Hey all, I have a 2011 Hatchback (purchased Oct 2010) with just over 40k miles on it. Over the last few weeks I have had a sputtering issue on cold start (car is left in garage for few days without driving) where I get cylinder 1-4 misfire CELs at times. I have searched around an seems that sputtering on start is either a carbon of HPFP issue but I had a few specific questions about things.

The problem only happens after the car sits for a few hours in cooler (60s-70s) temperatures and once warmed up it runs totally fine. The CEL is becoming a bit more common but does not trigger every time once I have cleared it with the Torque app.

So my questions are:

1. Does this seem like just a carbon issue? I am thinking a new set of plugs, oil change, and some Seafoam might help improve the issue.
2. I am out of the maintenance warranty now, would this repair (carbon, plugs, coil, etc) not be covered at all at this point by the dealer?
3. What would be your first steps to self-diagnose the issue?
4. I have used the torque app to check the fuel rail psi. Even with sputtering it seems as though the HPFP is operating correctly (70-90 psi while sputtering and while running normally)
5. Has anyone had their HPFP replaced by the dealer after maintenance warranty has already passed? Is it in an easy enough spot to DIY replace?
6. Would walnut shell blasting really be needed after 40k miles of 'aggressive' driving? That seems to be too soon compared to others postings.

I have an appointment with the dealer on Thursday but just want to walk in there with some understanding around the potential issue.

Thanks all!