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Cummins 4BT Jeep Wrangler TJ Diesel Conversion

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Tagged As: 4BT, Cummins, Diesel, Diesel, How To, Jeep, TJ, and Wrangler

Back Story

The Jeep Wrangler is the quintessential off-road 4x4, a vehicle capable in its own right, but able to be modified and customized in almost every conceivable way. The most commonly seen Jeep modifications are usually larger tires, a mild lift and perhaps a winch. But given a long enough timeline, the Jeep modder always has a continuous itch to start looking at the next big thing for their Wrangler ... and its usually more engine power or getting lost fuel efficiency back.

I already went through my power phase with a RIPP supercharger on my 4.0L, but as gas prices rose towards $4 a gallon back in 2011, a mere 14-15mpg was making the Jeep less and less ideal as daily driver. Unfortunately my 4.0L met its demise that winter when my engine "popped" and suddenly lost power on the highway. Some driveway diagnostics revealed the third cylinder was only holding 40psi compression compared to the others at ~150psi. I had her towed to my local 4x4 mechanic and lost hope when he described my 4.0L as a "boat anchor." He discovered a ring on the third piston busted and gouged the cylinder walls. My first thoughts were, well, maybe I can just bore the cylinders for a 4.7L stroker. Apparently the walls were gouged too deeply to simply bore them out. Further investigation showed putting a sleeve into the cylinder was not an option either. Faced with the requirement to either scrap the vehicle or replace the engine, I began thinking. Put another 4.0L in? Put a Golen 4.7L stroker in? Perhaps the LS1 swap?

The rest of the world has been enjoying a diesel edition of the Jeep Wrangler for awhile. During a trip to Morocco, I saw their government using diesel Wranglers everywhere.

And then the silver lining hit me, with high gas prices and a dead engine, now was the time to shoot for the fuel efficiency of a diesel conversion and drop a Cummins 3.9L 4BT into my Jeep. Putting a diesel into a Jeep piques the interest of almost all Wrangler owners with the promise of low RPM torque, dramatic fuel efficiency gains and the tell tale clack clack clack clack diesel sound. Back in 2008, Chrysler teased Jeep Wrangler owners with the possibility of a diesel crate engine, an improvement based on the 2.8L CRD found in 05-06 models. A number of companies offer turnkey conversions such as a Cummins 4BT from Bruiser Conversions or a Volkswagon TDi from HPA Motorsports. The convenience of turnkey solutions usually run at least $16K but regular patrons of forums like JeepForum, Pirate4x4 and 4BTswaps know that if you're willing to put up with the diesel conversion aggravations yourself, the price can come down quite a bit.

Information on the Cummins 4BT

Cummins engines date their historical origins back to 1899 when their inventor Clessie Cummins began building steam engines as an 11 year old boy. Two decades later, he produced his first diesel engine for the Cummins Engine Company. His engines quickly began making a name for themselves by setting records for fuel efficiency and speed. There was even a Cummins diesel Indy Car that while slower than the pack, completed the race without ever stopping for fuel. The company continued to foray into diesel industrial engine applications and commercial trucking where Cummins engines began to dominate. It was in the late eighties that Cummins introduced the six cylinder B-Series diesel engines for the Dodge Ram and a stripped down four cylinder version for commercial vehicles. For more information, Allpar has an excellent article on the history of Cummins engines since their inception.

The B-series engines began life for industrial diesel applications such as tractors, loaders and other heavy equipment to include marine and electricity generation. As described by Allpar, "The B was built just like any other Cummins, with 18 wheelers in mind. There were no shortcuts taken by Cummins, nor any modifications to make it work. Dodge had to make the truck work around the engine." The design continued to be refined throughout the nineties with upgrades to fuel delivery, electronic management and other improvements. The original B-series were entirely mechanical and required only an electrical impulse to keep the fuel solenoid open and a throttle linkage cable. With its high compression ratio, the B-series engines were among the few to leverage direct injection and start easily in cold temperatures without the use of glow plugs, grid heaters or starting fluids. The Cummins 4BT evolved as a shortened version of the 6BT and were often found in diesel step vans or diesel delivery trucks.

