Check to see if your Jag has an auxiliary cooling pump. It may be bad and has created an air bubble in the system. This happened on my XJ8. Same symptoms. We replaced the pump, and flushed the system, especially the heater core. Everything in the system needs to flow and circulate. Also, make sure you don't have a cracked thermostat housing. In the later models they are made of plastic and have a reputation for cracking. Lastly, be sure you bleed the system after any work. This is imperative. Best of luck.

Rob
XJ8 VDP 2004
ex: XJ6 1996

 

I'd like to throw in another vote for the fan clutch. I just replaced the fan clutch on my '88, and the difference is night and day. The temp gauge doesn't go past the top of the N now.

 

One of the causes of overheating is the centrifugal advance in the distributor has a habit of seizing up. To check it pull the distributor cap and see if you can turn the rotor about 1/8 turn. If not it might be seized.

 

Series 3 E Type V 12 overheating had become a chronic problem until we did the following:

1. Replaced the factory original radiator core with an aluminum radiator from Wizard Cooling. Radiator shop will do an inspection on the original core.
2. Replaced the original fans with new higher volume electric fans (retaining the original shroud)
3. Sealed the top of the shroud across the bonnet - blocking shroud bypasses to the original air intake snorkels (raising the air pressure forward of the radiator/ increasing air flow)
4. Ducted the port and starboard fresh air intakes directly into the air cleaners (will add some photos)

Now it runs normal/subnormal temp even hard running.