Cold header tank? 4.0 litre facelift car

[CliveRothwell]

OK, so a while ago the car seemed to lose a header tank of water after a 300 mile run. Topped it up and noticed that the expansion tank was cold, even after a run of 20 miles to work. It continued like this for a couple of months. but no further loss of water. Still concerned, thinking it might explain a lowering of the MPG figure ( from 25mpg down to 23 mpg) - as if it was running rich, thinking it was cold so it enrichened the mixture - I replaced the thermostat for a new Jaguar part. No change to the header tank. I then replaced the pressure cap on the header tank and again, initially no change even after several miles. I switched it off and went for the book again. A final check on the header tank, and it was HOT at last! So, any thoughts anybody? I have always assumed the header tank / expansion tank had to run hot and was sealed by the pressure cap, under pressure, to increase the boiling point. So why didn't the changing of the pressure cap work before the car was switched off?
Over to you for comments please!

Cheers

Clive

[Brakebuster]

this could be a simple airlock of the system ,

or signs that the pump is not working correctly , have you checked the drive belt and its tightness onto the pump ? and or system pressure when running ?

BB

[steve_m]

23 mpg.
Luxury!