I started getting P-0128 code which said coolant temp not up to temperature.

Attached are photos of replacing thermostat. You can see the seal tear. Photo of removed pieces does not show the larger of two springs. I used a small gear puller and a 12mm socket to compress the spring in order to fit the spring retainer in place. I tried to get a photo of the gear puller in place, but it was a two person job holding everything in place. The large spring was too stiff for me to compress it by hand.

It took almost 45 minutes to get the retainer cross bar in place. I must admit that the frustration of trying several times to get everything in place had me cursing the ancestry of the engineer that designed such a complicated device. Took an hour and a half total. Three bolts secure the thermostat housing in place. The bolt heads were 8mm.

The XJR uses a multiple piece thermostat that was very different from the common one piece unit in my 99 X308

 

Not sure, mine was assembled.

But sure appears to look the same as mine did