Wanted to share my most recent experience in hopes of helping someone else resolve a similar problem. The car has 108K miles on it, I am religious about changing the oil at the 50% marker. Started with the check engine light and stabilitrak lights. Scanned the computer and had P0300 and P0302 codes. Most of the time these issues can be resolved with new plugs and coils. I replaced both. A few days later same codes came back. Swapped no2 cyl with no4 cyl (plug and coil). Days later..same codes. Thought possibly a bad injector. After researching what was involved in changing injectors, I wanted to make absolutely certain that the injector was the culprit. Using a scanner, I was able to determine that the injector was NOT the problem. Next part of the diagnostic was to perform a compression test on all cylinders. All of bank 1 were between 120-125. Bank 2 cyl 2 was 75 the other 2 125. In speaking with another GM trained mechanic, He said that these motors, being direct fuel injected, have a tendency to clog the valves and valve seats with carbon. He suggested removing the intake manifold ( which is fairly simple 6 - 13 mm bolts and 2 10mm bolts on the rear) and inspect the cylinder through the intake ports. Number 1 and 2 cylinders had significant carbon build up. Number 2 was so bad, I scrape off large chunks with a screwdriver and blow gun to start. I purchased 2 cans of ACDelco Top Engine Cleaner part number 10-3007 (1052626). Rotated the engine to close the valves and loaded the cylinders with this cleaner. Let sit 20 mins. Blew out while covering the port with disposable rags. Repeated the process again on no 1&2. Once that was done performed the same cleaning on the remaining cylinders. Once all done blew out each cylinder again in both the intake port and sparkplug hole. Reassembled.

Caution : I highly recommend wearing a face shield when spraying and removing product. Also only start the car after this procedure in a well ventilated area or outdoors. The initial start will create significant smoke.

This process worked for me. The car runs like new, no miss at idle and no codes!