Hello! I recently purchased a 1998 XJ Sport (4.0 I6, Auto, 4x4) The original motor was warped crazy and so I had it replaced with another 4.0 I6 from a 1996 XJ. This 1996 motor has 160XXX and the 1998 body has 130XXX. Upon start up, which only happens after a rough and long crank, it will idle relatively fine, and will advance to about 2000 RPMs and then sputter and backfire very rapidly. The spark from the plugs, which are new, is also relatively orange. Compression is good, however cylinder 3 is about 105psi (the lowest of them all) and to my bewilderment, the original engine which had bad compression showed the lowest in cylinder 3 as well. Please any and all help would be appreciated! Thanks.

 

First of all, your spark should be bright blue. Any other color will cause a lot of problems. We need to know what the compression is on all cyl. If I'm not mistaken the compression should be about 135. But, don't hold me to that.

 

Compression should be between 120-150 psi. The acceptable range of difference should be no more than 30 psi from what I have been told although I like no more than 20 psi. I would also do a wet test on cyl number 3.
With some of the problems that you are having and seeing that you just did an engine swap I would double check and clean all your grounds.