Loss of coolant, unable to find leak.... HELP!!

[crazycowell]

I have been having a problem with my coolant system, I keep losing coolant (as in the computer keeps telling me its low because it is!!) and I cant find the leak...

Things I have tried:-

The header tank cap
Looking for any water under the car after it has been ran, parked for a while etc
Pressure test and exhust gas test
theromsat replacement

I have even tried putting the leak stop stuff in, but that still doesnt work!

I did see that after putting the leak stop stuff in (that has that brown colour to it!) that the liquid does come out of the over pressure/flow release tube from the header tank.

I have asked at a garage I go to (know the owners!) and they cant think of what it might be!
If I'm going on long journeys I can get 60 miles before I get the light on the dash

So if anyone has any ideas on what it could be I'm open to suggestions....

Many thanks!!

[mikeybutch]

Are you getting good pressure?My A4 came with a slight leak which was from a hairline crack in the header tank which you could only see if you released the tank and looked at the back.Easy to release and check .Trouble is slow leaks are often steam which leaves little trace.Otherwise I would check hoses and clips for tightness.Hoping its not a head gasket .Any sign of water in the oil?

[GRBspeedster]

Check oil level to see if it is rising, ie water getting into system. Any build up of sludge in the oil filler cap?

[crazycowell]

I have checked the header tank and there's no split in it, when we pressure checked it we had it to 3 times the normal pressure and couldn't hear/see anything and the pressure held!
There's no water in the oil as its not milky or anything and the level is staying the same!

This has a lot of people stumped!

[mikeybutch]

So how much coolant are you losing when you say you have the light come on after 60 miles?Is the level then well below min?If so it has to be going somewhere and theres a leak where it cant be seen .Could it be the heater matrix any sign of dampness inside in that area?