Coils & Plugs at 50,000 MilesAs part of my 50,000-mile service last month, I replaced the plugs and coils again. The last replacement was at 25,000 miles (instructions).
The coils are one revision newer - the last ones I installed were the "C" part number, these are 07K 905 715 D.
One of these things is not like the other.... (the coils):
The coil I pulled from cylinder 4 had a lot of corrosion near the sparkplug end:
The NGK BKR7EIX plugs with 25,000 miles of usage:
Notice the corrosion around the back of Plug 4, corresponding to the corrosion on the coilpack:
When I posted pictures of these in the 2.0T forum, I was accused of negligently getting water in the plug recess due to pressure-washing the engine. The fact is that I have never pressure-washed the top of the valve cover, nor caused any water to get around the coils that would cause this.
Looking at each of of the plug recesses, you can see some degree of corrosion around the top of 1, 2 and 3, but significant corrosion down in 4. I'm going to keep an eye on the new coil in the No. 4 spot to see if it starts developing corrosion, too. This is probably just a cosmetic problem, since it didn't make it past the threads of the plug nor did it interfere with the functioning of the coil.
The plugs were purchased from an eBay vendor for $37.00, and the coils were obtained from World Impex.
As I experienced before, installing the new plugs and coils had an immediate improvement in idle smoothness. After nearly 2000 miles of driving on the new parts, the idle is still smoother than it was prior to the change.