Jerry's 2.5 to 4.0 TJ Engine Swap

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Jerry's 2.5 to 4.0 TJ Engine Swap

Postby Jerry Blair » Mon Jul 19, 2010 9:54 pm

Add 60 HP and 95 ft/lbs TQ

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My 2001 TJ is well equipped with a Currie D44 high-pinion front end, Currie HP9 rear end, 4.56 gears, ARB lockers, armor, etc. It also had the 2.5 4-cylinder engine with less than 80,000 miles. It runs great and has never let me down but it just doesn’t meet my needs any more. I want to be able to haul my family (me, my wife and two kids) along with a small trailer full of camping gear and bicycles all over the US in search of family adventure. I could do that with the 2.5, but from experience, I know the 4.0 would be much better suited.

I could have had a V8!
Every expert on the web says, “It is as much work and money to swap the 4.0 as it is a V8. You might as well swap the V8.” Really? How many of those folks have actually swapped both into a TJ? I don’t know any of them personally so I don’t know. I know the 5.0 and AOD I swapped into my YJ required WAY more work than this swap. Why? The only welding required for the 4.0 swap is engine perches on the frame. Everything else is simply parts swapping. No adapters or radiator (4.0 and 2.5 use same radiator) to buy, no engine or transmission mounts to fab (I actually reused the 2.5 mounts), no wiring to splice, no fuel pump issues (2.5 and 4.0 use the same pump), no exhaust to fabricate, no gauge issues, etc. Simply cut off the old engine perches and weld-on the new ones, bolt up the engine, plug in the wiring and go. HOW could it be easier or cheaper?

As I mentioned, I had previously swapped a Ford 5.0 and AOD into my 1995 YJ and considered the 5.0 for my TJ. It is a great engine but the engine bay was TIGHT. Everything fit, but maintenance became a pain. Not any worse than most new cars but it sure took the fun out of tinkering. Yes it ran well but the automatic took a lot of the fun out of driving. I don’t know what the horsepower was for the 5.0 but I spent a lot on engine work and performance parts. I swapped in 1 tons at the same time and honestly wasn’t impressed enough by the power difference to call it a success. The little 2.5 in the TJ had 120HP and 140 ft/lbs TQ. My new-to-me 2002 4.0 is 190HP and 235 ft/lbs TQ.

The appeal for me with this TJ is simplicity and ease of service. I went WAY overboard modifying my YJ and I wanted the TJ to adhere to KISS principles (Keep It Simple Stupid!). I know it is easier to just buy a TJ with the 4.0 but I got a great deal on this Jeep. All the suspension and axle work was done and it had everything I wanted except the 4.0 and AC. I’ll address AC later.

I spent well over $4K on the 5.0 engine swap not including the Atlas II transfer case and I wanted to keep this swap as inexpensive as possible. I’ll keep a running total to let you know exactly what you might expect if you are willing to spend a little time tracking down parts.

What’s Involved?
The key to making this swap as EASY as possible is: GET ALL YOUR PARTS FROM ONE DONOR VEHICLE! I can’t stress this enough. My new 4.0, the NV3550, wiring harness and computer all came from a 2002 TJ with approximately 60,000 miles and cost me $1000 from a local jeeper that swapped a V8 in to his TJ buggy. As luck would have it, though, a lot of folks know I have various parts available from time to time and one contacted me to see if I had a 2.5 4 cylinder available. Mine only had 80,000 miles and he offered me $1000 for it! Seemed like to perfect time to start my swap!

My 2001 TJ and the donor TJ are from the same model run. This makes it MUCH easier than swapping parts from XJs, ZJs, YJs, etc. This swap is very difficult or nearly impossible if you start trying to make a 91 XJ engine fit a TJ. There are simply too many problems/changes to address. Read the links to other 2.5 to 4.0 swaps if you doubt me. If you plan to do this swap, I’ll stress again how important it is to keep the model and year as close as possible.

I used the two links below for reference. Both are good. I also used a FSM from 2001 and 2002 TJs to double-check my thoughts. ... php?t=1632 ... ay-576008/

Frame and Engine Placement.
The 4 and 6 cylinder frames are different. Specifically the engine perches are in different places. Not a big deal and not worth the effort to swap frames. Simply purchase the frame mounts from Jeep or build new ones. Originally I had intended to buy the new mounts from Jeep but after pricing them (just over $100 for the set of 4 pieces) I decided to make my own. I know everyone claims the original frame mounts cannot be used, but I found they worked just fine. It does require a little trimming to fit, but not difficult if you have a bench vise, a reciprocating saw, 4 ½” angle grinder and a cut-off wheel. You’ll also need a welder. I used my 220V Miller 175.

