So, I just changed the timing belt, tensioner, rollers, water pump, thermostat, and valve cover gasket on my '08 A4 2.0T. It has 154k miles and this is the first time I've changed them. After starting the vehicle cold, everything sounded great, but once the engine warmed and the low idle kicked in, the idle start hesitating every 3 seconds like it wants to conk out. There is a loud air sound coming from PCV area, but no detectable leaks. If I put my ear directly on the PCV flying saucer, it sounds like it is internal to that chamber, but it is still pretty loud in the valve cover also. I've checked the timing marks 6 times and they are still lined up. So, I'm leaning towards PCV, but I can't understand why it would have gone bad by just nudging it out of the way to replace valve cover gasket.

The following is a video of the rough idle. Suggestions?

 

Spray some brake cleaner or some other flammable lubricant in the area where you hear the air leak while it's running. If the RPM's increase your leak is there. It really doesn't take much to throw it off. The diaphragm on the PCV valve on my A8 ruptured and was sucking air bad though it causing a lean condition.

 

Thanks for the reply. I sprayed starter fluid and didn't detect a change. I ended up popping off the PCV flying saucer cover and finding a torn diaphram. I put a nitrile glove over the circle, closed the cover, and cut away the excess glove. The idle returned to normal. Picking up a new PCV tomorrow.

 

That's how you do it.. Some of those "pancake" PCV valves have a small hole on top. If you can feel air being sucked into it, the thing is leaking and should be replaced.