CJ 4.0 Engine Swap


To upgrade the motor in the Scrambler and improve the drive-ability and reliability. Provide additional torque and power without sacrificing gas mileage. Sounds like the ultimate paradox doesn't it?


1981 Jeep Scrambler with factory 258 (4.2L) inline 6 cylinder. All original smog and emission equipment still intact. Approx. 70K miles on motor. Motor has normal blowby problems and burns no oil. Runs good for a BBD carb, but not good enough. Overall, the term power plant would not apply as the motor has no power and could only plant you in the seat by running off a cliff.


$2000 +/- 10%


Option 1: Take 4.2 as is and add the MOPAR MPI kit and do a 4.0 head conversion. Also upgrading the ignition to the HEI system and adding a header. The rough cost on this option is @$2300. Issues with this option are the head bolt differences (7/16" on 4.2 and 1/2" on 4.0 head) and the overall uncertainty with this short block.

Option 2: Find a 4.0 donor with low miles at a reasonable price and perform a swap. Rough cost of this option is @$2000. Issues with this swap are dependent on the donor vehicle, others will arise of course.

What to do?:

Deciding between the options is not easy. Option 1 may sound easier, but it entails the outlay of a large amount of cash to get the MPI kit. Option 2 could encounter the same problems, so let's examine the mission again. My goal here is make the Scrambler dependable and reliable. Option 1 relies heavily on the soundness of the 4.2 short block, which would not be touched in the upgrades. Option 2 affords me a new(er) long block if I can find the right donor. Option 2 comes with the MPI kit from the factory if right year is found. Plus Option 2 has better 4.0 head already, naturally. Sounds like we have a winner!

Course of Action:

After examining all of the facts, Option 2 is my choice. So I start by calling around to local salvage yards with nothing but laughs from the owners. I initially was looking only for a 91 and newer 4.0 due to the HO(High Output) distinction. After I discovered that I wasn't going to find one down the street, I decided to narrow my search to a 95 donor. I know that sounds counterproductive based on the lack of prospects, but I felt like if I called the salvage yards with a specific year, I may have more luck rather than saying 91 or newer.

So I begin to look luck locally. I decide to widen my search area and go to and use their online yellow pages. Called 7-8 yards and finally got a prospect. 94 with 81k miles and no accessories or harness....thanks, but no thanks.

Did I make the right choice here.......patience grasshopper....patience.

Finally a break.....I used a locator service and within minutes start receiving replies. After wading through the replies I called the most promising ones. The price is outrageous for what I need. Most places want extra for the motor accessories and harness, which I need. Finally I come across a reply from Wagner's Truck and Van in PA. Called them up and BINGO!!!!!

Wagner's had the dream donor......96 XJ with 30K miles. Included in the price was all the accessories, harness, ECM, and all under-hood components. After discussing the donor with a known guru on the swap, I decided the make the purchase.

Removal of 4.2

Not a whole lot to say about this step. Take it out and remove all hoses and unneeded items. Items to remove include the following: starter, solenoid, charcoal canister, IAC solenoid (81 model), battery cables. None of those items will be used in the swap.

4.0 Prep

Motor arrived on a wooden palette and a tire. Looks like a shadetree job! O2 sensor was not there. From talking to my source, I know that I need to change the water pump to a wrangler pump and pulley. So I stripped the pump, ps gear, fan, fan clutch, belt, etc. off. Next I installed a turbo ciry hi-flow thermostat housing w/ 160 degree thermostat. Put on new (not re-manufactured) 95 wrangler water pump. Got a used 95 wrangler water pump pulley and fan blade and used the XJ fan clutch. Everything bolted right up with no problem. Reinstalled all accessories and used 95 wrangler belt and belt routing.

The motor mounts were torched off from the frame by the salvage yard so I had to remove the chunks of frame, then the mounts and the engine brackets. One of the fuel line quick connects was broken as well so they were scrapped.

The XJ donor was an auto so I removed the flex-plate and the backing plate (the 4.2 backing plate will be used). Part of my motor deal included a free (remember this for later) 4.0 manual flywheel. So I took the 4.2 flywheel bolts (longer than flex-plate bolts) and bolted the supposed 4.0 manual flywheel up to the 4.0. I ordered a pilot bushing to fit the input shaft of my tranny (T-176) and had to machine it down to fit the crank of the 4.0. This was not hard at all since the bushing is very easy to manipulate with a drill press and some emery cloth. After a little trial and error, the bushing was a perfect fit.

