My mate has just got a 1999 3.2 petrol xj Jaguar,drives lovely but he has had to top up the coolant/water 3 or 4 times in about 500 miles,there is no steam or smoke,no loss of power,filler cap is clean and no overheating,as we are not mechanics not sure what or even where to look at when he pops it around for me to scratch my head at....any help would be appreciated....the engine has done 86k

 

Park it over a piece of cardboard overnight, and try to identify any leaks. If it's not burning it, it's leaking it. It's just a matter of finding out where. If you don't see any drops on it in morning, run the car until it warms up, still over the cardboard, and see if anything is dripping. You might end up having to take it to a shop and have them test it to see if it's leaking somewhere above where it's not able to drip down while stationary.

 

Coolant leaks on a Jaguar can become a major disaster if not corrected. The only effective way to locate a leak is to do a pressure test of the cooling system. Your leak could be anything from a weak pump seal to a pin hole in a hose that will only get worse. You also need to have the coolant tank CAP tested.
Either get a pressure testing kit or have a radiator repair facility conduct a pressure test. You most likely are loosing coolant while running at system pressure and it is only going to get worse.

 

...along with the others good directions, check the coolant tank for a crack and the connectors...the 'O' rings can leak. Look under the vehicle to see if moisture is wicking along under it, ...like the frame and/or floor pans. If that coolant reservoir is leaking, it will run down the firewall and back.
How much is he putting in to top off btw?