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I removed the front grill to make pulling out the 2.5 and transmission easier. Removing the front fenders would have been even better because I’d have had more room to work. It isn’t necessary to remove the fenders, but I’d recommend it. Of course I didn’t do it.

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Next I used the reciprocating saw with a 6 inch, 18 tpi blade to trim off the old frame mounts. I used the angle grinder to clean up the frame. I didn’t grind any deeper than seen in the pics. The next hurdle was fitting the new engine into place.

My new 4.0 came pretty complete, but it was missing the transmission mounting plate that bolts between the transmission and the trans mount.

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The mounting plate from the old AX5 is similar to the NV3550 but not exact. I needed to drill new mounting holes and add a ¼ inch spacer to make it fit properly. You could avoid this all together by making sure your donor has the mounting plate. In the picture above you can see holes used for the old AX-5 transmission.

Next I loosely bolted up the transmission to the skidplate. Be sure to slide the transmission as far back as it will go to give you room for your fan. Don’t tighten the mount to the skid yet as you want to be able to move the engine. Then position the engine side to side in the chassis. I removed the exhaust y-pipe to give me more room to work and make things easier to measure. My engine is approximately 2 inches off center toward the passenger side and about an inch higher than stock (think MM lift). If you are using the stock mechanical fan, now is a good time to put the grill back on and position the engine so that the fan is squarely in the shroud and that your engine is positioned far enough rearward. Even with my electric fan, I put the grill back on at this point to be sure I wouldn’t run into trouble later.

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Finally marked for final trimming.
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Once the engine is in position, tighten down the transmission mount and recheck clearance around the engine and that your fan is centered in the shroud. Bolt your engine motor mounts (stock or aftermarket) onto the engine and tighten them down to keep the bottom parallel with the ground. I used some light cardboard to make templates for the rough cuts on the stock frame mounts. The 4 cylinder frame mount is farther rearward before the frame bends upward for the axle so they are taller than the 6 cylinder mounts. This gives you plenty of material to trim to make them work on your 4.0. The pictures above show the process on the passenger mount. This step really went quickly and easily. After the engine was properly positioned, it probably took only 40 minutes to trim the mounts until they fit well enough to tack-weld in place. Now would be a good time to triple-check engine position to see if you are ready to burn in the mounts.
Last edited by Jerry Blair on Thu Feb 27, 2014 9:30 pm, edited 4 times in total.

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Postby Jerry Blair » Mon Jul 19, 2010 10:03 pm

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Now carefully remove the grill/engine/transmission and fully weld the frame mounts. The passenger side of the frame has plenty of room to work. The driver side is a little more difficult due to the brake lines and the steering shaft. If you unbolt the steering shaft you will have more room to work. I had heard that it is necessary to move the steering pillow block for this installation, but I didn’t. Maybe when you are using the factory 4.0 frame mounts you do. I suspect, also, that if you are using the 4.0 mounts, a lot of the engine positioning will be taken care of for you.

Frame and welding….finished.

Did I need to swap ALL the under hood wiring or just the engine harness? None of the write-ups I read online were clear on that. There are three harnesses under the hood: the engine harness, the power distribution harness and the front grill harness. My research through the factory service manuals indicated that it should be as simple as swapping the engine harness and the computer.

I needed to check the Pin Out diagrams for C103 Black Connector and C104 Gray Connector for the 4 cylinder vs. the 6 cylinder. These are the connectors in the center of the firewall above the engine that connect the engine harness to the power distribution center harness. The C103 connector is identical on 4 and 6 cylinder and across model year 2001 to 2002. On C104 it appears the differences between 4 and 6 are that pin 2 uses a DB/WT wire on the 4 cyl and a DB/OR on the 6. Also, Pin 14 is empty on the engine side of the 4 cyl, but contains a VT/OR wire on my 6 cylinder (this might be CA emissions specific. It applies to the downstream 02 sensor heater relay). I have a P0161 code after the swap. I haven’t checked to see if the 02 sensor is actually bad or if this is a wiring connector issue. (As of this writing, I’ve only been finished a week.) I also found the C104 dash side pin 2 has 20 gauge wire for 2001 and 18 gauge wire for 2002 color is same. I don’t expect this to be an issue.

What does this gobbledygook mean to you? It means plug in the engine harness and the computer and go! Don’t sweat the wiring.

The previous owner of my new 4.0 did me few favors in selling me a “wiring harness”. As mentioned earlier he took this out of his TJ buggy. He sold me the power distribution center harness and the engine harness. For some reason though, he had cut the plug to the headlights out of the harness and he had cut the fuel pump wires out of the engine harness. This caused a little hassle, nothing major but still, a distraction.

The other wiring issue is hooking up the alternator and starter feeds and grounds from the power distribution center. The 4-cylinder battery cables were a little tight in the 4.0, but I was able to make them work. There is a little stretch from the engine grounds and the alternator feed but well within useable limits.

