This is extremely common for a direct injected engine. The only real option is to get it cleaned either yourself or through a reputable shop. Catch cans do nothing to avoid this problem. I just recently ran meth/water injection in my car, which cleans the vavle head and manifold, completely eradicating carbon build up. Although i did do this early, my car only has 7k miles on it
In sum, the only mod to give you piece of mind is meth/water injection, my kit cost me 300 clams and is a stage 1 Snow performance kit . And once again CATCH CANS DO NOTHING FOR OUR ENGINES!
i bought it from BSH
http://www.bshspeedshop.com/bshstore/
Won't meth make you run rich unless youre tuning for it?