Hello,hoping someone can help, 2003 x type petrol, loosing 2 litres coolant every 20 miles,no visible sign of a leak! The thermostat and housing have been changed and also replaced the reservoir filler cap.
The engine oil is clean and oil filler cap totally clean and no missfire from the engine.
Any thoughts gentlemen please?

 

I would hazard a guess, it's water pump. They seem to be pretty hard to see leaking while driving. Stationary idling should show it (at temperature).

 

This is based on your statement of "no visible signs of leak" = the normal reservoir failure.

 

Thank you for your reply, the water pump area looks completely dry even at temperature and running, do you have any more thoughts please?

 

That's still A LOT of coolant in that short of a distance to not see any signs of a leak anywhere. Maybe try having someone rev it up a bit while you are looking around the engine bay. Looking at both sides of the radiator, the water pump, the top nipple om the reservoir and the bottom of it, thermostat housing, hoses and such. A leak like that is too big not to show itself.

Maybe also check with the bottom cover removed. It would make any leaks or drips more apt to hit the ground and maybe easier to follow to the source.

 

On my 2002 I discovered a leak right in the bottom of the reservoir,
its plastic you know.
You did'nt state how many miles on the car.

Walter

 

I think the reservoir failure on so many Jags is partially due to design, but a lot are from improper filling and/or topping off a low reservoir. I've read Thermo's instructions on the proper method and would never of thought to do it that way. His explanation of the vacuum created if done the "normal, always done it this way" certainly points to causing or hastening the reservoir failure (cracking).