I have 2 XJ's, a 1995 and 2001. I get great MPG using them and can feel the difference. Also I have them in my Harleys. No doubt you can get a Bad e3 Plug in a heartbeat. With everything being made in China and they do their best to save money for their pockets and cut quality. My friends have experienced this issue with Champion and others. The first set I put into my 1995 XJ I got a bad one in CYL #1. I drove the Jeep to O'Reilly's Auto-Parts and got a brand new set, no questions ask. They have 100,000 mile warranty. On my Harleys I found the Tips coming loose like a lot of other Harley Riders have had happen and I called e3 about it. They ask if I would send them the Plugs so they can research this issue. I told them, I have Red Loctite the tips, they are now fine. The only thing I can figure out is the High Performance Harley Coils Fries the compound holding the tip on, to much current.
The 1995 XJ after 50,000 miles on the second set and the 2001 XJ after 30,000 miles no problems.