Finally got around to swapping out the 2008 XKR's thermostat which I believed to be the cause for the code P0128 - Coolant Temperature Below Thermostat Regulating Temperature.

First, went to local auto parts store and got a thermostat. Item was completely wrong. The XKR's thermostat housing acts as the mount for individual thermostat parts (plunger, springs, etc.).



So the OEM kit is recommended... (new parts on left)



I also got a new OEM o-ring. I was skeptical of needing a new one, but compared the old and new and there was a substantial difference in width (New on the left). I'd recommend replacing this as well.



To get the old thermostat parts out, I used the palm of my hand to push down plunger and spring and turned until the thermostat cross brace cleared the tabs in the aluminum housing. I'd recommend taking the housing completely out of the car. (3) 8mm bolts holds it on one side and a hose on the other. It can be done leaving the hose attached but is easier if completely removed.

Upon inspection I could see that the plungers rubber seal was compromised. This was preventing the thermostat from closing completely.




Replacing the thermostat parts was pretty simple after paying attention to orientation of parts when removing old pieces from the housing. I found the easiest way to get the cross brace back into the tabs in the aluminum housing was to press down with the palm of my hands again and twist. There is a little resistance so I aligned with a pair of pliers.



I cleared the code and all seems well. I will monitor for a few hundred miles to be sure code does not return.

Not sure how much this service would cost, but it is pretty easy to DIY.

 

Well done!
You're lucky with the XKR, as the housing is metal. With the XK its best to replace the whole plastic housing at the same time as the thermostat.

 

Yes, in my original research I read a lot about the plastic housing. Was happy I did not have to buy it as it would have been an added expense.

 

Sorry for the late followup, but after a bit of driving over the past few months, code has not returned.

I probably should have replaced all the coolant when doing this. Car has about 42K miles so still pretty young and service was up to date when I bought it. I am worried about coolant longer term if it is dexcool. I may just flush and replace with better alternative.

 

Just did mine today. No signs of problems but it was time to do a coolant drain and fill and figured at 54K miles and nearly 10 years - where did the time go?- it wouldn't be a bad idea for little cost to replace the petcock, reservoir cap, fill washer and thermostat & seal. Didn't take any pics but even though it appeared to have been working correctly (no temp issues) the gasket showed age/wear and the rubber on the thermostat was indeed compromised as seen in the pic above.

 

I'm at around the same mileage and plan to flush and replace coolant. I had a thermostat issue that was throwing a code and fixed that a while back. Would have been a good time to do it then, but I needed to get it on road quick.

I am not planning on using dexcool and will use an all makes coolant. I had dexcool in another car that had higher mileage and it was starting to gel up. What do you plan to replace coolant with?

 

I used dexcool, always have in Jags. Never any issues.