Firstly many thanks for everyone's Camber/Caster guidance. Got a selection of shims and will head back to the alignment shop.

I've had my 1985 XJ6 since 2016 and am stilled thrilled each time I drive it (Spring/Summer/Fall but never in the rain). Even on really hot - plus 28-30C days, the needle never rose above 180-185F except for the occasional hot soak startup after a highway run, but once started, the temps would drop back to the 185F mark. The car has never, ever overheated at all, retains a constant fluid level and has no coolant leaks and Oil's clean also.
This year was no different, with the exception that recently, with ambients around 5-10C, she takes forever to reach even 150F and if highway driven, will never reach the usual 180-185F mark. So I'm assuming I've got a thermostat stuck open (fail-safe type), unless It never had a thermostat to start with and I never drove it in those lower temps, which I'm pretty sure I have done.

So I've ordered a "high performance" 180F thermo

And a 180F "fail safe"
https://motorad.com/products/?partNumbers=7228-180

Which one would be preferable to install?
Note the rad fan seems fime when performing the "hand spin/stop" routine and is not locked "on".

 

Neither seem to have the jiggle pin feature, which you want.

I just installed a MotoRad 233-180JV and it works fine. The jiggle pin can be seen in this pic:



Use extreme caution and patience when removing the bolts to the thermostat housing. They're often seized. Go very slowly with lots of back-and-forth. If you break one it isn't a crisis but, still, you'd rather not have to deal with it at all

Cheers
DD

 

Furhter to Doug's point, the jiggle pin is in a small hole, ,it is the hole, not the pin, that is important, as it allows air to bleed into the radiator and thus out of the cooling system when the thermostat is closed.
If you have got the thermostats, then you can just drill a 1/8th inch hole in the rim, inboard of the sealing rim.
The thermostat must be installed with the hole uppermost.

 

I would first check if the thermostat is stuck open, you should see it if you get it out. you also can test them with a thermometer and water heated in a pot.

On my v12 i had exactly the same problem, turned out my system it was not proper bled.

Faulty temperature sensor can be a problem too.

 

The other thing that is vital on a V12 is to make sure the foot on the bottom extends far enough to close off the bypass port. That along makes a tremendous difference in operating temperatures. When hot, you want a distance of 42mm from the foot to the flange of the thermostat. The OEM Waxstat brand typically only go to 38-39 and that's not far enough.

The ones for a 1995 Ford Crown Victoria with the 4.6 engine are the correct size.

See a photo here: https://www.jaguarforums.com/forum/x...8/#post2725628

Edit: Just noticed the car is a 6 cylinder not a V12. However, the same thing applies, both engines take the same thermostat and have the same distance to the bypass port. However, the 6 cylinders do seem to be less prone to overheating than the V12.

 

And down here, and some other places.

Dayco DT18A. Suits Ford and many other brands.

I have attached the PDF, and yes, I know its V12, but the stat specs are the same, only you got one on a 6cyl.