Coolant

[M4rkL]

Can anyone enlighten me as why i lose water on journeys say over 1hrs drive, but not travelling to/from work, no sign of water anywhere under the car or inside footwells. On a long journey the A3 using most of the water in expansion tank. As i said it only looses water on long journeys.

[Magnet]

Hello Mark,

To lose a reservoir full of coolant on an hour’s run is worrying. It could be as simple as a failing/failed reservoir (pressure) cap - but although this is not uncommon on other marques, I’m not aware that it’s an issue with Audi, but….

Let’s colour in a bit more detail:- what is the one way distance commute to work? Can you travel this accumulated distance for a week without any coolant loss? What would be your typical max. and average speed on your commute? 
With the ‘say an hour’ - would average speed over this travelling time have any influence on the amount of coolant loss? Couple you, for example, run the car on the motorway for half an hour at the legal limit and not experience your emptying of the reservoir? 
Suggested test:- take the car on your near to 1 hr. troublesome  run, Park up, but leave the engine running, and get an assistant to rev the car to a steady 2000 rpm and hold it at that rev. for 2 minutes, while you observe the smoke from the exhaust. You will be looking for any evidence of white vapour smoke. 
Does the car overheat ( any degree over normal) during your troublesome hour run? 
Any evidence of emulsion on the oil dip stick? 
Perhaps you could come back to us Mark. 
Kind regards,

Gareth. 

[M4rkL]

Hi Gareth, can travel over a month to from work (12 mile return) use no water, new cap fitted, had both water system pressurized  and head tested, both ok, car runs at proper temp, no overheating, no water deposits on dipstick or oil cap. As i said only uses water on 1hr+ journeys, also after half hour running engine near side blows cold air, off side blows hot air, checked footwells for water from heater matrix and nothing snuff dry. Its puzzling 

[Magnet]

Thanks Mark, 

That does indeed colour in more detail, and apologies for suggesting what you have already done in terms of the cap. 
The coolant loss on a reasonable run has to be explained by either a leak or being burnt in the combustion process - it has to be going somewhere!

If it were mine, I would definitely carryout the exhaust emission investigation I suggested after a long run. 
Kind regards,

Gareth.