man I am a inline 6 fan for sure. The 4.0 motor is probably the greatest of them because it has all of the great things the 225 slant 6 and the 258 4.2 had. plus like he said computer controlled, and fuel injected. I have put over 200k miles on all of the 4.0 motors i have had and no complaints.. easy on the high revving you can break a flexplate and around 200k you will probably need main bearings. thats about it.. oh and about the better ride and fuel economy from factory i disagree with. both times i have built a XJ I have gotten better ride and gas mileage. mainly because of lift and gear choice. rustys offroad 4 1/2" kit with springs instead of AAL made my XJ ride great with 32" BFG m/t. I put 4.10 gears in and got 18mpgs highway. I then installed a K&N cool air intake with 8" filter. Bought a flowmaster exhaust kit from rustys for $100 bucks and the throttle response and gas mileage were great. It pulled alot harder from 3500 to 5500 rpms. and in the end I ended up getting 20mpg highway..