The motor mounts from the 4.2 were removed and trashed and the 4.2 motor mount brackets were transferred over to the 4.0 with new 4.2 motor mounts. The brackets are a direct bolt on with only one small bit of work needed to find a metric fine thread bolt for the rear driver's side bracket bolt. The brackets bolt up in a different spot than the 4.0 brackets, hence the problem with the metric bolt. Ok, at this point she is ready to set in.

Scrambler Prep

The 4.0 wiring harness and the 4.2 harness were shipped off with the backing plate, bellhousing, and 4.0 crank sensor to be worked up for the swap. The work to be done to the harness and bellhousing would take @ 3 weeks with shipping time, so I had some time to get other things done. First order of business was to strip out the metal gas tank and all OE gas lines. The tank was replaced with a 15 gallon tank from MTS tanks and the lines were plumbed with 3/8" aluminum line from Summit. Metal lines were run all the way to the fuel rail with only 2-3 inches of rubber FI hose used to connect to the rail. The pressure line was kept on the passenger's side and the return and vapor lines were routed down the driver's side. I obtained some 1/2" line bolt downs to hold the lines to the frame. A used fuel pump from a Ford van was mounted right after the passenger side rear shock with a pre and post filter. Since I had so much time before my harness/bellhousing arrived, I stripped what was left from the inner fenders and firewall and painted them flat black. Also took the time the coat the frame again with some POR-15 Chassis Coat in the front. Other parts that were ordered at this time include Taylor Stainless Steel Battery Cables and Ground Straps, TurboCity air tube w/ K&N Filter, Jacobs 8.5mm plug wires, and a throttle cable from a 95 wrangler 4.0.

Quick Summary

At this point, all parts have been ordered and the harness/bellhousing has arrived after a lengthy delay (not discussed here). The Scrambler is ready for the donor....all the pieces are in line, so let's get to it!

4.0 Guzinta Scrambler

First step is to get the motor into the Jeep. Standard stuff here. Just takes a while to get mounts, tranny, and bellhousing lined up. No big deal, just had to use a little force to get all to line up at the same time.

So the motor is in the Scrambler now...ahhhhhh. Next step was to install the harness. No big deal here due to the hard work performed by the folks at the harness shop. All wires fell perfectly into place and plugged right in as expected. Only took 30 minutes to install the complete engine harness back to the firewall. Next came the new battery cables and ground straps for the engine, body, and grill. The ground straps are very important on a FI vehicle as are the battery cables. Do not skimp here only to have something bite you later. Next came more minor details...heater hoses, rad hoses(installed grill too), plug wires, turbo city air tube. Last detail was to finish the connections between the new fuel lines and the fuel rail.

Houston, We Have a Problem we are...everything is done (except minor little issues that won't keep it from running). Turn the key and after a few engine rotations she fires right up. The 4.0 is running VERY RICH and stumbling terribly on throttle movement. Let it run for a few minutes, had to hold the throttle though as it wouldn't idle. So I shut it down and began to doublecheck all of the connections to make sure they are made. All was well so I chalked it up to the motor having been sitting up for a year or so. Tried to crank her again and it just laughed at me. After numerous attempts it was clear it was flooded and the plugs were fouled. At this point I'll make a long story short. I went through 2 more sets of plugs and checked every sensor in the errors. Even had a friend check it with a DRBII for error codes.....none. But the DRBII gave one very interesting piece of information...the computer was being told that the engine was running at 7000 RPM at idle. So it was trying to dump the fuel to it to keep up, hence my richness and fouled plugs.

There was only one thing that cause this....the flywheel. Remember the reference to the "free" flywheel that came with the motor? Yeah right...there ain't no such thing as a free lunch!

Yanked the transmission back out examined the flywheel. Not knowing that much about the 4.0 flywheel I had to refer to the manual. The manual showed a flywheel with 12 notches but mine had 60 (or 61, can't remember) notches. So the CPS was going haywire and reporting bad info. After some legwork I found the correct flywheel and put it on. Bolted everything else back up and fired her up. Purred like a kitten!

Buttoning Up

The 4.0 was now running perfectly. Had a Flowmaster installed with 2" pipe all the way out. Also had the A/C lines fixed to adapt to the 4.0 compressor, this A/C is cold!. After driving the Scrambler for a while, the only problem I can report is a bad IAC motor. The 4.0 drives like a dream in the city or on the highway. Gas mileage has increased and the power difference is wonderful. It just hiccups, no hard starting, no bogs, no sputters......AHHHHHHHHHH!!!!!!!!

The total cost of the swap after all of the extras I bought (and I bought many) was @$3000. This inludes the exhaust work, the a/c work, the harness/bellhousing, the donor, and all the other parts I bought. Was it worth it? Every single penny.