My wife has a 1997 A6 Quattro that is having issues. We have owned the car for just over a year and it has had a bit of trouble with hard starting since we got it but it hasn't been a real major issue. The other day she was driving and it started to run real rough and lost so much power it was undriveable. It still would idle okay, had good oil pressure and temp was also normal. I got the car home and started to try and figure out the problem. The engine had kind of a ping sound up top and then a second ping type sound joined in. It is definetely not the tick like a lifter makes or a deeper thunk like a rod. It actually sounded like there was something on top of the piston. I fully expected to find a timing belt issue and bent valves but the timing appears to be good as far as I can tell and the compression is between 125 and 135 on all the holes. I have pulled the intake off and the tops of the pistons all look good as far as damage. Of course I can only see what I can see through the spark plug holes. The odd thing is that all of the pistons have a layer of carbon on the top except the number 2 hole. The top of this piston is shiny and clean as if the injector has just been blasting it or something. I've done a fair share of turning wrenches in my day but that was mostly pre injector days. I am wondering if anyone has seen this or if it's possible for injectors to fail and make this noise. I will also attach a picture of the plugs. The number 2 is the worst one. Any help would be greatly appreciated.