It's the crank sensor!
Join Date: Nov 2007
Location: Central Minnesota
lifted92xj is correct. It's going somewhere.
Any sweet smelling white smoke coming from your tailpipe? Check your oil for anything milky and a high level on the dipstick. You can have the coolant sniffed for the presence of hydrocarbons as well and yes, DEFINITELY pressure test your system. My money is on a bad headgasket sorry to say....
It may be worth looking at your spark plugs, if one is much cleaner than the others that's a surefire coolant leak cleaning it.
Good luck and post back what you find.
P.S. Your 2000 has heads that are more prone to problems as well.....they started using a different head in post 1999 XJs which I believe that some people have trouble with cracking. Keep that in mind. I believe the number on those heads is the 0331.
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!