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Swapping a 4.0L Into a CJ-7

Posted in How To: Engine on August 11, 2016
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Photographers: Trenton McGee

It seems like everyone and their brother has put a small-block V-8 into a CJ-5 or CJ-7, and those that haven’t done it have heard about someone doing it. We’ve covered swaps like these many times. There are a lot of benefits to putting a V-8 in a CJ, but it’s not the only viable swap out there. Few people will fault the durability and reliability of a Jeep’s own 4.0L, and long-nose CJs were designed to accommodate the 4.0L’s predecessor, the 4.2L, right from the factory. In fact, the inline sixes share the same block and external dimensions, which means lots of stuff can be swapped between the two engines (accessory drives, engine mounts, and so on). On paper, swapping a 4.0L into a CJ is a piece of cake, yet somehow we’ve never covered one in these pages until now.

We decided to find out if a 4.0L swap is as easy as it seems, so we plugged a 4.0L scavenged from a ’96 Jeep Grand Cherokee into our ’84 CJ-7. Our CJ was equipped with a pathetic high-mileage four-banger that couldn’t maintain 65 mph going downhill with a tailwind. Because it seemed so easy, we even had grandiose visions of getting it done over a three-day weekend. That didn’t happen, but we did complete the swap in about two weeks, working on it in the evenings and over a couple of weekends in between our real job. We also did it without ever picking up a welder or making any custom modifications, so it’s a fairly easy bolt-in that doesn’t require any fabrication skills. What follows are the highlights of what we did and some of the problems we ran across, so you can decide for yourself if it’s worth the work. As with any engine swap, there are pros and cons, but in the end, we did end up with a very reliable, torquey CJ-7 and no longer had to live in fear of maintaining freeway speeds.

