The 1987-90 YJ Wranglers launched the EFI era with throttle body injected 2.5L AMC four-cylinder engines. The legendary 4.2L inline sixes remained carbureted through 1990. In 1991, Chrysler switched to MPI on the 2.5L engines and replaced the 4.2L with the XJ Cherokee's popular 4.0L MPI inline six. The '97-'06 TJ Wranglers offered the pushrod 2.5L and, in the last years of production, a spunky DOHC 2.4L four as the base engines. The majority of TJs received the 4.0L inline six option.
The 2.5L four, derived from AMC's rugged inline six-cylinder pushrod designs, proved its exceptional stamina for two decades, powering many CJ models, Wranglers, and XJ Cherokees. Despite lower peak torque and horsepower than the sixes, the 2.5L is a peppy engine, easily handling most low range four-wheeling chores. The 2.5L develops its peak torque and horsepower at a higher, less efficient rpm than the 4.2L and 4.0L sixes. This is most noticeable when climbing grades or at higher altitudes. Under such conditions, the four-cylinder often uses more fuel than a six-cylinder engine! Large tires, a winch, a hardtop, gas cans, and hefty aftermarket bumpers can inflate the curb weight of a YJ or TJ Wrangler by 700 pounds or more. At that weight, or if you plan light towing, the inline sixes do a better job.
Given the ready availability of recycled '91-up 4.0L MPI six-cylinder Wrangler and XJ Cherokee engines, converting a YJ or TJ to a six might sound practical. However, such a swap presents a major challenge: the frames are different between four- and six-cylinder models!
The Wrangler departed from the earlier CJ frame design. For the AMC-era CJ four-, six- and eight-cylinder applications, each year and model uses the same frame. The CJs use
bolt-on frame brackets attached at different sets of frame holes. This allows engine interchangeability--unlike the YJ or TJ Wranglers. YJ and TJ frames have the engine mounts factory-welded in position for either a four- or six-cylinder model.
Once the engine mounts have been corrected, the four to six swap will also require new fuel lines, the correct MPI fuel pump, a six-cylinder MPI wiring harness, and a 4.0L engine computer. One approach is Mopar's EFI Conversion Kit for `81-'90 4.2L sixes. This kit uses original equipment 4.0L MPI components and provides a new, easy to attach engine wiring harness, a coil, the computer, a fuel pressure regulator, all new sensors, injectors and fuel rail, a throttle body, an air cleaner, and an intake manifold. The Mopar package offers a user-friendly option to the recycled 4.0L Wrangler or XJ engine's used computer and wiring harness, which requires diagrams and splicing into existing chassis harnesses.
These swaps require 4.0L six-cylinder exhaust components, transmission mount and plate changes, and possibly some driveline changes or floor pan modifications. The radiator, engine fan and fan shroud must be changed to six-cylinder types, and an air-conditioning type radiator is always advisable. Before undertaking this swap, consider the costs, labor, and needed skills. Selling your four-cylinder YJ or TJ and purchasing a clean 4.0L Wrangler may be a reasonable alternative. Do your homework. Before plunging, check out the steps involved!
No Easy Task:
Changing a 2.5L or 2.4L four to a 4.0L MPI six requires tools, welding skill and attention to detail. The project is comparable to a V-8 swap. The option of subletting this conversion to a shop could prove costly and tie up your Wrangler for a lengthy time. This is not a weekend task!