Troubled years ran from its debut in 1987 through the 1990 model year, when the Jeep 4.0 was equipped with an engine management and fuel injection system made by Renix, a partnership between Renault and Bendix. Although previously used on the 2.5L — another highly reliable old Jeep engine — the system was more complex than the Mopar computer and FI system that appeared in 1991. The Renix system had a knock sensor, and the computer managed engine timing automatically, although this could be tweaked slightly by adjusting the mount points of the crankshaft position sensor. Furthermore, the 1987 Renix 4.0 was the least powerful version of this engine, producing 173 horsepower and 220 pound-feet of torque. The next year, higher-capacity fuel injectors boosted output marginally but did nothing to expel the electronic gremlins.
The Renix system was unable to store engine fault codes, making troubleshooting difficult. Chrysler abandoned it beginning with 1991 models, replacing it with an in-house engine management and fuel injection system. The intake and exhaust manifolds were also redesigned, and a larger throttle body and higher capacity fuel injectors went atop the intake.
[Featured image by CZmarlin via Wikimedia Commons|Cropped and scaled|Public Domain]
