Today's Tribune (auto section; Motormouth) has a question/answer regarding carbon buildup in direct injection motors. "Will fuel additives also benefit newer direct injection systems?" The answer was added as "Yes, up to a point. While the additives will do nothing to remove carbon buildup on intake valves, they may help to keep the fuel injectors clean. Dirty valves are a growing problem that leads to rough idle and cold-start hesitation. Although there is some speculation as to the cause and possible control of carbon buildup on direct injection engines, we have seen nothing convincing yet."
Which led to think that DI is really new to production vehicles which points to a greater rate of buildup when compared to non-DI engines. When rebuilding the 3L on my 89 Caravan (175,000 miles), I inspected the valves and found them to be almost spotless. I would think that this sudden recognition of buildup points to a new problem that DI has introduced. Are we getting a 1-2 mpg increase but sacrificing performance and reliability in turn? I did not know of this supposed problem until reading the paper.