I don't doubt there is much info out there on XK8 coolant temperature gauge readouts. I realize the indication is massaged and if "normal" it reads in the "middle", etc. I have seen it pegged a couple of times when I lost coolant for one reason or another, the indication coming simultaneously with other panel warnings.
I replaced the thermostat this summer (the old one was about 6 years old), along with fluid and radiator hoses. Coolant level is perfect.
I have a feeling that the thermostat is either defective or I may have gotten one with a different temperature range. The coolant temperature gauge takes longer than before to come to the "middle" normal indication. When cruising at highway speeds, the needle actually backs off the middle and sags almost 1/4 of the way down to "cold". Once in stop & go traffic, it goes back to the middle. Now that it is winter, the few times I've had the car out on a cold day, the gauge almost never gets to the middle. Any thoughts or recommendations?

 

Have you tried replacing the coolant temperature sensor? Mine had similar problems with the temperature gauge going cold at highway speed and then going back to the middle when I slowed down to city speed. I replaced the cross over pipe with the new sensor already installed and haven't had the problem since. Easy job, took about a half hour and the pipe with sensor cost about 50 dollars Canadian. Part #AJ85885. David Yau posted a great video on how to do the job,
.

 

I bought a Bluetooth OBD I I that I run through my phone. It gives me real time temperature read outs as well as several other readings. Works well for me

 

which one did you buy ?

Z

 

I bought a cheap plug-n-play OBD display to go on the dashboard. For the price, its surprisingly good.




https://www.jaguarforums.com/forum/x...-gauge-210411/

 

I got one called Blue Driver. Easy to hook up and down load the app. I can plug it in and go for a nice long cruise and I can read the real time temperature on my phone display. It will also download and graph the pertinent data for later viewing. Works well with my 2001 XKR.

 

I have the Autool X-60, very similar. Most of the time they are a good alternative to hooking up a bulky reader

 

I also have the Autool x-60, now installed in a couple of my cars. I mounted mine in the Jag on the left side of the instrument cluster, near the A-pillar, so it is out of the way yet easily readable. My interior is Tan, so I painted the A-60 to match the dash. I like that you can also read and clear codes with it, and you can leave it plugged in all the time. Won't read as many codes as Torque or other apps, but get's the basic job done. You can also set warning tones, so if the temp hit's a certain number, you will get a warning.