Just a little story for everybody to learn from.
My Touareg has been plagued by various exhaust problems for two years now. First I started to smell exhaust fumes in the cabin, for no obvious reason. Then the the turbo needed replacement, because the car was underboosted. Then I had some gray smoke problem when accelerating hard. Then the engine got gradually louder and louder under load. Ultimately I also had to replace both the turbo actuator solenoid and the turbo pressure sensor, which seemed to mostly cure the problems, but I still had too much noise when launching from standstill in the first few gears. Because the idle was also pretty rough when the engine was cold, I figured this was due to EGR problems. VCDS also seemed to confirm this, as the EGR control values were pretty much all over the place.
So, because of this, this week I pulled the whole intake assembly (including shut-off air flap, EGR valve and intake manifold) on my Touareg, to have it all thoroughly cleaned. The assembly turned out not as bad as it seemed at first sight, none the less the cleaning made a huge difference. However, when I took off the intake manifold, to my horror this is what was waiting for me:
It turns out that the dealership messed up my car the last time they took the intake manifold off, because when they put it back on, they obviously misplaced one of the gaskets around the intake ports. The slipped gasket got bent, cracked, and was partially blocking the intake port of cylinder 5, which didn't get proper air flow because of that. This was two years ago, at the 160.000 kms / 100.000 miles maintenance service of the car, when they had to remove the manifold, so they could replace the rubber clutch and freewheel on the generator.
This must have been the primary reason for all the engine/exhaust problems I've encountering during the last two years. It explains the sudden appearance of exhaust smell in the cabin; the boost problems; the carbonization; the engine getting loud under load; the gray smoke during heavy acceleration. Interestingly enough none of these throwed any codes (even the boost condition appeared very rarely), and the situation even improved slightly on its own - most likely because the gap at the place of the missing gasket was partially sealed up by soot deposits (which are very noticeable on the block around the problematic port in the pictures above).
On a side note, and unrelated to this problem, but still very much related to the point, the dealership had me since replace the front brakes when it were actually the rear ones that were worn and triggering the pad warning (yeah, I didn't even look until after, because it didn't even occur to me that they'd have mess even this up), and had regassed the A/C multiple times, when it was actually the pressure sensor that was bad. They also broke the holder clips of the plastic trim covering the cabin filter, which (ie. the cover) started to rattle during rides because of that, until I glued back the broken holder back together with some epoxy. And of course I also complained several times to them about the exhaust/engine problems over the years, but they never found anything particular what could have caused those. Well, now I did.
Lesson learned: never trust the dealership, do whatever you can yourself, and if you can't, then always doublecheck whatever they supposedly did your car! Just because they're professionals they're not really better at their craft - if for nothing else, then because they just don't care.