I did a compression test and found 180-185 across the board. Leak down test passed as well. At this point, I’ve replaced the injectors, HPFP and did a carbon cleaning. Also swapped the plugs to the one step colder Denso non-projected.
According to
@Hoon, rail pressure drops on is38 GTI pump gas tunes is common. The HPFP can’t keep up but there’s enough time for the injectors to spray the requisite fuel and as long as it recovers in the upper RPM, it’s fine.
Looking in cylinder one now, it’s clean. There is none of the build up that was there before and no signs of further detonation on the piston. I stuck a piece of towel down there when the piston was TDC to check for oil in the cylinder and it was dry as a bone.
So, I’ve been driving and spirited at times and will keep an eye on it. I guess there’s no way to be sure what caused the detonation in Cyl one. It could have been a weak/dirty injector, weak HPFP, or truly needed a colder plug.
The last piece of the fuel system I haven’t touched is the LPFP. I’m not opposed to replacing it, but unless I see further evidence in Cyl one indicating the new parts didn’t solve anything, I’m inclined to think this was not a fuel delivery issue based on what Matt said about the is38 GTI tunes.
If there are any flaws in my logic, let me know.