Click for Coverage
Due to the EU’s Global Data Protection Regulation, our website is currently unavailable to visitors from most European countries. We apologize for this inconvenience and encourage you to visit for the latest on new cars, car reviews and news, concept cars and auto show coverage, awards and much more.MOTORTREND.COM
  • JP Magazine
  • Dirt Sports + Off-Road
  • 4-Wheel & Off-Road
  • Four Wheeler

Swapping a Cummins in a TJ Wrangler

Posted in How To: Engine on May 1, 2017
Share this

Never before have so many folks been interested in an engine swap we’ve done. The new Cummins R2.8 turbodiesel is what many have been asking for, and there is a very good possibility this is the engine you will want in your next little off-road project.

To start, this is not the 4BT that so many have heard about. The 3.9L 4BT is tough as an anvil but also big and very heavy. The R2.8 is a smaller, more modern, and much lighter-weight 2.8L engine that’s perfect for stuffing in a small Jeep Wrangler—and that’s just what we did.

The electronically controlled common rail 2.8L is based on an engine family used overseas in everything from small Chinese pickups to large Brazilian Ford trucks, and with 267 lb-ft of torque and 160 hp it is just what the doctor ordered to replace our tired 2.5L four-cylinder gas engine in our 1997 TJ we call Tube Sock. This new diesel has more torque than the tried-and-true 4.0L Jeep straight-six as well but is comparable in weight to the I6. However, the mileage of the Cummins should be about 30 percent better than the gas engines and deliver similar power and torque at a lower rpm, all great attributes in a four-wheeling engine.

As of this writing, Cummins has not released prices or sales structure (where to buy), but we would guess the R2.8 package will retail between $8,000 and $10,000 and will be sold through the current Cummins dealers. The biggest unknown currently is the California Air Resource Board, which is reviewing the Cummins application for an Executive Order (EO) number designating what year vehicles the engine can be used in for a legal emissions-compliant swap. Last fall, Cummins demonstrated a 25 percent reduction in CO2 emissions and lower hydrocarbon emissions too in the R2.8 as compared to the 4.0L. Cummins wants to offer the R2.8 as a 50-state-legal crate engine, even if it means only in certain older vehicles. We will have an update soon if and when CARB makes a decision.

