leaking coolant from radiator cap

My car is leaking coolant from the radiator cap despite the temp guage reading only very slightly to the right of verticle/normal.

As daft as it sounds it seems to only do it when i stop the car (although obviously i cant check when i stop at roundabouts or traffic lights). MY reason for thinking this is that if it was loosing coolant at the rate it does when i stop (around half a cupful bubbling out within 30 seconds of stopping) i'd have nothing left by the end of the journey yet theres still some in the radiator.

Any ideas what could be causing this? Faulty pressure cap on radiator? head gasket on its way out and pressurizing coolant (exhaust isnt smokey though and im not using huge amounts of water)?

i checked it after my journey home from work. Immediately after stopping there was a tiny trace of having lost some coolant from around the radiator cap but within 60 seconds there was a steady trickle which lasted for 2 minutes and eventually there was a little puddle underneath the car as usual.

Any ideas?

Do you have a blue or black cap?

blue cap with VW audi stamp on it

Had a similar problem in my 1.1, although temp went somewhat higher. New radiator cap solved it. Replacement cap is only a couple of quid from GSF, so might as well get one as a process of elimination.

Good luck.

Cheers. That was going to be my next plan of action. Might invest in another temp sender as i dont have 100% faith in the current one. Think they're only about a fiver from GSF so its worth it just to cure my paranoia if nothing else.

new radiator cap fixed it. Not suprising really as i noticed that squeezing the cooant pipes i could hear air hissing in and out the cap. Still none the wiser why it was only loosing coolant when i stopped (and briefly after starting i later realised).

Im guessing that once the engine was running pressure built up in the system and caused whatever leak there was in the cap to plug itself. Once the engine was off presumably the pressure dropped and it started leaking again.  Anyone got a better explanation?