COOLANT LEAK PUZZLE HELP REQUIRED

[subtacex]

Hi Guys
Have an odd defect and looking for advice or if anybody has had a similar defect.
I have been losing about 1ltr of coolant roughly about every month or so and decided to look for leaks seeing as the weather is half decent again.
Done the cardboard under the engine bay after taking off the bottom engine cover.
Found the leak coming from the passenger side chassis leg at the back of the engine near the bulkhead.
Removed the scuttle panel cover and found below the reservoir bottle was wet
The hoses and pipe work into the heater matrix are fine,dry and in good order.
This had me puzzled and eventually I checked the reservoir cap which did not look to healthy
I replaced with a new one and took her for a spin today.

Engine response is as normal
temp is steady at 90
gearbox temp is norm
slight tinny noise,which is suspect is the water pump although my power steering pump is a little noisy as well,but the tinny noise is new

I checked under the car again and it looks as though the cap is passing/lifting possibly due to overpressure.
There is no white smoke from the exhaust and all seems normal there ruling out a head gasket! (I don't rag the car )
Just wondering if a faulty/failing pump might cause over pressure due to non circulation and the car is staying cool on convection alone
Any feedback would be most appreciated

[Shoppinit]

Definitely log the temps, with Vagcom is poss. On aircon unit if not. Maybe your thermostat is failing and local temps are getting high, but your gauge is still pinned to the middle position.

Hopefully not a head gasket issue. Any build up of pressure in the system? You would observe that when opening the cap after letting it cool suffisiantly.

Any bubbling in the return from the rad? Not sure how visible that is on the RS6.

[subtacex]

Hi Shoppinit

No pressure on opening the cap,It seems as though its a build up of pressure though,enough to lift the reservoir cap and then dissipate.
It only appeared as the weather has got warmer.....your suggestion of thermostat seems viable

[tmg]

I had something similar where there were small cracks in the tank itself which only opened up at full temp and under pressure, so a static pressure test wouldn't show it up. the giveaway was that the coolant level dropped to about two thirds of the way to the bottom of the tank (so 1.5 liters of coolant), and stayed there.

i could see the underneath of the tank was damp, so changed it and the cap (see here - viewtopic.php?f=44&t=115428 ) and that fixed it - how yellow is your tank?

t.

[subtacex]

Tank is just about see through ish, level goes from max to min and stays there with wet under the tank before draining away

[subtacex]

is the correct number for new expansion tank 4B3121403B ?

[Mɐʇʇ]

No. Assuming a RHD car, it's 4B0121403
If LHD, it's 8D0121403L

They're under 35 quid

[subtacex]

Thanks Mat

[el_ringo]

I had a tiny leak in my rad once. Only escaped when hot and was a bugger to spot.

[spurtle]

Late to thread but if you suspect you're losing coolant through the reservoir cap then try tying a plastic bag to the overflow pipe with a cable tie, ensure correct coolant level, then take the car out for a beating. Easy way to determine if it's the cap or something else.

Even if there's no big pressure build up it might be worth getting a sniff test done.