It was the high pressure fuel pump. After letting it sit overnight, the dealership was able to reproduce the issue and replaced the pump under the extended warranty. The car is driving like it did when I bought it two years ago. No more trouble starting, no more stumbling when accelerating when cold.
Thanks to everyone for your suggestions!
A car that does most of its driving will benefit from a "harder" drive occasionally... Known as an "Italian service" by some.
My car has over 200K km's on it and it always runs much better after a long motorway run (800km at 120km/h...) The normal home to work and back tends to allow the turbo to build up carbon...
So, a good run once in a while will help ...
Is it actually soot or is it oil? If it is oil and you didn't notice this behavior prior to the oil change, I'd suggest one of two things has happened: Either the shop didn't tighten the drain plug tight enough, or they over tightened it and stripped the threads. If it's loose, you should be able to tighten it without too much of a hassle. If it's stripped, ...
(Repeating what I put in comments ...)
I'd suggest you already know the culprit ... if there's a leak from the combustion chamber, you're going to have the noise you're talking about as well as leaked out combustibles, which means gunk. This would be the source of the black looking stuff around the injector.
More than likely the seals which go around the ...
My understanding is the reason DI engines get carbon build-up on the intake valves is because the fuel (with it's detergent package) get sprayed directly into the cylinder, not on the back of the intake valve like in a "normal" engine.
So, adding a cleaner will get injected exactly the same way (giving the intake valve a miss) and will likely not have much ...
Letting the valve soak over night seems to be a common way. Just make sure you only flood the pipes and not the actuator.
There are specific EGR cleaning agents (sprays) sold at a lot of places; might work better than generic carb cleaner, or not.
Somebody reported success using oven cleaner from the kitchen, but I'd go for soaking with EGR cleaner first.
2011 R57 S cabrio. I had identical issue. Misfires all cylinders. Replaced HPFP, coils, walnut blast. ALL FAILED.
Engine limp mode returns.
Finally checked fuel filter (75k miles).
Runs like new.
$30 for kit. 30 minutes DIY.
It sounds to me as though the temperature sensor is giving out a valid but incorrect reading so that the ECU of the car is incorrectly calculating throttle position against fuel requirements.
I'd personally start by investigating the voltage when cold of the temperature sender unit.
Start with plugs and wires. Especially if they've never been replaced, they are almost certainly causing your current problems. You can test the wires with a multimeter and visual inspection of the spark plugs can tell you a lot(you can give that a google, or we have a question here about what plugs are supposed to look like).
What it is actually doing is vibrating all the crud off. I have no experience cleaning that way but I now might get one.
What is really happening is since water does not compress this is sending pulses through the water which will transfer into the crud knocking it off using physical force. might be more effective with parts cleaner or electric contact ...
It seems to me that the only source for the carbon build up would be the PCV valve or maybe leakage from the intake valves, but given that most of the time air is being pulled in, I'd bet on the PCV valve. I'm thinking that what you're seeing reflects that pattern of air flow from the PCV valve outlet to the cylinders.
The wetness could be condensate from ...