This dynometer sheet for the Cummins 4BT's torque and HP curves was acquired from 4BTswaps.

The Cummins 4BT is a popular diesel swap into Jeeps due to its performance characteristics. Although a taller and heavier engine, the Cummins 4BT's length and width fit optimally with room to spare inside the Wrangler's engine bay. When properly geared, diesel's produce low rpm torque which is ideal for crawling, towing and other low speed scenarios. The Cummins 4BT's 16,000 pound GVWR allows it to push a heavily modified (literally heavy) Jeep with its horrendous aerodynamics along the highway with ease. So much ease in fact that diesel fuel economy can more than double for a modified Jeep. These benefits aside, the Cummins 4BT is not necessarily a no-brainer and was listed by JP Magazine as one of the ten dumbest engine swaps stating, "they sound good on paper, but when you've lived with one of these conversions for a little while the buzzing and belching can really test the temperament of some drivers."

Displacement3.9L, 239 cubic inches
Bore4.02 inches
Stroke4.72 inches
Compression17.5 : 1
Length30.6 inches
Width24.6 inches
Height37.7 inches
Weight745 - 782 pounds
Horsepower105hp @ 2300 rpm
Torque265ft-lbs @ 1600 rpm
GVWR16,000 pounds

This data was derived from

Sourcing a Cummins 4BT

There are a number of ways to acquire a Cummins 4BT diesel engine. The most common story is to find delivery trucks or other commercial vehicles that are no longer in service and being excessed out of a company's motorpool. People have been able to score Cummins 4BTs for as little as a few hundred dollars to several thousand. It's easier to get one at a lower price when the sale is for the entire vehicle as scrap. When the mechanic knows what he's sitting on, they'll usually scrap the vehicle themselves to a salvage yard and sell you the diesel engine directly which raises the price. The positive side of buying one of these engines is that you can potentially save a lot of money. But it comes with a big risk in that you have absolutely no idea what how well that engine was taken care of and it comes with no warranty whatsoever.

Front view of a Cummins 4BT still mounted in the remains of a bread truck chassis.

There are some rules of thumb to buying a used Cummins 4BT ... heed them! Definitely read about the engine buying experiences from others on forums such as to see what problems people have encountered. Some of the most pertinent include:

  • Ensure the vehicle has its engine serial number tag.
  • Make sure the bill of sale receipt include the engine and donor vehicle's serial number.
  • Check for an emissions certified Cummins 4BT and the appropriate engine tags if your state requires it.
  • Visually see the engine fire up and operate - but ensure appropriate levels of coolant, oil and diesel are present first.
Side view of a Cummins 4BT after being exposed to the elements for an undetermined amount of time.

An alternative, albeit far more expensive option, is to purchase a remanufactured Cummins 4BT directly from a Cummins power systems supplier. I had already bought mine at this point, but was having it serviced at my local Cummins location and just asked about the availability of rebuilt engines. I was quoted $7500 and a short wait time to have one shipped in if I was interested. The positive angle to taking this approach is you know the engine is in excellent working order and it may come with a short warranty. That comfort is countered by a steep price tag ... but it can be worth it.

Parts & Other Considerations

So what does it take to convert a Jeep Wrangler to diesel with a Cummins 4BT? A lot - you must consider engine mounts, transmission mating, driveline strength, fuel delivery, electrical issues, cooling, instrumentation ... let's begin.

Instrument Gauges

A Jeep Wrangler TJ's instrument cluster is not driven directly by the senders. Rather, electronic engine management via an ECU requires all of that sensor data to be directly into the computer which in turn dispatches the appropriate signals to the instrument cluster. What does that mean? Even though the Wrangler's ECU is not needed at all to operate the Cummins 4BT, it must remain active in order to get the stock instrument cluster to work. That or it will be necessary to remove the entire instrument cluster and replace all of the requisite gauges with something else.

