This is a GDI engine, 115K, with what looks like a lot of carbon build up. My experience so far with GDI is this is normal. We started with Misfire, P0300, on all cylinders. Engine runs rough at idle, Vacuum is 15" at idle. WPS shows VE is compromised, but not showing a fully burned valve. ( Recently did a Mazda Turbo GDI with a valve cut in half. ) We did a aggressive FI service with BG, even pulled the intake and soaked it. Interesting thing is, we fixed one bank. Good trims, no miss showing on bank 2. We got the VE, and trims to settle down on bank 1, but not all the way. The most interesting thing is, driving, it runs good, trims at +6, or under. It will not show misfire, or set code for it, unless you are decelerating. Vacuum is still low, 16.5", and WPS is better, but still jumpy. I pulled the valve cover just to see in the cam. Looks OK. Probe pictures from exh. side show a lot of buildup on the exh. stem. I believe the Valve guides/stems are worn, causing the issue that remains, and we are doing a valve job.

I am not looking for a silver bullet, just some talk about these issues, GDI, in general. Seems we are right back in the 80's, with Carbon cleaning. And, strategy, Why is it only seeing miss, setting code from a decel condition. At idle, you can feel the vibration/miss. It also looks like on decel, it is going open loop for a bit, trims go to 0's across the board. That is when miss gets counting. Regards. tr

Thomas Richardson
Owner/Technician
Master Tech Automotive
Richland, Washington, USA