The 4.0L inline 6 (I6) engine was introduced in the Cherokee in the late 80s and in the YJ in 1991. The cylinder design is based on the changes made to the fuel injected 2.5L I4 engine. The block and cylinder head are cast iron. It uses 7 main bearings and hydraulic lifters. The HP and torque gains are substantial over the older carbureted 258, but the peaks come at higher RPM. The fuel injected engine performs much better in off-camber, bouncy, and steep situations. It also calibrates better for changes in altitude. Fuel economy is also improved.

A good way to embarass yourself in front of your Jeeping friends is to refer to it as a V6. It isn't a V6, it is an inline 6 or I6. The "V" in V6 refers to the configuration of the pistons in the block. The only two V6 engines ever available from the factory in a Jeep are the 225 "Dauntless" Buick V6 in the late 60s and early 70s and the 2.8L GM V6 in 80s Cherokees and Comanches. If you are shopping for a Cherokee or Comanche, make sure you get the 4.0L and not the 2.8L engine, there is a world of difference.

The 4.0L engine used in the Wrangler from '91 until the present.

4.0L I6
Bore x Stroke	3.88" x 3.41"
Displacement	242 (4.0L)
Compression Ratio	8.8:1
Horsepower (net)	180@4750
Torque (net)	220@4000
Main Bearings	7
Valve Configuration	OHV
Fuel	MPI

The 4.0L I6 that is used in the XJ and ZJ is rated slightly higher than the YJ and TJ 4.0L engine probably due to a different exhaust system.

4.0L I6
Bore x Stroke	3.88" x 3.41"
Displacement	242 (4.0L)
Compression Ratio	8.8:1
Horsepower (net)	185@4750
Torque (net)	220@4000
Main Bearings	7
Valve Configuration	OHV
Fuel	MPI