Normally my analysis is better then this..... So perhaps I am looking for some kind of diagnosis decision tree.... So in early December (this year) the car went into limp mode, throwing a code. I can't recall that code, but I do recall reading that the solution could be a knock sensor (including making sure it was properly torqued).... I reset the code and nothing since, although I have driven perhaps 100 miles since. Now, about a month later we are noticing soot all over the back of the car... it could have been that for a while... I run my finger over it and its not really greasy/oily, more soot like. So we just did a oil/filter change (mobile 1 always) and it still seems to be still be generating soot. I captured some oil to send off for analysis (not sent yet)... so just looking for some trouble shooting ideas/process.

Knowing I have 110K on the engine and that the dealer had rebuilt the engine at about 80K (after they improperly installed a new water pump...) and so I am thinking the following is due/over due as neither have been done
1) I was already aware of the intake plenum cleaning issue/maintenance... so I have the CRC GDI IVD spray coming... figure I would try that (I am sure there are strong opinions here on that!)
2) I am thinking a turbo rebuild is due/over due. It does not seem that painful a DIY, but tell if if I am wrong... Seems it is mostly labor, and I'm handy, but I am open to suggestions on parts, if I should rebuild myself or source one... etc.,\

I am not sure if #1 or #2 could be a factor in the soot situation... Anything else I should be looking for on the soot situation? (again, sorry for not being able to supply more detail)

 

Wouldn't get to worried about soot. Not sure where to put this but not all of what you see is soot. It can be tire dust. The rear of some of my cars collect a lot of soot and the engine is fine. Soot is a normal byproduct of combustion and converter action. Cold weather. Cold starts. Rich running. Soot gonna happen.

Absent any signs of trouble from the engine, excessive oil or fuel consumption I would not worry about soot.

My limited experience with turbos is they generally let one know when it is time for a rebuild. Excessive oil in the intake from a bad compressor side seal is one sign. Or excessive smoking from a turbine side seal leaking is another. 'course, it is up to you if you want to rebuild the turbo.

Might add it is important to give the engine some idle time to let the turbo cool down some. And exhaust valves. This is really important if one has been driving the car in a spirited manner. My previous turbo charged car the owners manual recommended a 2 minute idle time to let the turbos cool down and I followed this unless I had been driving the car on surface streets a while after leaving the freeway.

I even give my 2018 JCW some idle time even though the turbo is water cooled which tends to have the turbo run cooler and cool down faster.

 

Thanks, agreed on the cool down. I had a rabbit convertible with the Callaway conversion and remember the cool down process from that (and the turbo timers that were available and likely still are).

I suspected the soot might be road material, as it did not seem greasy when I ran my finger over the soot to see if it streaked. To your point I think I will take a look at the plenum after the turbo to see if there is any oil in there etc. We had the fuel system rebuilt a few thousand miles ago by a Robert Bosch shop (car was at my kid's school, so I could not get it home... not that I could have done the work necessarily), but as I recall they replaced the in tank fuel pump, out of tank fuel pump (?) and some other work. In retrospect I should have checked to see if they took at look at the intake for the carbon build up... Not sure they would have been in that part of the engine bay or would have done that as part of the work, but it was about $3500.... Anyway, I will check the plenum for oil residue, and plan to use the CRC product for the valve cleaning anyway... seems like a maintenance. It would be great if the CRC product worked on the general cleaning, and if there is no residue in the intake plenum, then things seem good for the time being.

Much appreciated.