Page updated: 07/06/2001 08:11 PM

The following article details a conversion performed on my ’94 4cyl. YJ during the summer of ’97.  The donor vehicle was a ’92 YJ with the 4.0L HO motor and AX15.  All of the notes will be valid for ’92 - ’95 YJs, and most of the content is applicable down through ’87 and up through the TJs.  It is meant to provide a good explanation of what to expect, but falls short of being a step by step guide.  This was my first engine swap at the time so I’ll try to write it such that most howler monkeys can at least be prepared to tackle it – if I can do it, you prolly can.  I’m going to also assume you have a repair manual and experience of some kind for the detail work.  I’ll apologize for the lack of good pictures.  If you prefer, there is also a quick and dirty "things to consider" at the bottom of this article that should be reviewed if you are serious about the swap anyway.

The story begins in the city of Lexington where I was looking for power improvements for my Jeep that would allow me to use 5^th gear again.  Eventually, I decided it just “needed” a new motor.  The best deal I could find was in Montreal for an entire YJ drive train from radiator to lug nuts.  So my wife and I hooked a trailer to the back of the Jeep and set out to stay the week 1000 miles from home in French speaking Quebec.  We stayed with the guy selling the parts (Paul and I at left next to his YJ7) and his girlfriend in their apartment and worked out of the common area garage where I became known to his neighbors as the Garage Troll.  That, however, is a different story in itself.
 

First step was to remove the old drive train.  This is much easier with the grill removed, and we went ahead and removed the fenders and hood for better access since we planned to drop everything in as a unit.
 

If you are tight on space, you can stack all the panels in the Jeep… we only had space during the day when the neighbors weren’t home.

Note: You can’t reuse the AX5 tranny.  All jeep AX15 transmissions have a 23 spline output shaft, and AX5s have a 21 spline output shaft.  Which means you either need to buy a 23 spline input gear for your NP231 t-case, or buy a used 23 spline NP231 and sell yours to offset the cost.  Since my drive train was complete, this was not a concern. 

Taking the motor out isn’t as hard as it may sound.  The only special tools you’ll need besides the usual tools and jacks are an engine hoist (which can be rented) and maybe another howler monkey.  Remove all the hoses and ground straps and such, but you may keep the block wiring and its sensors attached to the block as these won’t be reused – disconnect it from the firewall.  Set the power steering reservoir to the side and remove the tranny shifter at its base (push and turn the retainer plate) – stuff a rag in the hole.  Of course, you’ll need to remove the skid plate and driveshafts. Secure the hoist chain directly to studs or bolts on the front and rear of the head or block.  Be sure not to hook to any of those aluminum brackets!  Because you’ll have to replace the tranny anyway, it’s a little less work to remove the whole pig (motor, tranny and t-case) as an assembled unit. Just take it slow and check your manual for more details.

This is when you ask yourself “What ARE you smoking??”

Note: Pay attention to your donor engine/tranny if they are from different vehicles as there are slight changes though the years.  I know at least that the pilot bushing changed sizes in ’92.  There is an adapter bushing that lightly presses into the crank, so in a worst case you may have to take some measurements and have a bushing machined.  Any machine shop should be able to do this for cheap.The master & slave cylinders changed connections in at least ’93, prolly other years as well.  Additionally, the slave cylinders changed to external in ’94 so the throwout bearing is different and requires a shift fork.

The frame will need to be prepared to accept the 4.0L.  The motor mount brackets are not in the same location, so cutting and welding will be required.  The old brackets cannot be easily moved either – they sit on a portion of the frame that is not level.

Mounts could be fabricated, but the easiest thing to do is buy new ones from a decent dealership.  We could only buy one side and had to cut the other mount from the donor vehicle.  Welding these brackets was the first real-world welding I had ever done… sheesh, no pressure… we only had to drive it 1000 miles back home.
 

Determining the location for the new brackets is not particularly easy, but probably only needs to be accurate to within ±¹/₈ ” or so.  They are mounted on a curve in the frame and there’s no obvious flat surface to measure from.  For that reason, I can’t give a concrete number, but I measured from the shock towers as they are in the same location through all YJs.  Just use a consistent measurement technique, and quadruple check it!

Note:  If your skid plate does not have the slotted holes for the transmission mount (early YJs?) then be prepared to drill new holes after everything is mounted up.

Dropping in the new drivetrain…

If you don’t feel comfortable moving the whole drivetrain on the hoist (or there’s not enough room) then mount the motor first, then the tranny and t-case.
 

If you are dropping in the whole pig, it’ll sit at kind of a steep angle on the hoist.   The motor is very long and will use up every bit of the length of the engine bay once the fan is on.  Don’t be concerned when the rear of the valve cover is very close to the firewall.
 

I had to lift the rear of the drivetrain while Paul guided it in and operated the lift.  It’s not too bad, but removing the bumper and winch would have helped a lot. 