The time had come for the carbureted 2.5L in our CJ to go away. With nearly 150,000 miles on the odometer, it was beyond tired and leaked oil out of every orifice. Knowing the swap was going to be a big job, we spent the hour it took to remove the front clip from the Jeep. This made it much easier to work on wiring, plumbing, and everything else that needed to get done.
We picked up a complete 4.0L with an engine harness and computer from a ’96 Grand Cherokee on Craigslist and also sourced a used AX-15. The stock T-4 was a marginal tranny that wouldn’t bolt up to the 4.0L anyway. If we had to do it over again, we would have pulled the engine ourselves, as a lot of the fasteners and other stuff we needed were missing. As a result, we conservatively made 20 trips to the hardware store getting various bolts and fittings. We also did a quickie rebuild to the engine that was covered in a previous article. If you read that article, you know the rebuild ended up not going all that well.
One of the most intimidating aspects of this swap is wiring the fuel injection. We used a factory engine harness from the donor and got our hands on a factory service manual, then whittled down the harness to only what we needed to make the engine run. While tedious, it’s not particularly hard if you can read schematics and pay attention. The rat’s nest in the box is what we cut out of the donor harness. Though we retained the power distribution center from the donor harness, we used just three circuits and relays: the fuel pump relay, ASD relay, and starter solenoid relay. In the end, there are only three connections needed from the original harness to the EFI harness: constant power, starting power, and start/run power.
If your recipient is an ’80s CJ like ours, you’re going to have a mild mess of wiring to clean up. The majority of it is related to the feedback carburetor system and its computer, which may have been spawned from the devil himself. The good news is that most of the later feedback stuff was added-on to the core harness and is pretty easy to remove. Aside from constant and switched power, you’ll retain the temp and oil pressure leads, tach lead (if applicable), and the reverse light leads that go to the tranny. Everything else goes away. We swapped the oil and temp sensors to the new engine and retained the original gauges. The headlights and parking light leads also stay, but that’s all in a separate loom. Overall, our stock harness was in great shape despite its age.
We debated on which engine mounts to use: the early 4.2L (258ci) mounts (left) or the later 4.0L mounts (right). The 4.0L block is almost identical to the earlier 4.2s, so if you’re starting with a six-cylinder Jeep, then the mounts are a direct swap. The frame horns on a 2.5L Jeep like ours are different, but both 4.2L and 2.5L mounts simply bolt to the frame. We found a set of 4.2L engine and frame brackets (top) cheap, so we ended up going with the earlier mounts for an easy bolt-on solution rather than fabricating frame mounts for the later style. However, note that the early rubber mounts are failure-prone and the later ones have a much better reputation.
Our AX-15 came with a flywheel. It also came with a clutch that looked like it was in good shape, but we couldn’t bring ourselves to use it. We had the flywheel resurfaced and invested in a new clutch and pressure plate. We also had to install a pilot bearing in the crank since our donor was originally in front of an auto. Don’t forget that you need the aluminum plate and inspection cover that sandwiches between the engine and tranny.
We retained the original Dana 300 because you can’t get or swap in a much better transfer case. All stock Dana 300s have a 23-spline input that matches the output shaft of the AX-15, and they share an identical six-bolt mounting pattern. The clocking on the AX-15 is different, however, so you’ll need a clocking ring to mate them together. We used an inexpensive adjustable clocking ring from Novak Enterprises. There are four possible positions from 0 to 27 degrees, while the factory Dana 300 clocking is somewhere around 23 degrees. We ended up positioning the transfer case at the 9 degree position. The clocking ring simply bolts on the front face with no other modifications needed.
We toyed with the idea of rebuilding the AX-15 and the Dana 300, but in the end, time constraints and budget limitations forced us to roll the dice. We knew the transfer case was fine, and the AX-15 proved to be flawless. We swapped the funky Torx external hardware on the bellhousing for normal Grade 8 stuff, but were careful to torque the bolts to factory specs since the AX-15 bellhousings have a reputation for cracking. It’s also a good idea to leave the crank position sensor out until the drivetrain is installed to avoid damaging it. We stabbed the whole assembly into the Jeep as one unit.
A low-profile belly pan is part of the long-term plans for the Jeep, especially since we clocked the transfer case much higher than stock and now have the room. But we chose to reuse the stock pan, at least temporarily, again due to budget and time constraints. Since CJs came with a variety of drivetrain combinations, there are a variety of positions available with the stock pan. We slotted some existing holes in the pan (red arrows) and also drilled holes (green arrows) to accept the TJ-style transmission mount we used because it was cheap and bolted to the AX-15.
With the engine stabbed we could focus on plumbing and wiring. We left the pared-down harness unwrapped and determined how it should be routed just in case we had to adjust the position of any leads. It looks a little bit like chaos here, but figuring out the routing turned out to be one of the easiest jobs of the project. We didn’t wrap the harness in wire-loom until we fired up the engine and verified everything worked properly. The computer and power distribution center ended up mounted to the passenger fenderwell near the battery.
