It appears the intake valves are carboning up bad. My guess is with no gas contacting them perhaps they are running a lot hotter. Anyways service bulletins have been sent out to various makes dealers. Apparently the manufacturers have been trying to keep it as quiet as possible.

Symptoms are poor idle and running. Cure is removal of the carbon. I wonder if periodic water injection might work? A small nozzle hole into the intake not disturbing the map sensor might work.

Bmw is using walnut shells somehow. Kia does not disclose their cure but recommends a good gas with a cleaning agent and additives to the gas basically on a permanent basis. The real cure apparently is too get a little of the injected spray to hit the intake valve. Probably a head redesign.

I think direct injection is here to stay on gas engines. The best guess is that the manufacturer can increase fuel milage by fifteen percent with higher low end torque. Or can choose instead to boost overall power using the same amount of gas.