Naturally, there are nuances. For the most part, things will just work:

  • Voltmeter - this works fine
  • Fuel Level - this works fine
  • Oil Pressure - ummm ... we'll get to that
  • Water Temperature - this works fine
  • RPM - depending on your year, this will work with a kit
  • Speed - this works fine (but probably needs calibration)
  • Odometer - this works fine
  • Check Gauges / MIL - be prepared to live with them illuminated or remove the bulbs

For most Wrangler TJ's, the instrument cluster will work fine using a kit from JD Jeeps. They sell an "exciter ring" that mounts to the Cummins 4BT's crank pulley such that the stock crankshaft position sensor will be appropriately triggered. At the time of my installation, this kit only worked on '92-'04 models and did NOT work for the '05-'06 (which uses a different ECU entirely). As such, I opted to add an Autometer Diesel Tachometer that triggers from the alternator. The Autometer gauge uses a sensor attached directly to your alternator which is pretty easy to install.

This is an installed exciter ring kit sold by JD Jeeps to make the stock CPS sender work, allowing the existing instrument cluster to function.

The stock oil pressure gauge in the Wrangler TJ is an idiot gauge. It does not show instantaneous pressure, rather it only displays the presence of pressure (upright and center) or a complete lack thereof (dropped left into the red). This fact has been belabored ad nauseum on various Wrangler web forums so just accept it, the gauge is useless for monitoring real oil pressure. Simply drop a new sender into the Cummins 4BT and wire up a real oil pressure gauge. Especially with diesels, its important to know the idle and running oil pressures are at the appropriate levels. An Autometer oil pressure gauge spanning 0-100psi will suffice for this application.

Installing a real oil pressure gauge is highly recommended in order to properly monitor the Cummins 4BT's operating conditions.

Even after putting in a 26 tooth speedometer gear into the transfer case, the Jeep still puts out the wrong speed. The 26 tooth is the smallest gear made but can only accommodate 32" tires on a 3.07 gear ratio. If you are building for economy you're probably still off and will need a calibration "patch". I accomplished this using a SuperLift Tru-Speed module spliced in-line to the speedometer wires leading into the ECU. The nice part is that it has a switch allowing you to choose between two tire sizes (presumably on-road and off-road) and set independent adjustments up to 10%.


It goes without saying that a Cummins 4BT is a much bigger engine than your existing 4.0L. You need at least a 4" lift (not a fake body lift) to accommodate the increased size mostly because the Cummins 4BT is taller at 37.7" in height. A body lift will not help at all, the top of the Cummins 4BT fits fine in the engine bay. The oil pan, however, is much lower and unless you plan on scraping it off on a rock, you need 4" of real lift.

The Cummins 4BT weighs approximately 750 pounds which will significantly affect your suspension. My 4" lift was compressed in the front to be somewhat akin to a 2.5" lift. Granted, I also have a Rock Hard Extreme Duty front bumper and a 100 pound Warn PowerPlant mounted ... but if you're the kind of person to consider mounting a Cummins 4BT, you probably have a heavy Jeep, too. Make sure your springs are stiff enough to handle the increased weight. It's also important to ensure your suspension can handle the increased weight in motion. For instance, I replaced my stock sway bar long ago with a Currie AntiRock system and while the additional body roll was just noticeable before when turning, its very noticeable with so much extra mass up front.

Another element of suspension to consider is the design itself. I swapped out the control arms from a 4" Superlift awhile back for a Rusty's Offroad Long Arm upgrade. This kit utilizes radius arms in the front which cleared the new engine mounts, but not by much. As a matter of fact, I'm not sure I could have used these engine mounts had the old upper control arms actually still been in place. With the radius arms, the springs were compressed under the weight such that I had a mere 1.5" of vertical travel before the arms would strike the engine mounts. This absolutely must be addressed through properly sized bump stops and longer/stiffer springs. The engine mounts themselves can be sourced from JD Jeeps and allow you to adjust the Cummins 4BT placement within your engine bay.

The engine mounts are stout, but the clearance between them and the suspension arms can get a little tight. Learn from my mistake and make sure yours are flush with the bottom of the frame and you'll get back about an inch of clearance.


