The Perfect Jeep-Diesel Transplant

Jeremy Taylor’s Cummins QSB3.3-powered 2006 Jeep LJ is a lesson in how it’s done right. Read on.

Stuart A. BourdonPhotographer, Writer

Jeremy Taylor has owned a few Jeeps, and as a matter of fact, this is the seventh. Having learned quite a few lessons along the way, especially from the repower of the sixth—a 1997 Wrangler he swapped a mechanically injected Cummins B3.3 into—he had a new idea for the 2006 LJ Rubicon. This time, he would transplant a QSB3.3 electronic common rail Cummins diesel that would require full integration of the Cummins ECM and the factory 4.0L ECM.

First Steps

Axis Industries in Edinburgh, Indiana, was his first phone call. An adapter system to mate the standard SAE 4 (industrial) bellhousing to the stock NSG370 transmission consisted of a billet aluminum bellhousing adapter that bolted to the Cummins and had an auxiliary bolt pattern mimicking that of a 4.0L Jeep block. The second part of the system was a hardened crankshaft adapter to mimic the axial spacing and bolt pattern of the 4.0L crankshaft. Axis was working on CNC cut and bent brackets for the engine and frame side of the engine mounts; Jeremy chose a hydraulically damped “pancake” style used on early Ford inline engines that provided good isolation for the vibration common to inline four-cylinder engines.

After installing the 4.0L flywheel and a new LuK Pro-Gold stock replacement clutch, he assembled the engine/trans combo on the floor and bolted the mounts together (engine side, rubber mount, and frame side) before hoisting it all between the frame rails. Bolting the transmission mount to the stock belly pan/crossmember that locates the engine and maintains stock driveshaft lengths was next.

The second part of locating the new powertrain happened at the front of the chassis, where an angle finder is used to set the “power angle” of the engine. This is the downward slope that prevents cavitation and loss of pressure as oil flows back toward the sump under acceleration. Because the TJ Wrangler was a driver-side drop front axle arrangement, the stock engine was offset to the passenger side to allow for front driveshaft clearance. With the QSB3.3 still in the sling and the centerline matching that of the removed 4.0L engine, it was just a matter of tacking and burning in the frame-side brackets to the cleaned chassis rails.

Accessory Placement

Getting the FEAD (front end accessory drive) configured to use the stock 4.0L alternator and power steering pump that were chosen for availability and easy factory integration was next. Again, Axis had billet aluminum pulleys and brackets to mount the stock Jeep components and convert the old-fashioned V-belt to a tensioned serpentine system.

Next was the fuel supply. The stock stainless steel 5/16-inch feed line was reused and plumbed into the Cummins integrated fuel filter at the engine side. The return line was a vapor vent line from the 4.0L fuel injection system. At the fuel tank side of the chassis, a FASS fuel systems fuel pickup kit was cut into the topside of the plastic tank and provided a pickup and return separate from the stock fuel pump. Knowing that diesel fuel would degrade the stock fuel pump, Jeremy removed it from the fuel pump cartridge, leaving only the fuel level sender on the cartridge. Using available AN-to-Mopar quick-connect fittings, the stock fuel lines were connected to the new FASS pickup.

Check Time

At this point the tub was placed back on the Jeep to check for firewall clearance. The front grille had to be modified, and the stock A/C condenser removed to fit an air-to-air intercooler. The 4.0L radiator is adequate for the little Cummins so a stock Mopar unit was sourced, and Jeremy began work on the coolant plumbing, utilizing a pre-formed upper hose from an early 1990’s Honda Civic, and the stock lower radiator hose from the 4.0L. The lower hose required an internal spring to hold the shape of the hose and keep coolant flowing. The QSB3.3 had ports for auxiliary coolant temperature and flow ports, and after looking at the engine’s coolant flow diagrams, Jeremy picked up the inlet/outlet and the adapter from the pipe thread fittings to barbed hose connections for the auxiliary coolant temperature and flow ports. The stock heater core lines worked just fine with a little bending and forming to match the lines of the new engine.

After fluid came air. The fenders and inner fender wells were installed to again check for clearance. A couple different mandrel bent tubing pieces were used to lay out the charge-air piping from the turbocharger outlet to the intercooler inlet, and intercooler exit to intake air horn. Some cutting and tacking eventually led to welded and leak-checked pipes that were then sent out for powdercoating before re-installation.

