I bought a new to me B7 S4 a couple weeks ago, knowing it had a CEL for misfire, so the price was decent. The car has 180k miles, but absolutely looks brand new inside and out. The codes were P0300 (random misfire), P0306 (cylinder 6 misfire), and P0308 (cylinder 8 misfire). A quick google search brought up coilpack issues, so I figured it would be an easy fix. I bought 4 new coil packs and replaced all the coilpacks on the left bank. I also changed all 4 plugs on bank 2 while I was there, though the ones that came out looked fine (ignoring the soot on them from the misfiring cylinders). After doing this, I still had the same codes, so I took it to a local independent shop called Eurowise and had them spend some diagnostic time on it.
Eurowise performed a smoke test and found no vacuum leaks, and could easily duplicate the issue, but couldn’t find the cause. They said everything looked normal and the timing was good, but I never saw for myself what it was. They offered for me to leave it so they could do further diagnostics, but I’m pretty mechanically inclined, and not nearly as financially inclined, so I took it home. They did provide me with a paper copy of the VAG-COM scan results, which I’ve attached as a PDF below, but I’ll put the codes here:
005495 Right Engine Mount Solenoid Valve P1577 - 004 - Open Circuit, Freq 1, 778rpm
000768 Random/Multiple Cylinder Misfire Detected P0300 - 001 - Intermittent, Freq
After further netterwebz research, I started reading more and more about timing chain guide/tensioner/adjuster issues.