I have a 2001 AUG motor from a Passat in my A4. I decided it was time to replace the plugs and once I took the first coil out of the motor, I saw a rusty colored corrosion on the metal part of the coil. The inside of the plug "silo" was also corroded. I took the plug out and the threads of the plug were also corroded and rusty. I then proceeded to take out the 2nd coil out. This one was perfectly clean and looked good. My question is, do you think it's electrolysis, a cracked block, or a bad o-shaped plug gasket that fits under the valve cover?

 

Thats my first thought was the gasket in the valve cover. Its not hard at all to replace and certainly wouldnt hurt...

 

good, i plan on doing that this winter at some point anyway. oil dripping on the cat. no fun.