Has anyone seen or heard of this problem and could anyone suggest anything to look for prior to putting the engine back in service.
Did the spring overheat? The break occurred while coasting to a stoplight. The car had been on a road trip 2-3 weeks before the spring broke but showed no signs of any problems (i.e. overheating, low oil etc.). Incl. pic of custom valve spring compressor. You likey?

 

Look closely at the break. See if you can see a material defect or a scratch on the outside. Springs can break (just ask anyone with H&R lowering springs) but is is very rare for a valve spring.

 

If you were replacing one valve spring you dont need to take the head off string wire is a great thing

 

Thanks for your input. Glad to hear someone say it is a rare occurance. I haven't found any mention of it happening on any blogs or web searches. Wish I could determine if it is a product defect (as far as I know the spring has 150k miles on it) or if its a symptom of a problem in my individual engine. Maybe I should try sending a pic to Audi engineering
And yes... I'm trying to do the job without removing the head... the other pic shows my "custom " valve spring compression tool. Keep your fingers crossed!

 

broken valve spring is a new one on me as well. trying to get a good look at your pic of the broken spring, based on how it looks to me, i suspect a material defect (or you somehow managed to exceed the working specifications of the spring)

nice work on your spring compressor. last custom one I remember seeing was a concoction of PVC pipes

good luck. keep us updated, especially if you get the new one in there without taking the head off.