My jag is 23 years old 45,000 miles runs like new all of the time. One exception from time to time the temp gauge buries needle at super hot. Put in about 1/2 cup of coolant and all is well till it happens again. Seems to me thermostat problem but could use some more knowledgeable advice.

 

You should probably check out the X100 forum. This section is for the 2006-onward, aluminium cars.

 

How often do you have to put in the 1/2 cup of coolant? Are you filling the black tank right to the bottom of the neck? You need to keep on doing that until it stops going down as it sounds as if your atmospheric expansion tank in the wheel arch is empty. This is not unusual and you will find threads in the X100 forum about how to top it up directly. If repeated fillings do not address the shortfall, it sounds as if you have a coolant leak but it's not enough to lead to overheating which might be caused by a faulty thermostat but is more likely to be caused by a faulty temperature sender. You presumably appreciate that the temperature gauge in the dashboard is not really a gauge at all.

Your post is in the wrong forum so you need to move it for more appropriate advice.

Richard

 

Could also be air trapped in the circuit and a leak, even small, can cause overheating as the circuit's not sealed and the seal's (pressure) what keeps temps down.