Hello , I have a pristine 4 .0 2002 xk8 second owner is myself and I have an issue with the coolant level reservoir depleting ...once a week I put one small bottle 10 oz bottle of water in the reservoir.This has been going on over a year and I have taken the car to numerous shops and every time it’s a problem not solved and a new part and expensive labor bill.I have had the reservoir replaced, the cap replaced, the radiator replaced, the water pump replaced , the water serpentine hose replaced, the fitting plastic fitting behind the block, thermostat housing...and it’s still using a bottle a week...i drive the car about 66 miles a week tops ...and no signs of water on the garage floor., no water in oil, no overheating, just a warning on the dash coolant level low..
Hopefully some here can assit me and advise me what to check...thanks guys

 

Have you performed a cooling system pressure test? That would be your most appropriate action before spending any additional money on your issue....

 

Yes that was done several times and no pressure is lost

 

When you perform a cooling system pressure test, the only component not present in the equation is the reservoir cap. Are you certain your cap is functioning properly? There has been a rash of failing caps lately (mine fell apart in my hand a couple of weeks ago)....

 

cap was replaced with OEM

 

Remove the right side fender liner and see how much fluid id retained in the overflow tank, and whether there is indication of a leak from the tank.

 

to RJ's point, if I recall correctly, a leak in the overflow tank wouldn't show up under a pressure test and might not leak until you're on the road. If you top the car off, then let it sit, will the coolant level still drop?

 

Also, for what it's worth, I've had problems with the thermostat cap leaking before (it actually only happened when I replaced the thermostat and therefore the cap as well). But I assume that would have shown up under a pressure test.

 

I have the exact same issue, never knew about the overflow tank. I’ll be taking a look.