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!
