I searched all the posts for coolant leaks but did not find my exact problem.
Mine is leaking at the thermostat housing or the housing the thermostat goes into, both are plastic. I can not find the "valley pipe" not sure if 2000 4.0 has one.
I thought about just removing the thermo housing and replacing the gasket but I thought I would get some advice here first.
I did not pressure check, but pretty confident one of these two are leaking. Leak stays in this area in attached photo, You can see the orange puddle below the housings.
Is there a special seal around the thermostat or is there an o-ring?

 

https://www.autohausaz.com/pn/547_20...saAtMxEALw_wcB

https://www.ebay.com/itm/Jaguar-S-Ty...-/271180234404

Rock Auto has one for $40.99 (URO brand)

The AJ26 or AJ27 for the X100 or X308 will NOT work. (The air bleed fitting is different)

MAKE SURE IT IS FOR THE AJ28 4.0 X200 engine.

 

Thanks, I will order and replace.

 

Got the new aluminum thermostat housing install and all looks good no leaks.
The two back bolts were a bit difficult to get out.
I didn't have 8mm crows foot so I bent a wrench that worked out great.

 

I replaced mine a few years ago with aluminum. No issues there. The worst coolant leak comes from the overflow tank under the cowl cover. Not a good design and replacing it is a bear. I've done both. If you see water coming out behind the driver's front wheel where the drain is and it is not raining or just washed, check it for antifreeze. If you touch the water on your tongue and you taste AF, then your tank is most likely cracked. If it is, do NOT go cheap Chinese for a replacement.