The 1987 RENIX 4.0 made 173 hp (129 kW) and 220 lb·ft (300 N·m) of torque. In 1988, the 4.0 received higher flowing fuel injectors, raising output to 177 hp (132 kW) and 224 lb·ft (304 N·m). In comparison, "this engine made more power than the Ford 302 V8, Chevy 305 V8, Chrysler 318 V8, as well as any of the 6 cylinder engines the Japanese were putting in their trucks... and it had comparable or better fuel economy."[5]
In 1991, Chrysler redesigned the RENIX engine control computer and raised the intake ports approximately .125 in (3.2 mm) for a better entry radius. Chrysler also enlarged the throttle body and redesigned the intake and exhaust manifolds for more efficiency, and the fuel injectors were once again replaced with higher flowing units. The camshaft profile was also changed. The net result was an engine that made 190 hp (140 kW) and 225 lb·ft (305 N·m) of torque. Badging on most Jeeps equipped with this engine read "4.0 Litre HIGH OUTPUT." The new cam profile combined with altered computer programming eliminated the need for an EGR valve and knock sensor, but made the engine more sensitive to alterations, especially where emissions are concerned.