It should go without saying that properly functioning brakes are of the utmost importance. The most obvious thing to consider with a Cummins 4BT upgrade is handling the increased weight of the engine. That said, the increased weight from the Cummins 4BT would be akin to just adding two passengers, however, in this case that weight is almost directly above the front axle. Besides that, there is more stress on the brakes from increased rotational mass from the beefy tires.

That doesn't mean there is nothing to consider with regards to the Jeep's brakes. Diesel engines, especially when turbocharged, do not generate vacuum. Without vacuum, the Wrangler's brake booster in the master cylinder will not function at all, effectively rendering the brakes useless.

There are two simple solutions for this. The cheapest is an electric vacuum pump which is easily sourced. Plumb the Jeep's existing vacuum lines to this pump and all vacuum operated devices will function again. A more reliable and powerful solution is to put a vacuum pump onto the Cummins 4BT's accessory drive. While the accessory drive can be used for a variety of devices, it is common to find an air compressor in this location. JD Jeeps supplies a vacuum pump that can be easily substituted for an air compressor that will quickly solve the Jeep's vacuum needs.

An electric vacuum pump in conjunction with the Cummins 4BT's accessory vacuum pump provides guaranteed vacuum for braking. Opting for a mechanical vacuum pump on the accessory drive is optimal.


Just like the absence of a vacuum pump affects brakes, the Cummins 4BT does not have a native steering pump. As a Jeep that only goes in a straight line isn't very enjoyable, it's fortunate that mechanical vacuum pumps for the Cummins 4BT allow a power steering pump to be attached in-line. This component can be sourced from JD Jeeps.

On a Cummins 4BT, the power steering pump is mounted to the rear of the accessory vacuum pump.

Fuel System

A very simple change for a diesel conversion is to alter the fuel filler spout. The diameters of diesel and gasoline nozzles are different. Forgetting to make this change before leaving the mechanic will quickly leave a Jeep stranded and unable to pour diesel into the tank. Just make sure the narrow neck for gasoline is removed and the diesel pump nozzle will fit right in.

Don't forget to modify the fuel inlet for diesel otherwise you'll never get diesel into your tank at a service station.

In addition to modifying the fuel neck, it is necessary to prepare your fuel tank to handle diesel fuel systems. The stock fuel pump is completely useless now. The 4.0L receives fuel pumped to it from within the tank itself such that the fuel lines are pressurized. The Cummins 4BT uses a lift pump on the engine itself to suck the diesel through the fuel lines. As such, you only need a tube descending to the bottom of the tank. The diesel fuel is passed from the lift pump on to the pressure pump, in this case a Bosch VE that pressurizes the diesel for the injectors. Diesel that is not used is returned back into the fuel tank unlike the 4.0L. So a second fuel line needs to be run from the Cummins 4BT back to the tank to handle the return. In order to retain functional use of the fuel gauge on the dashboard, it's necessary to leave the inert gas fuel pump in the tank in order to leverage the existing sending unit. JD Jeeps has a kit to convert your stock pump but this can also be done with some simple fabrication.

As a replacement fuel gauge, a programmable unit will probably be required (if NOT using the stock sender) such as an Autometer 3310. Jeep, in their infinite wisdom, opted to utilize very non-standard ohm values for indicating empty and full from the sender. Most standard gauges use one of three ranges: 0-90Ω, 73-12Ω or 240-33Ω. Jeep uses 20-220Ω. The wire going into the PCM is for tapping is C-26. A programmable unit allows the gauge to function correctly (and probably better than the stock fuel gauge which everyone complains about showing E with 5 gallons remaining. NOTE: My fuel tank looks different from stock because it is a 31.5 gallon Genright Safari tank which gives my diesel Jeep approximately 900 miles of range.

The fuel tank itself needs to be modified to support diesel. Primarily in that the stock fuel pump is not appropriate for a diesel application.


