Oil Leaking from Oil filler cap
 

98 A6 Quattro 30V, 151K

I have oil leaking out of the oil filler cap. I have replaced the rubber gasket and the oil filler cap itself. Any ideas as to what else could be causing the leak? Is this a common issue? There was not leak for months after I replaced the cap but the leak is back.

How much oil is leaking?

If it is just a little - live with it
If it is a lot begin looking for a stuck PCV valve or excessive wear on the cylinder walls causing blowby.

I can smell the oil burning and I can see the leak from the cap.

Audi release a new oil cap a couple years back that solved this issue. It vents properly. I had to change mine a year or so ago, my pcv system was plugged, and after changing that, it got better but still leaked. I picked up the new oil cap and the problem was solved. @ your mileage, replace both.

I replaced the oil cap with the new one and I am still getting the leaks. Also I tried to see if the pcv valve was bad and I was told that the car did not have one.

It does, but not in the traditional sense.

see here :http://www.partsgeek.com/catalog/199...ther_hose.html

So you replaced this part on your car and you didn't have issues anymore?

Yes.

I replaced the breather hose and the oil cap and no longer had a leaky oil cap.

I find it odd your new oil cap is still leaking. Is it leaking around the seal, or out the breather hole in the top of the oil cap?

It is leaking out of the seal.

There are vented caps that function as pcv - open air.