Jeep started producing the venerable 4.0 in 1987 as an upgrade to the previous 4.2L inline 6-cylinder design. The basic block dimensions lasted until the demise of the TJ Jeep in 2006… that’s damn near 20 years! These 12 valve gas pots can be described as a thick skinned, reliable workhorse with a fan club that can only be described as “devote”. The fans though, will attest that while the design of the basic short block is rock solid, its ancillary equipment leaves a lackluster sheen. The factory exhaust systems are what bring the 4.0L down a notch, but we’re going to raise it back up.