Wired Up

The final engine integration was the most time consuming, and after pouring over wiring diagrams from a Chilton’s manual for the Jeep Wrangler and the included Cummins wiring diagram, the two systems were wired together. The end goal was to fool the Mopar ECM into believing the stock engine was still in place. Common sensors such as water temperature and oil pressure were duplicated through both ECMs, while a tachometer output from the Cummins ECM was piped into the Mopar ECM circuit normally receiving the camshaft position sensor. This was critical so the Mopar ECM would believe that the stock engine is running by having it verify a signal from both the main and backup speed sensors, or crankshaft and camshaft speed. This duplicate sensor setup can be eliminated by splitting all of the Cummins sensor circuits, but the dual-sensor method is more robust due to duplication of signals and hardware.

The last key component of the re-wire was to get constant power, key power, grounds, and a throttle position sensor wired into the Cummins ECM. Jeremy chose to keep the stock pedal and cable assembly, using a throttle position assembly from an earlier Dodge Ram to change the mechanical pedal motion to an electronic signal. Then the stock Jeep starter solenoid circuit was extended and connected to the new Cummins starter, ensuring that the stock Jeep ignition switch would actuate the cranking mode on the new engine. Once the electronics were right, there was the tedious process of eliminating unnecessary original wiring.

Details, Details

After a quick test fire to ensure the engine was operable in the chassis, the final details to guarantee the installation was reliable were next. Because diesels don’t create engine vacuum, Jeremy had to use an alternative to provide brake boost. An OEM-grade electric vacuum pump and switching circuit used on late-model Chevrolet Corvettes, Dodge Ram Ecodiesels, and even the Jeep JK chassis (technically a Hella brand UP model pump) provided quiet operation due to its rotary design versus a diaphragm-style aftermarket pump.

One nice feature on the Cummins ECM is the ability to control up to two electric fans that turn on at pre-set coolant temperatures. Jeremy used one of these outputs to control the aftermarket electric fan/shroud combo attached to the Jeep radiator. Jeremy told us that even during slow-speed crawling in the heat during the Moab Easter Jeep Safari, the stock cooling system’s fans never kicked on and temperature control was flawless.

With air, fuel, coolant, and vacuum tackled, the final component was exhaust. The exhaust system for this Jeep was created with the same cut-and-tack system as the intake piping, using sections of pre-bent mandrel tubing carefully pieced together to maintain good linear flow. An aftermarket muffler for a four-cylinder engine was used, keeping the rumble in check so as to not override the perfect pitch of the turbocharger while spooling.

The difference between a Cummins engine and any other automotive-specific engine is the communication language employed in the ECMs. Cummins sells into a vast variety of OEMs and commercial manufacturers, thus employing the industry standard J1939 communication protocol. Chrysler, on the other hand, will use proprietary messaging since it’s always its engine talking to its chassis. As an added assurance, a J1939 CANbus gauge was chosen for any diesel specific monitoring (in addition to the stock fully functioning gauge cluster). Ametek Inc. was chosen for their compact 2G gauge that offers a built-in yellow check engine lamp, red stop lamp, and ability to review active and inactive fault codes, as well as data monitoring such as turbocharger boost.

Finally, we would be remiss if we left out the last detail of Jeremy’s successful conversion…the fuel filler neck. It only takes the first trip to diesel pump to realize that the diesel fill nozzles are larger than a gasoline nozzle. To accommodate this, the fuel filler neck has to be removed and modified to accept the new go-juice.

Why This Jeep

When we saw this 2006 Jeep LJ Wrangler was not only super clean and sturdily built, but also had what is likely the sweetest diesel transplant we’ve seen in we can’t remember how long, we were so excited we almost forgot to ask Jeremy Taylor if he could spare some time for a full feature photography session. Who wouldn’t like a Wrangler with a small-displacement diesel that purrs like a kitten and runs like a lion? We learned a lot talking to him about the rig, and we hope you have too. Now, if the Jeep execs read this and learn something…

Hard Facts

Vehicle: 2006 Jeep LJ Wrangler
Engine: 2012 3.3L Cummins I-4 diesel
Transmission: NSG 370 6-Speed Manual
Transfer Case: NP241 OR
Suspension: Clayton radius long arm, AEV 3.5-inch progressive springs, Bilstein 5125 12-inch with custom shock mounts (front); Clayton double-triangulated long arm, AEV 3.5-inch progressive springs, Bilstein 5125 10-inch with custom shock mounts (rear)
Axles: Rubicon Dana 44, Ten Factory allow shafts, Moog bearings and ball joints, 3.73 Rubi air locker (front); Rubicon Dana 44, Ten Factory alloy shafts, 3.73 front Rubi air locker (rear)
Wheels: 17x8.5-inch Walker Evans 31 Series beadlock
Tires: 37x12.50R17 BFGoodrich Mud Terrain KM2