by Larry Shepard

(taken from Jeep 4.0 Engines: How to Rebuild and Modify)

In the early 1990s, Mopar Performance engineered two kits that used the 4.0 hardware to convert the earlier 1981–1990 258 (4.2) engines to multi-point fuel injection, which offers obvious advantages for off-road activities. These fuel injection conversion kits have been very popular since their introduction. If you are interested in this conversion I strongly recommend that you go to the Hesco website; they have a detailed instruction sheet plus helpful tips. 

A lot of parts are in these kits, making them very complete. These kits were based on the 1995 4.0 hardware, which is now 18 years old and may no longer be stocked by Mopar Performance or Mopar/corporate dealers. Hesco appears to cover the gaps.

This graph compares the stock 4.2 engine (or 258 engine) with a production carburetor system and with a fuel injection kit. The peak horsepower number isn’t as high as the 4.0 engine’s because the basic engine was left as a production 258.

Torque is actually more important for off-road engines. This torque gain is pretty spectacular when you consider that the 258 engine itself wasn’t changed, only the fuel injection was added