Fortunately, the Cummins 4BT engine does not run particularly hot. The factory service manuals define the operating temperature between 170° and 190° Fahrenheit. The stock radiator will be more than sufficient for keeping her at the right operating temperature. It's a simple matter of getting the appropriately sized hoses to from JD Jeeps to connect the Cummins 4BT with the stock Wrangler radiator.

Some Cummins 4BTs are equipped with a clutched cooling fan driven from the serpentine belt. Even if present, this will not likely be very useful as there is a large space nearly a foot long between the radiator and the front of a Cummins 4BT in the Jeep's engine bay. Again, since the engine operates at a relatively cool temperature (when compared to the stock Jeep's 210°), an electric fan mounted flush to the radiator with a fan shroud will be sufficient. I used a Flex-a-Lite from Quadratec. In the summertime, its possible to hear the fan kick on every now and then when idling but otherwise it almost never activates. The Cummins 4BT has maintained a very stable 175° operating temperature at all speeds.

An electric Flex-a-lite cooling fan is more than adequate for cooling a Cummins 4BT using the stock Wrangler radiator.

Air Intake

The air intake may also need to be custom made. Cummins 4BTs come in two variations with low mount turbos and high mount turbos. Just get some appropriate piping (I'll admit, I'm not keen on the non-Mandrel bent pipes I have) and a decent air filter. Unlike a gasoline engine where the throttle controls airflow, a diesel always operates at full airflow so any restrictions here will cripple the engine's performance. JD Jeeps sources intake kits for both low and high turbo variations.

Image borrowed from JD Jeeps depicting a mandrel bent intake for a high mount turbo.


It's possible for any muffler shop worth their salt to make your new exhaust system. You'll need at least 3" piping to handle the diesel. Exhaust truly is a custom project in many cases due to the variations in lifts and rear articulation found in most Jeeps. JD Jeeps has an exhaust kit that will handle most situations.

In talking about exhaust, this is an appropriate time to discuss emissions. Depending on what state you live in, doing this conversion might not be permissible at all. My diesel Jeep, for example, is completely illegal in California but I don't live there so I don't care. Fortunately, one aspect the Socialist Republic of Maryland made easy (unlike their draconian gun laws) was this conversion - the VEIP staff visually confirms your vehicle is now diesel and grants you a permanent exemption to emissions testing forever forward. The guy literally looked in the engine bay and said, "Looks like a diesel ... Smells like diesel, too." Nice!


The transmission turned into an unexpected additional expense. If you're intending to increase the output power of the Cummins 4BT, then a heavy duty transmission such as the NV4550 is vital. At stock power output, however, a medium duty transmission as normally found in the Jeep Wrangler is sufficient. The challenge becomes mating the Cummins 4BT to that transmission. This is where it becomes important to know which version of the Cummins 4BT you've purchased which is something I was unaware of prior to starting.

The Chevrolet version of the Cummins 4BT can be mated with a six-speed NSG-370 with an adapter from Novak. There's a write-up on JeepForum showing the successful mating. I contacted doc_dyer from JeepForum and he informed me that you need to request a special adapter with 10 degrees of rotation. This only works with the Chevrolet manual flywheel.

I had the Ford version of the Cummins 4BT which, at the time was unfortunate as there were no solutions to be found for mating to an NSG370. I was faced with two options, using the Borg Warner T19 that came with my Cummins 4BT or purchasing a transmission that was known to work. The T19 was adaptable to the NP231 transfer case and capable of handling heavy duty torque. On the flip side, I had no idea what condition it was in plus it was limited with a 1:1 4th gear which would have restricted my highway speed severely. Another unknown presented by the T19 was related to fitment - would it shift the driveline requiring new driveshafts, etc?

Who knows what condition this dirty old transmission that came with my Cummins 4BT was in? As an industrial vehicle, it could have been abused by its drivers, lacked proper maintenance, etc.

I opted instead to use a known good transmission that has shipped with the Jeep Wrangler before. This would guarantee me a proper driveline fit and known compatibility with the Cummins 4BT. Instead of the NV3550, I chose the manual 5 speed AX15 which I was able to obtain rebuilt from a transmission company out in Missouri. It has a good reputation, is easy to have rebuilt, was utilized on the Wranglers for years and provided me an overdrive. Once again, JD Jeeps was a quick source for the requisite bell housings and adapters needed to install the Cummins 4BT.