Wrapping it up.

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The entire engine/transfer case is approximately 1 inch rearward in the chassis. This did not cause any difficulty during my swap though. With my HP D44 front axle I once again reused the stock TJ driveshaft. I also have a HP axle in the back along with a SYE and the driveshaft I have been using for years also worked for me.

My donor did not have the bracket for the transfer case that attaches to the shifter. I was able to modify the 4 cylinder bracket by drilling a hole. After this pic was taken I removed the bracket again and installed the green bushing. This also required a modification to the tcase shift rod. I cut it and lengthened it about 2 inches.

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Here is a list of things I used for the swap and what they cost me:

*2002 TJ 4.0 Engine with accessories, NV3550 transmission, computer & full underhood harness (The headlight portion of this harness is cut as well as the hood light. This is ok as I only need the engine portion. I later learned the fuel pump portion was cut as well), exhaust Y-pipe with cats and 02 sensors, EVAP canister and hoses. $1000

*23 spline input shaft for the t/c $40 from an NP231 with a cracked case I purchased. I also bought a tube of Permatex anaerobic sealer to reseal the case. This is the best I’ve found for preventing leaks and avoiding large globs of sealer in your tcase. $17

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*Flexalite #485 TJ Fan (I bought this for another project and didn’t use it) $300

*2002 TJ fuel rail (the previous owner used my engine with a YJ rail instead of the TJ fuel rail) $100 used with 6 extra injectors

*Fuel lines for 4 and 6 cylinders are slightly different. I made the 2.5 line work by using my brakeline bending tool to straighten the last bend in the line (at the frame) from 90degrees to about 45 degrees. Works great. [pic]

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*Power steering lines can be reused. Just cut the return line (low pressure) to the new length and use a bench vise and a length of pipe to bend the high pressure line to clear the resevoir.

*Radiator hoses. I was able to cut about 9 inches out of the upper radiator hose and make it work with the swap. The lower hose required purchasing a new 4.0 hose. $20

*Heater hoses are not the same between 4 and 6 cylinders. My choices were to buy new ones or splice in a short section of copper tubing. Since I had copper tubing at the house and I’m inherently cheap, I did the splice. It took about 10 inches of tubing. $3

Total $1480.00

Credits: Items I sold from my 4cyl:

4 Cylinder Engine ($1000)
Ax-5 Transmission sold locally on craigslist ($400)
21-spline input for NP231 on ebay ($50)
I kept lots of the accessories as spares: power steering pump, alternator doesn’t fit, AC Compressor, etc.

Total Credit ($1450.00)

In order to complete the swap you will need a few special tools in addition to what is found in most tool boxes.

Engine hoist
Fuel line tool[pic] to release the line from the fuel rail
Bent 15mm box wrench to make it easier to reach the transfer case bolts.
Snap ring pliers to disassemble transfer case (you will also need a replacement seal and slinger if you do not already have a SYE and are not planning on installing one)
Large channel locks make radiator hoses easy to remove

So, am I happy with it?


After driving the Jeep with the swap for nearly a month, I’d rate the swap a success. I replaced the bad O2 sensor and that solved the check engine issue. I actually reused the one from my TJ located behind the catalytic converter under the Jeep. It wasn’t used in this swap. I put the bad one back in that hole to avoid an exhaust leak.

The 4 cylinder always got me where I was going. The difference now is that I rarely need to shift gears. The extra torque makes driving effortless.

Is it a racecar? Certainly not, but it is considerably more fun to drive. Especially with a total cost of 3 days work and only $30 cash out of pocket.

On a difficulty scale of 1-10, I’d rate this swap a 6 if you know how to use the special tools required. I’d rate it an 8 if you don’t.

My conclusion?

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It is absolutely not “just as easy” to do a V8 swap. “Difficulty of labor”-wise, it might be close if you buy a kit from AEV, Advance Adapter or Burnsville Offroad, but the cost would be much, much higher. If you already have your 4 Cylinder TJ set up like you want it or you are just attached to the thing, go ahead and do the swap. It IS worth it.
Last edited by Jerry Blair on Thu Feb 27, 2014 10:09 pm, edited 3 times in total.

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Postby GreenJPTJ » Tue Jul 20, 2010 8:53 pm

I guess this is the right thread to reply to. When are we going to see you on the trail?

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Postby Jerry Blair » Tue Jul 20, 2010 10:38 pm

I had hoped to make this month's ride, then realized I had a prior commitment.

Jeep is ready. Need to make the time! :biggrin:

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Postby MY87redYJ » Sun Aug 01, 2010 5:26 am

what it didnt take you 6 years to do this swap? WOW :staun: :rollinglaugh:
its on the road finally only 2 1/2 years later. (note to self before doing work make sure you have the time to do it.)

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