how to track a Coolant loss on A4 1.9 TDI 130, 2001.

[A4-Paris]

Hello All, I just joined the forum today. I own an A4 1.9 TDI 130, from year 2001, with 280.000 kms.
Two weeks ago it signalled an empty coolant tank, for the 1st time ever. I refilled (obviously). It has since continued to lose coolant, specifically when riding on motorway for 200kms straight (avg 2300rpm), where the tank would be emptied again. Hence it seems that coolant is 'pressed out' when engine is hot and running at 2300rpm or above.
Yet, no apparent leakage when car is parked. The oil is perfectly clean. No leak appears from the hoses when checking the hood.
So I have no idea how the coolant disappears...

Any hint on how to track this loss problem ? Many thanks in advance.
O.K

[zollaf]

welcome to vwaf.
i would start with a pressure test, where the cooling system is pressurised to around 15psi. this will highlight any leaks. this test can be carried out for 24 hrs if required to find the smallest of leaks. you can do it with the engine cold, then start it and warm it up. remove the timing belt cover to see if the water pump is leaking, and feel around the carpets up front to rule out a heater matrix. this would also leave the windscreen greasy. presuming you have the original red oat coolant, you will look closely for any red staining indicating a leak. if no leaks are found then you can start to suspect a head gasket or porous head.

[A4-Paris]

Hi All, Hi Zollaf, thank you very much for your advice. I did my homework. First I put a bag around the coolant tank... and yes ! did find the bag filling up from the expansion tank... So the coolant is expelled out from the tank.
Second, I had the cooling system tested under pressure, at the garage. They confirmed no leaks from the hoses. But they could not be sure whether the water pump was the suspect or the head gasket. They told me the pressure in the feedback hose (into the expansion tank), quote : "was kind of low". The calorstat (temperature detector in French) works fine. That's why they suspected the water pump wasnt doing its job as it should...
Voilà. The status of my investigation.
Would you have any advice as to how to check if the water pump is truly the culprit ? Be aware that it has surpassed Audi's recommended mileage... by quite a bit.
Thanks for any bit of advice.

[A4-Paris]

Oops, let me correct one detail. Actually the water pump had been changed and has only run 40.000kms as of today (i.e 25000 miles).