Drive Train

On a dynometer, my Jeep in stock form peaked at 162 ft-lbs of torque (at the wheels) at around 4600rpm. When supercharged, my Jeep approached 300 ft-lbs of torque which exceeds the Cummins 4BT in stock form which puts out 286 ft-lbs of torque at 1700rpm so I knew my setup could handle the torque. It is not unheard of for modified 4BTs to put out even larger numbers. That kind of power can quickly destroy some of the weaker drivetrains put out by Jeep over the years. Now is the time to install an upgraded driveshaft like a Tom Woods capable of handling greater torque. If you are running anything less than a Dana 44 such as a Dana 35, just stop. The Dana 44 will handle the stock Cummins 4BT but if you intend to modify the motor, opt for a more durable axle from the beginning. Do it right or you'll be stuck on a trail with a bunch of broken parts.

These are the dyno results for my old RIPP SuperCharger versus a stock 4.0L Jeep engine. My driveline could handle the horsepower and especially the torque so I knew it was capable of handling the stock numbers from the Cummins 4BT.


Following any Jeep web forum will reveal the most common advice to anybody after upgrading is to re-gear. If an engine isn't operating within its appropriate power band, its simply wasting fuel (probably bogged down). Re-gearing is absolutely essential to properly running a Cummins 4BT in a Jeep. As per the dynometer charts from above, for a purely stock Cummins 4BT, the sweet spot with diesel is approximately 1750rpm for maximum torque and minimal fuel consumption. If you've already re-geared your Jeep after a tire upgrade, then its highly likely you're turning 2700rpm at 65mph meaning ... you absolutely must re-gear.

The selection of gears will be dependent on your intentions, of course, and will also be affected by your other hardware. Does your transmission have an overdrive? Do you drive 55mph or 70mph? What size tires are you turning? For my Jeep on 35" tires with an AX-15 transmission - I chose 3.07 gears based on the charts below. That puts me in the Cummins 4BT sweet spot for 55mph highways and 70mph interstates.

This chart depicts RPMs at various speeds for 3.07 gears and a 5 speed AX-15 on 35" tires.

This chart depicts RPMs at various speeds for 3.07 gears and a 5 speed AX-15 on 33" tires.

Another commonly overlooked hit to your wallet with regards to re-gearing are the carriers. Since I had already gone to 4.56 gears for my 4.0L and had Ox Lockers ... I basically had to get new locker components that functioned on a smaller carrier. The breakover for gearing is commonly around 3.73 so its highly likely a new carrier will be needed for the Cummins 4BTs gears. Maintaining your existing lockers just pours salt into the money wound.

Air Conditioning

It is possible to retain your air conditioning using a custom bracket from JD Jeeps. It requires a Ford style Cummins 4BT in order to fit properly and a 97-06 TJ. The kit mounts your stock air conditioning compressor on a bracket above the Jeep's stock alternator. It was necessary to use the Jeep's alternator to fit under the bracket. This modification requires that your stock ECU is properly working because it serves as the voltage regulator for the alternator and is required for engaging the air conditioning compressor's clutch. The modification will also requiring a whole new refrigerant plumbing array to handle the air conditioning's refrigerant.

NOTE: Although my mechanic tried to adapt this bracket to my Jeep, it was never installed successfully. The 05-06 ECU would not stay active for voltage regulation. The ECU would deactivate management functions shortly after starting since the sensors could not detect the 4.0L running. I ended up opting for the Cummins 4BT's native alternator which put out higher amperage than the stock Jeep alternator anyway. As a side effect, the Cummins 4BT's alternator was too large to fit under the bracket so my air conditioning was lost.