One of the tricks to swapping in a 4.0L is figuring out how to give the computer a vehicle speed signal (VSS), especially if the recipient is equipped with a mechanical speedometer and you want both to work. The solution is to nab one of these gizmos at your local wrecking yard, which retains the mechanical drive for the speedo but also has the necessary sensor for the computer built-in. Renix-injected Cherokees and YJs allegedly have them, but we didn’t have any luck finding the sensor on either one. We nabbed this one, plus a spare from an early ’90s Dakota, at the local Pick-a-Part. You’ll need to splice in the sensor’s connector (make sure you grab it), but the color codes from the 4.0L harness to the connector will likely match. We also needed a 7/8-inch to 3/4-inch adapter to hook up the CJ speedometer cable, which we sourced from a speedometer specialty shop online for under $15.
Cooling can be an issue with 4.0Ls, and we didn’t want to take any chances since it can get pretty warm in the Southwest. We chose a Flex-a-lite radiator for the swap, since the aluminum radiator itself is huge and incorporates a really nice electric fan with built-in mounts. Best of all, it simply bolts right to the stock grille mounts. Flex-a-lite warned us that the radiator wouldn’t work with a 4.0L conversion (it does work with AMC and most other V-8s), but we figured there had to be a way to make it work since CJ-7s came with 258s, which are the same dimensions externally as our 4.0L.
Flex-a-lite was right. It didn’t take a brain surgeon to figure out there was no way the fan supplied with the kit was going to work once we mocked up the radiator. There was also no way that a mechanical fan was going to work because there just wasn’t enough room. With our tail between our legs, we went back to the company and they swapped out the fan for one of their thinner 16-inch Trimline fans (PN 118). Even though it’s not quite as powerful, it should be able to keep up with our stock engine. The radiator did fit perfectly in the grille, though, and we didn’t have to drill any new holes to mount it. We plumbed the rest of the radiator with lightly modified TJ hoses, which worked perfectly.
The clutch hydraulics were another area where we had some choices. We nabbed the complete master and slave from a TJ with the donor AX-15, and we had the original CJ stuff. Our initial plan was to use all of the TJ stuff, but that proved to be a little complicated because the TJ master cylinder and pushrod weren’t compatible with the CJ firewall and pedals. We ended up using the CJ master cylinder (top) with the TJ slave (left). The bore of the TJ slave is a little smaller, but the stroke (travel) of the CJ master is a little shorter than the TJ version. It worked perfectly. We used a special fitting from Novak to convert the funky TJ slave fitting to normal 1/4-inch inverted flare and connected the two with a custom hose from a hydraulic shop.
For the fuel system, we mounted a Ford E2000 external fuel pump to the frame near the tank, then used a Wix No. 3737 fuel filter from a Corvette that has a built-in regulator to bring the E2000’s 80-plus psi down to a 4.0L-friendly 45 psi. We used high-pressure rubber hose between the pump and the filter/regulator, and we used low-pressure hose for the return line back to the tank. We plumbed both to the original pickup and return ports on the tank. We modified the stock 5/16-inch steel fuel line to accept inverted flare fittings on both ends and then ran a second short length of high-pressure rubber line between the frame and the engine.
We knew we were going to lose some rear driveshaft length and were going to need to modify both driveshafts with the swap, but we were really surprised when we compared the before and after measurements. According to our notes, we lost 5 1/8 inches of rear ’shaft length and gained the same amount in the front. This could be a deal-killer for some people, as our final length of 20 1/4 inches doesn’t leave a whole lot of length for a taller Jeep. We called the experts at JE Reel with our measurements, and they recommended going with new ’shafts, as well as converting the rear to a CV joint at the transfer case. The company’s high-quality driveshafts arrived a couple days later, were exactly as ordered, and bolted right in. Though not really needed now, the CV will be important if and when we decide to lift the Jeep higher than its current 2 1/2 inches.
Not surprisingly, we had to modify the transmission tunnel cover to work with the new location of the shifters. Luckily, the modifications were limited to just the cover, and we only had to lightly trim back a lip on the floor itself. We were going to reuse the stock transfer case shifter, but that proved difficult with the reclocked position of the case, so we dug out an old twin-stick shifter we picked up during some horse-trading years prior and put it to use. A small patch panel was all that was needed to seal up the floor.
The last piece of the puzzle was figuring out an intake and air filter for the engine. The stock Grand Cherokee stuff wouldn’t work and the TJ-style intake routes the filter over to the passenger side fender, where things were already pretty crowded on our Jeep. In the end, we opted to modify a cold-air intake system from AIRAID that was designed for a YJ with a 4.0L. The intake tube positions the filter on the driver side fender where we had more room, but it interfered with the power steering pump in its intended forward-facing position. We rotated the intake tube 90 degrees and pointed the filter at the firewall, then secured it with some of the brackets included with the intake kit.


Fife, WA 98424
Phoenix, AZ 85050
JE Reel Driveline Specialists
Novak Enterprises Inc.
Whittier, CA

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