Just a quick tidbit I learned today. I had been having misfire issues, with random misfires, and misfires on cylinder #4 on my 99.5 A4 Quattro 1.8T. So in standard troubleshooting fare, I swapped the packs on #3 and #4 to see if the misfire moved with the pack.
I also checked over the connectors of my other coilpacks to ensure everything was tight, when I noticed the ground screw was a little bit loose. 1 10mm wrench and about a full turn later, everything was nice and snug, put the cover back on, then started her up, let her run a bit, then drove it around to see if I was still having issues.
None. Problem fixed.
So word of advice, when you are having misfire issues, make sure to include checking that ground wire first. May save you some time, headaches, and money.
Now, I didn't think to take a picture, but I found an ECS coil pack hold down one to show



hope this helps someone.

 

Good tip. Grounds cause more problems than most realize.

 

it's amazing the headaches you can have from one loose connection

 

Well.... even better tip. Audi coils that bolt down.... like older 1.8t and 2.7t more than rarely go bad. Not nearly as much as the other kind.

but yea, grounds cause alot of issues and can have you chasing wires forever