as well, but ended up not needing it)



picked up a free Renix 4.0 from a buddy of mine and drug it back before starting



got it on a stand and began tearing it down



along with the pile of parts I had been gathering for the swap



put new freeze plugs in it and had to pull the head off…..the front exhaust stud was snapped off inside it and had to take it to the shop after I snapped off a drill bit inside of it trying to drill it out



got the head back from the shop the next day, hot-tanked and stud drilled out and refitted with a helicoil





flipped it over, put a new Felpro 1 pc pan gasket on it and installed a new RMS



flipped it back over, cleaned up the head bolts and installed a new head gasket and reinstalled the head

installed an HO A/C bracket and compressor (to be used for OBA on the next project) and an HO power steering pump



put a Felpro VC gasket on it, reinstalled the VC and the manifolds







moved on to the AX15, installed a new slave/throwout, CPS, output seal







cleaned up the dust shield, installed the new flywheel





had to switch flywheel bolts because the engine I’m using had an AW4 behind it previously and the flexplate bolts were too short for the flywheel

then the fun started with the pilot bearing…….the one in the new clutch kit didn’t fit the crank, so I tried a few different part #’s but none fit both the tranny input and the crank. Ended up having a brass bushing machined, let it sit in my freezer for a week, then tapped it in. Went in without a hitch.







got that done and went to install the new clutch kit and lo and behold, I was given the wrong kit. The part # on the box was correct, but the factory had packed an AX5 kit in the box, not an AX15 one. Took it back, pulled another one off the shelf and it was correct…..got it back to the shop and noticed the pilot bearing was different. Pulled it out and what do ya know….it was the right one! Pulled the brass bushing back out of the crank, and installed the bearing and new clutch





got that all done, hung the tranny off the engine (replaced the stupid external torx bellhousing bolts), ready to install



went back to the MJ, swapped in new 4.0 frame horns for the old 2.5 ones, ran the 4.0 wiring harness and ECU (eliminated the C101 connector in the process, too)





swapped HVAC motor/fan because the connector was different and I didn’t want to splice anything if I could help it



also swapped the heater control valve because the 2.5 is lined up differently because it sends coolant through the intake manifold on it’s way back to the block

got the new driveline put in last weekend and got it bolted up





since my harness I was using came from an auto MJ, I had to splice the NSS wiring and the backup lights



got all the wiring harness reconnected, then started with the little things, like the tranny cover plate and gas pedal, both different





took a break from the swap for a minute and did some other stuff, like installing a RE trackbar brace and recoating the floor with bedliner





also installed one of 5-90’s wiring sets. Got all that done and had fun with getting the tranny crossmember installed and getting everything lined up. I didn’t take any good pics of this, but for anyone swapping in an AX15 into their rig that wasn’t there in the first place, the tranny crossmemeber and a few pieces of the TC linkage are specific to the AX15 only. Couple trips to the junkyard and I had that all sorted out and had the whole driveline bolted in place, correctly. Started filling fluids, got the exhaust intermediate pipe hooked up, and hung the transfercase





Here’s one shot of the crossmember that shows how it’s diveted out for the AX15. If you use a regular crossmember, your TC linkage will not line up and your cat converter will be kissing the floorboard

put the battery in it last weekend and switched the key to check for fuel leaks (I had to splice in the 4.0 fuel lines to the 2.5 hard lines on the framerail). Fuel lines were fine but the pressure regulator on the end of the rail was raining gas, so I had to swap it out. Engine woudn’t turn over but I could hear the starter click so I put my yellow top on charge for a couple days. Went back and still no worky, so on a hunch I swapped out starter motors with an old greasy spare one I had, and it worked.

Put a new cat converter on it, and swapped the 2.5 fuel pump for a 4.0 one. Got fire in the hole on Saturday afternoon. Went down yesterday and hung a glasspack off the cat, threw the front DS on it (rear one is at the shop getting cut down, am picking it up today after work), and took her for a small maiden voyage around the parking lot. I still have to reinstall the front clip, airbox, and a few vacuum hoses, but other than that she is running strong:



Jeff

got the shortened rear shaft back from the shop today after work, pressed a set of UJ's into it and hung it. Will do a full test ride tomorrow....it's beer and bedtime now.

Jeff

Nice swap. I think you should have went with the external slave set up with the AX-15 but what you have should work nice. I put the AX-15 behind my 2.5L. I plan on keeping the 2.5L in mine mostly because you don't see too many 2.5L XJ's and in my opinion the 2.5L is better than the GM 2.8L that was offered in '86.

I wanted to go external, and will in the future (next plan now that this is in and running is to start building a stroker, so it'll all be coming back out again ) but currently am under both a time and budget constraint. That storage facility is costing me more in rent than I spent on all the parts I used for the swap and I need to get out by the end of this month to avoid another month's rent. I used what I had on hand for now, but the funny thing is that I was at the junkyard last weekend and came across an external slave AX15 bellhousing. I picked it up and it's now in the *new* parts pile, for the next build.....

Jeff

Jeff, awesome job!

When you mention 4.0 frame horns... the motor mount brackets? Do they simply install in the same position or was there measuring/drilling/welding involved?

thanks

The frame horns I'm talking about are the brackets that bolt to the framerails that the mounts sit on (check my fourth post, seventh pic down). The 4.0 ones are different, and attach at different spots on the framerails than the 2.5 ones. But, all the holes and one stud on each side are already there to accept either set, so no cutting/tapping/fabbing is needed....just fit them up on the rails and insert bolts/nuts in the appropriate spots. Nice thing about XJ/MJ frame horns is that they are bolt on, whereas the TJ/YJ ones are welded.

Jeff

awesome

ran it tonight with the rear shaft in place.....the thing is downright dangerous

Gonna shake it down this weekend on a small run at a local friend's place, I'll post some pics up. Need to bleed the clutch a bit more first, though. Didn't want to fubar the new clutch package, but I did spin the 33's some from a dead stopiratefla

Jeff

nice work!

Forgot to post this earlier……pic of the difference between the 2.5 and the 4.0fuel pumps (2.5 one is the smaller one on the bottom). I was going to try and see if the 2.5 pump would have enough psi to fire but went ahead and swapped everything out beforehand. I wasn’t sure it was going to start to begin with, and didn’t want to add a variable to the equation if I had to troubleshoot.

differences:



because MJ fuel pump assemblies have a tendency to crack where the metal lines pass through the mounting plate, and they are universally unavailable anymore, I decided to add a bit of JB weld to the lines on the new assembly before putting it in as a safety measure:

sweet jeep. im actually doing the same thing to mine i have a donor mj with the 4.0 i have it all out and am going to start the install. i was just wondering if you had to modify the harness at all, im sure ill figure it out just wanted to maybe see if i can save some time

no mods to the wiring harness at all, it's 100% plug and play.

Jeff