Electrical System

As mentioned above, the Jeep managed its alternator from the ECU. An alternator requires regulation because as the engine changes speed, the alternator also changes speed. The voltage and amperage produced is a function of many variables such as the field strength and rotation speed. The regulator controls the electric field on the alternator to keep the voltage at a steady 14V. Unfortunately, the 05-06 Jeep Wrangler ECUs would not stay "alive" on certain functions without the 4.0L engine present and providing appropriate sensor inputs. The voltage regulator was one of the components that would go inert. At this point, there was no value in keeping the stock alternator so I kept the alternator that came with the Cummins 4BT and utilized an external voltage regulator.

External voltage regulator for the alternator.

Another expense to a diesel engine is the requirement for a new battery. Due to the higher compression on a diesel, the starters require much more power to turn the engine over. My Jeep's battery wouldn't even turn the engine so I opted for the Sears Die Hard Platinum P2 which has 930 CCA (cold cranking amps). Of course, a P2 battery doesn't fit the stock tray which would require some fabrication as all the aftermarket brackets are still for Jeep sized batteries. I know I've been harping on "doing it right" but I had to resort to using cinch straps to hold the battery down.

Diesels need a stouter battery with higher cold cranking amperage to drive the starter.

Installing the Cummins 4BT

Unless you have a private garage with a hydraulic lift, engine hoist, welding equipment and all the other tools necessary for conducting an engine swap ... you're probably going to need a shop to perform the installation. I chose Mid-Atlantic 4x4 and Speed for my Jeep diesel conversion and was fortunate in that Sam had done a diesel swap into a Jeep before. His prior project dropped a Cummins 6BT and an NV4550 into a Jeep TJ which clearly involved a lot more fabrication than mine so I knew I was in good hands.

After cleaning her up, this Cummins 4BT is looking a lot better.

Unless you bought a remanufactured engine from Cummins, yours may be looking rough depending on what vehicle it was pulled from. It's a good idea to break the engine down, brush off the rust and give it a good painting for future rust prevention. Mask off all the appropriate locations to make sure paint isn't covering gasket surfaces. Make sure you use a high temperature paint for the exhaust manifold.

A Jeep looks so sad with no engine.

Removing the old 4.0L engine is a fairly straightforward process. Make sure you tag all the appropriate hoses and loose wires during the removal depending on what you intend to re-use in the future. A significant number of the wires could be simply cut off at the ECU wiring harness as they'll be completely useless for a Cummins 4BT - wires for spark, throttle position sensor, intake air temperature, etc. Or they can be bundled up just in case you ever go back to a 4.0L. The Cummins 4BT will require a ground, crank hot for the starter, ignition hot for the fuel pump solenoid and the mechanical cable for the throttle.

After realizing the NSG-370 was not compatible, the Jeep looks even sadder with no engine and no transmission.

With everything cleared out of the way and all the requisite pieces on hand (the engine, the transmission, kit components, etc.) it was time to start installing. Using a known transmission for the Jeep was beneficial because it guaranteed the driveline remained in stock locations using stock mounting bolts. It is important to take the time during this phase to properly align the diesel engine with the transmission to eliminate potential binding. With the Cummins 4BT attached and centered, get the engine mounts placed against the frame and marked. Now the welding process should be easy and the mounts from JD Jeeps allow some wiggle room after the fact for getting everything centered and straight.

The engine is finally mounted but there is still a mess of wiring and other tasks to take care of.

In reality, getting the Cummins 4BT installed was the "easy" part. Despite having all the wiring labeled and only a handful of requisite connections to make, getting all the finer details finished took some significant troubleshooting. Once installed, the exhaust had to be custom routed instead of just passed out the side due to the long arm suspension. Then it was time to run the diesel return line back to the fuel tank. Swapping out the NSG-370 for the AX-15 did cause my transfer case linkage to misalign just enough that I could not shift into 4LO without using Novak's cable conversion.

We didn't realize the peculiarities of the 05-06 ECU until certain elements kept failing. For example, it took driving awhile and noticing the voltage slowly dropping to realize the alternator was not working which led to using an external regulator. Running the diesel under load also revealed a number of leaks that did not manifest during the ground starts which required repair. After a few hundred miles, everything had to be checked again to determine which attachments couldn't handle the Cummins 4BT's vibration. Ultimately, the Jeep's conversion to diesel required approximately 120 hours of labor spanning nearly six months for research, acquiring parts and troubleshooting.

