Comanche MJ: The 1986-92 Cummins-Powered Jeep

The 1986-92 Jeep Comanche MJ was one of Jeep’s better ideas. Essentially an XJ Cherokee with a pickup bed, the Comanche even had a factory diesel option for the 1986 and 1987 model years, though today those engines are about as rare as hen’s teeth.

Donald Bankston loved the Comanche but knew that even if he could find one with the original Renault-sourced 2.1-liter turbo diesel, its 85 horsepower wouldn’t be nearly enough to satisfy him. Like many Jeepers and dieseohaulics, Bankston’s philosophy is “if you can’t buy it, build it!” He paid $200 for a gutted 1992 4×2 MJ from New Mexico and set to work.

The Comanche’s limited engine compartment space left few choices for a diesel powerplant. Bankston focused on the Cummins 3.9-liter 4BT four-cylinder because of its compact size, simplicity, easy adaptability, and the fact that it makes good power with room to grow. The Cummins fours most commonly seen in vehicles are the 105-hp/300 lb-ft 4BT and the 120-hp/325 lb-ft 4BTA. The primary difference between the two is that the 4BTA has an intercooler (the “A” stands for aftercooled) and uses higher fuel rate settings.

Bankston scored a 4BTA but had to buy a 1987 Grumman-bodied GM chassis box van to get it. The engine was a 1993, indicating the van was one of the many that had been repowered. (Here’s a clue for 4BT seekers: many 1980s box vans on GM chassis originally mounted GM 6.2L V-8 diesels, but the commercial workload proved too much for them and many died young. A cottage repowering industry emerged and the Cummins 4BTA was a popular choice as it was powerful, economical and durable enough for the job—not to mention small enough to fit under the doghouse.)

The beauty of the 4BTA is that it’s basically a 6BT with two fewer cylinders. The mountings are the same as the 5.9L Cummins, or nearly so. Bankston used the rear adaptor plate, flywheel and clutch from a Dodge 5.9L application to mate his engine to an NV-4500 five-speed from a 2000 Dodge 3500 4×4. The rubber engine mountings are first generation Cummins-Dodge pieces, and Don fabricated the chassis brackets from 3/16-inch plate, forming them to fit along 9 inches of the front frame rails on each side. He took care here because the Comanche MJ combines the front unibody section of a Cherokee XJ from the firewall forward with a conventional ladder chassis in back. The fabbed mounts spread the 800-lb weight of the engine over a larger section of chassis, using many of the original mounting spots and a few new ones. That ensured no problems due to weight or torque.

The 4BTA is a tall engine. No firewall or radiator support modifications were needed, but in order to simultaneously close the hood and have the oil pan clear the front axle and track bar, Bankston needed 4 ½ inches of lift. He reckoned the oil pan and oil pickup could be modified, but that would reduce oil capacity. Besides, he wanted enough of a lift for 33-inch tires.

The exhaust system is 3-inch mandrel bent from the turbo back with no muffler. Bankston bought several tailpipes for 1998-up Dodge ¾-ton vans at $20 each. This gave him all the bends and straight sections he needed to cut, weld and fabricate his exhaust system.

To keep the Comanche Cummins cool, Bankston used a three-core HD cooling aftermarket radiator in the original mounts with a set of triple 9-inch electric fans. Bankston reports the engine runs cool as a cucumber in all conditions.

Wiring a mechanically injected diesel is easy and Bankston was able to re-use the donor truck’s original Delco 15 SI alternator and starter. The chassis wiring was a bit of a problem because of the electronic speedometer used in 1989 and later Comanches. The answer was simple: Bankston pulled the wiring harness and mechanical speedo from a rust bucket MJ parts vehicle.

Bankston was justifiably worried the original Jeep NP-231 transfer case wouldn’t hold up, so he built a heavy-duty NP-241 series case from new and used pieces, incorporating a 1 1/2-inch chain from the Dodge NP-241D, a six-pinion planetary set and 32 spline shafts. (The transfer case alone probably deserves its own article.)

The axles are the stock units, with a D30 high pinion converted to 297-size shafts and U-joints in front and a Dana 44 in the rear. Unfortunately, these lightweight axles haven’t held up well to the Cummins’ torque and Bankston’s heavy right foot, so they will soon be swapped out for Dana 60s. But the 4BT likes the 3.55 gears, and Bankston is happy with the 33×12.50-15 Goodyear Wranglers, which he has mounted on 15×10 Eagle alloys.

Once the engine was installed and the shakedown tests completed, he began tweaking the little Cummins. At present, it’s making a dyno-tested 162.4 hp at 2,374 rpm and 393.4 lb-ft of torque at the rear wheels. In a 4,050-lb truck, that’s “yeehaa” territory. Bankston has a hard time not boasting that the power bump cost him only $85. He simply installed a 3,200-rpm governor spring in the Bosch VE rotary pump, turned the fuel up and swapped out the original 16cm3 exhaust housing from the Holset H1C turbo with a used 12cm3 housing from a Dodge 6BT. The new setup spools a lot faster and makes about 30-psi maximum boost.

One of the best parts about the conversion has been the fuel economy. Driving like a bank robber on the run, Bankston has recorded a low of 24 mpg. Driving like Scrooge with a license, he’s recorded 36 mpg. His usual average in mixed driving is 26-28, including lots of short hops. So far, he has put some 20,000 miles on the conversion with no trouble. Don says the conversion is easily doable for any MJ or XJ Jeep owner with moderate wrenching skills. When asked for specific advice to prospective swappers, Bankston replies dryly, “Don’t test-run a diesel on the floor… they’re hard to catch!” DW