Our 1997 Jeep Wrangler that we affectionately call Tube Sock came with a 2.5L gas engine, an AX5 manual transmission, and an NP231 T-case. We previously swapped in a set of Currie RockJock 44 front and 60 rear axles (“Mayhem to Moab, Part 4,” Nov. 2014) and then wheeled it with 37-inch tires. All this was done just prior the 2014 Moab Easter Jeep Safari. In addition, this little Jeep was a returning project on Dirt Every Day, our web series you can find on YouTube and Netflix, where we covered building Tube Sock and taking it to both Moab and later Hawaii for some muddy wheeling action.
After a meeting with a representative from Cummins, we concocted a harebrained idea to put a small diesel in our Wrangler and attempt to drive it underwater. Yes, underwater, like 12 feet underwater. That fever dream turned into reality and can also be seen on Dirt Every Day, but even cooler is the little R2.8 that we got to do research and development testing for Cummins with. The R2.8 is the first in Cummins’ new crate engine initiative that will hopefully eventually include inline-six and V-8 options as well. As of now we have Version 2 of the crate engine in the Jeep. The underwater engine came out just fine, but the Cummins folks wanted us to upgrade to a version nearly identical to what they will sell. Because of this, some of our photos will show different engines, but the tech is the same, and we will concentrate on the latest version if possible.
Step 1 of any engine swap: Remove the old engine. We pulled not only the 2.5L four-cylinder but also the AX5 manual and NP231 T-case. We sourced a rebuilt NV3500 and Atlas transfer case from Advance Adapters to put stronger components behind the little Cummins. Always remember to clean, repair, and paint any firewall problems when the old engine is out because it will make putting the new engine in that much easier.
We took the carcass of Tube Sock to Advance Adapters, where we had done the engine swap, so the company could develop an adaptor from the R2.8 to the AMC V-8/I-6 pattern to work with a Wrangler AX15/NV3500 bellhousing. This uses an adapter ring from the Cummins circular pattern to the AMC pattern. We also incorporated an adapter between the Cummins flywheel and the Jeep 4.0L flywheel. An Atlas with a 4.3 low buttons up our new powertrain.
The production R2.8 crate engine will have an EGR system and will be designed to run an intercooler. There are provisions on the engine for a vacuum pump to run your power brakes and HVAC controls, and the lift pump and high-pressure CP3 fuel pump are mounted on the engine so you just have to have a pickup tube to your fuel tank.
To start with, the first-gen adapter kit from Advance Adapters uses two flywheels. The factory Cummins unit is installed on the engine, and then has an adapter/spacer that mounts between it and a neutral-balance factory Jeep flywheel from a 4.0L straight-six gas engine application.
The second flywheel is for the clutch, while the first is for the starter. The spacer between the flywheels makes up the distance of the Cummins SAE-3 bellhousing-to-AMC bellhousing adapter. Advance is trying to work out another solution that uses only one flywheel for both the starter and clutch, but for now the first-gen adapter system works fine.
With the clutch and dual flywheel and adapter there is a fair bit of mass behind the engine. But for slow driving and rockcrawling, adding mass to the flywheel isn’t always a bad thing. We had actually added a heavier inertia flywheel to our old 2.5L gas engine for additional low end grunt, and found that the little Cummins also chugs right along with the heavy dual flywheels.
The TJ frame was cleaned of its factory 2.5L engine mounts, and new Advance Adapters mounts were installed to the frame. Finding the perfect engine mounts to reduce or eliminate the diesel vibration is a challenge. We are still fine tuning, and it’s getting better with every setup we try.
The R2.8L works perfectly fine with a stock 4.0L Wrangler radiator. We purchased a PT Cruiser intercooler from RockAuto and stuffed it in front of the Wrangler radiator. We had to modify the grille and intercooler a fair bit to fit, and we think a custom-made intercooler for a Jeep grille would be a much better but more expensive alternative.
The high-pressure CP3 injection pump on the Cummins is also a lift pump, so there is no need for an in-tank pump of any kind. We removed the in-tank, high-pressure gas pump guts from the factory TJ sending unit and simply dropped a hose down to the bottom of our GenRight aluminum tank. An inline fuel filter keeps the fuel clean.
The gas tank filler neck won’t fit the large diesel fuel nozzles at the pump, so we knocked out the reducer and cleaned out all the gas from the tank so we could reuse it all.
The R2.8 is an electronically controlled engine. It has a wiring harness that runs to an ECU and a drive-by-wire throttle pedal. It can even run electric fans and gauges if desired. The harness is very simple, and Cummins is still fine-tuning it for as universal a fit as possible.
The front accessory drive supports a power steering pump and a vacuum pump for brakes and accessories, and could fit an air conditioning pump if so desired. There is also an oil cooler and alternator. Plans are in the works for a remote oil filter option.
The completed engine installation fits the TJ engine bay as if designed for it. It’s a little bigger package than the stock gasoline four-cylinder but fits well. Ever-so-slight firewall massaging was needed on our setup. We sourced some battery wires from Painless Performance, and plenty of intake and turbo hoses and elbows from Summit Racing Equipment. We went with a mechanical fan but are not sure if that will be an offered solution or if electric fans will be standard. The air filter is tucked under the hood, and the engine purrs with a little diesel rattle as expected.
We did have one major hiccup. The oil pan of the R2.8 is made of a polymerlike plastic to reduce noise. We’d recommend a good skidplate because we smashed ours the first trip out like total rookies. Any standard TJ engine skid should clear the pan of the R2.8, or there may be a metal oil pan in the works.
While we were swapping the R2.8 in our TJ, Cummins was doing a similar swap in a bone-stock TJ. Cummins pulled the 4.0L out and dropped in the R2.8 to do data acquisition. Its diesel TJ was third-party tested to verify fuel economy and realized mileage numbers of 30 mpg on the highway and 25 mpg in the city! A stock 1997 TJ with a 4.0L is rated at 16-18 mpg highway, 13-15 city. A 2.5L 4-cylinder TJ isn’t much better with 18-19 highway, 15-17 city. (Data from Of course, mileage with big 37-inch tires will be much different than stock. (Stay tuned for our long distance mileage testing of Tube Sock).


Summit Racing
Akron, OH
GenRight Off-Road
Simi Valley, CA 93063
Painless Performance Products
Ft. Worth, TX 76105
Advance Adapters
Paso Robles, CA 93446
Columbus, IN 47202
Madison, WI

Connect With Us

Newsletter Sign Up

Subscribe to the Magazine

Browse Articles By Vehicle

See Results