About 800 miles after installation, I had to have the crankshaft and third piston replaced ... (the possible price you pay for getting a Cummins 4BT out of an industrial vehicle). Upon cracking her open, we discovered the engine had actually been overhauled somewhat recently with new pistons, etc. The problem arose from a clogged piston cooling outlet that melted - why they're made of plastic I have no idea - which in turn caused a bearing to overheat and melt. After the painful cost of getting that fixed, she's been running perfectly ever since.

The Cummins 4BT Wrangler Experience

An odd point to maintaining a Cummins 4BT that I've never seen anybody mention is oil consumption. The factory service manual actually specifies that normal consumption is 1 quart per 400 miles. That seemed crazily excessive and I never would have believed that without reading it in the manual myself. After enough miles to confirm it, I now count myself lucky that my engine only seems to burn a quart every 900 miles. Keep it lubricated!

Thus far, I have nearly 18,000 miles on my Cummins 4BT powered diesel Jeep Wrangler. After fixing the leak in my VE Fuel Pump, I regularly get 30mpg when I keep the speed at 55mph in 4th gear. At 70mph in 5th gear, the diesel gets 27mpg. I would imagine a 2nd generation Cummins 4BT with electronic management could do even better. But combined with the 31.5 gallon diesel tank in my Jeep, I can make most long road trips without stopping for diesel.

My concern in the beginning was having "dump truck" acceleration - she's definitely slower from a standstill on diesel than a gas engine but it's not that bad. My Cummins 4BT doesn't plume out black smoke like some but at night, you can definitely it smoking during acceleration when the trail vehicle illuminates it with their headlights.

The vibration and noise take some getting used to (they don't call it the "paint shaker" for nothing), but to be honest, it doesn't bother me at all. I was recently in a friend's TJ and thought, wow, this is so quiet! A Cummins 4BT powered TJ feels loudest at speeds below 40mph. Speeds above that, while really the same in terms of engine noise, do not feel as loud (from the engine) because the usual Jeep noises from tires, wind and a soft top start to become a factor. If you make a lot of mobile phone calls, you should consider getting ear bud headphones that you can put a shooting headset over - that makes the calls perfectly easy to hear at the expense of looking ridiculous.


I'm not even going to declare a total cost for converting a Jeep Wrangler to diesel aside from saying it was still considerably cheaper than purchasing a turnkey solution. Hopefully, this article has conveyed that converting a Jeep to diesel properly is not just a matter of acquiring an engine and dropping it in. So if you're looking to do it on a budget, just stop. Either find something else to spend your money on or wait until you have the funds to do it properly. It may only be cost effective if your existing engine is actually dead and you're already facing those sorts of expenses.

The Cummins 4BT fully installed leaving me with a sweet, diesel powered Jeep Wrangler.

Was it worth it? At $3.60 for diesel and 30mpg, I get 8.33 miles per dollar compared with 4.11 miles per dollar on gasoline at $3.40 and 14mpg. Considering that I hear most other Jeep's modified similar to mine were at 12mpg that's more like an improvement from 3.52 miles per dollar. I do tend to get more mpg than the typical driver simply as a result of the huge factor driving style plays into fuel efficiency. It's going to take nearly half a decade, but for the distances I drive and the fuel savings I actually get, this Cummins 4BT will actually pay for itself. Although I haven't done it, Cummins 4BTs are known to run on waste vegetable oil which means it could be possible to really save fuel money with some up front investment into equipment.

300 miles with 3/4 of a tank remaining is pretty sweet.

All that said, the conversion certainly draws attention to you as well. People try to "help you" at the gas station because they think you're dumb for putting diesel in a Jeep. There are also plenty of Jeep and diesel enthusiasts who recognize that unique diesel sound and just want to check out the modification. A Cummins 4BT powered Jeep is simply a really cool and unique vehicle to have.


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