I took my A4 to my father in laws earlier for him to look at, and he was tinkering around the engine bay as you do, now as the engine was running he took off the oil cap and the engine started to run lumpy to the point it stalled, it also done this when he pulled out the dipstick too.
Should this happen?
Also pressure built up when he removed the cap too, like when you undo the petrol cap ... ???
Is this normal?