coolant disappearing

[fredthomas_1963]

hi I have an s type 2.7 tdi 2007/8

my coolant just disappears, uses about a gallon of water on a 50 mile run, now there are no puddles under the car, there's no steam nothing to indicate where its going,

i put a new radiator cap on and drove it 25 miles and all seemed fine except when i turned the engine off the auxiliary pump kicked in and the water level dropped to below minimum, any ideas what is going on

[Big John]

Hi Fred

Only 2 possibilities. It is either leaking:
1) externally, such as venting through the radiator cap as steam  or through a leaking hose or radiator or heater core, all of which should be visible.

2) Internally, Either directly into the cylinder to be expelled through the exhaust, or into the lubrication system which should show as "mayonaise" in the Oil.

I would check the Oil for signs of coolant (likewise coolant for signs of oil) and then recheck for leaks when standing still as that is a LOT to be losing without an external leak.

Good luck

John