Radiator Leaking from Rubber Bung?
 

I have a radiator leak that is coming from what looks to be a rubber bung located underneath the left side (passenger side) of the radiator that is nearest the engine. It only leaks when at pressure and won't come out by hand so reluctant to wrench it out without a replacement.

I can't find any drawing or reference to it in the works manual, local radiator repairers don't seem to know of this and Audi parts are not aware of its existence.

Has anybody else had this problem and if so what was the solution?

Thanks

Photos are always good and left/right depends which way you're looking from. ;)
And which car? You list a D2 and a D3......

If we're talking about a D2, do you mean where the blue radiator drain screw is (as shown on the photo below)?:


https://i.imgur.com/7UMdLyC.jpg

Just wrote e reply and lost it so here goes again.

Car is a D2 and left is nearside (if you drive on the left). I've attached a couple of pics one dry the other when engine up to temp. I'm wondering if this bung is a fail safe should the engine exceed operating pressure. It's rubber and very pliable so can't really grip it unless I want to damage it.

I think thermostat works as I hear hissing when reaching operating temp and assume it's the thermostat opening. It may be the the filler cap pressure valve can't check it but can replace it.

Any thoughts will be appreciated.

The stat will not be hissing. Its possible that thats the coolant filler cap as you say and the spring is lifting slightly as it is exceeding max op pressure. If that is the case then you need to investigate why it is exceeding. Do you have any other signs of overheating? I would suggest you go the "D2 - Common Faults-Tech Dump" thread and pull up the appropriate service manual to identify the plug.

Thats one of the two mounting bungs for the radiator. If its leaking from there there's a tiny chance it's leaking from the thermo-switch, but most likely you have a hole in the radiator.