Finally, we all gathered around and marveled at a job half done.  Little did we know, there was more left than we suspected… 

Once the motor is installed we started attaching it to its new host.  The driveshafts are a different length.  The rear needs to be about 1” shorter and the front 1” longer.  You could have them retubed for around $60 each or get them used.

Note:  The pinion yoke on the D35c changed to a larger size in ’94.  We didn’t know this until the whole damn thing was almost complete, but luckily we had a donor yoke and replaced the one on my axle.  First time I’d done that too!

The exhaust from the cat back will be reusable, but the pipe from the header to the cat is different on the 4.0L. 

I’m not sure about the O₂ sensor or coolant temperature sensor, but I don’t think any of the other sensors will transfer over.  Certainly, the engine wiring harness and computer need to be changed.  It’s possible that you’ll need to change the under dash wiring harness.  According to Paul’s Chrysler buddy, it has a different part number, but Al and I couldn’t find any difference in the factory service manual.  I went ahead and changed mine out since it was available, and I thought it would look cool to have all my gauges in SI units (geek).

The 4cyl radiator probably won’t be able to handle the extra heat generated by the 4.0L, but I didn’t test it out.  I changed mine out with a regular ol’ CJ rad and the motor stays cool even now that I’ve stroked it out to 4.6L (another thing to write about later J).

If you have the 2.5LMPI (’91 and up?) then the existing fuel pump and tank are the same, but now is a good opportunity to do the gas tank conversion for an extra 5 gallons or so.  If you have an older 4cyl that’s throttle body injected or carb'd, then you’ll need to work out a fuel pump that delivers high enough pressure or drop in the tank and pump from the donor.

There are some other little things we didn’t discover until we thought we were just about done and finishing up.  The radiator hoses are a different length (duh).  The intake tube leading to the air box is different.  The quick disconnect fuel pressure and return lines leading to the rail (2.5MPI) are too short – either buy new 4.0L ones ($$!!) or splice in some high pressure fuel line. 

Any engine swap has some surprises - even when it's just a replacement.  This should help you eliminate most of them, but be prepared for a lot of work and more time than you expected.  Even as beginners we did the entire job in four days and nights, but sleep was not something we could even spell by the time we were done.  It’s reasonable to assume completion within a week’s time if you prepare things in advance.

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The quick list...

1. Can't use the 4cyl's AX5 tranny - it won't bolt up and even if you could find an adapter, it wouldn't last.

2. If you get a stock AX15 to use, the input gear for the NP231 t-case is 21 spline for the AX5 and 23 spline for the AX15. This gear can be swapped out in around 4 hours. When I did my swap, I changed the whole drivetrain and dropped it in as a whole asm. so I'm not sure if the t-case linkage is the same but it looks very similar. I know the 4.0L to AX15 pilot bushing changed in '92.

3. You need the wiring harness and computer with the 4.0L. Make sure it includes *everything* like MAP sensor, etc.  I was told I also needed a new under-dash harness (different year motors).  I'm not sure that's true, but I did change mine anyway. Swapping the harness was pretty easy - all the plugs are made to mate correctly.  The service manual for '94 makes no distinction between the 4.0L and 2.5L underdash wiring of the same year.  I think the only thing you'll be able to use from the 4cyl is the alternator.   I used the longer 4cyl intake tube to attach a K&N cone and rest it on the fender.

4. The motor mount brackets are in a different location on the frame.  Cut and weld required... Try to get them new - we cut one off an old frame and that was just too much work.  I don't think you could cut and reweld the old ones.  They sit at a different elevation on the frame so they're different lengths.  There's no distinct place to measure on the frame, but the shock towers on the 6cyl and 4cyl are in the same location so you can measure from there.  I think I took this measurement about 30 times before I felt good enough to start welding.  It's a looong motor so it sits really close to the firewall...

5. Drive shafts will be a different length - modify your old ones or get some for the 6cyl.  The rear will be about an inch shorter, and the front about an inch longer.  I think I still have one for the 6cyl left over from when I went to a CV driveshaft.  BTW, the yoke and u-joint size for the rear axle changed in '94.

6. Probably want to change the radiator, the 4cyl rad is pretty thin for a big 4.0L - I put a CJ rad in mine and have nothing close to a cooling problem.  You might make the original rad work depending on how hot it gets in your region... judgment call.

7. Exhaust is different between the motor and the cat, but the same in the rear section.

8. If you don't have the MPI 2.5L, you'll need a new sending unit/fuel pump.  If you do have the MPI 2.5L, now is a good time to do the gas tank conversion for an extra 5 gallons.  Also, the high pressure and return fuel lines under the hood (with the quick disconnects) on the 2.5L are not long enough to reach the 4.0L.  Ordinary high pressure fuel lines are fine to splice in here (that's what I did).