Originally Posted By: Danh
Yes, it is common with DI engines though some brands are worse (look at Ford EcoBoost exhaust tips) than others. DI, while generally emission-friendly, does produce more particulate matter than port-injected engines and this is likely what you're seeing.
There has been speculation that standards may have to be modified to limit particulate emissions of DI gas engines, which in turn could require particulate filters like current diesels.
During the week after I took delivery of my 3.5 direct-injection EcoBoost Explorer, I noticed carbon deposits on the stainless-steel tailpipe tips. It was easy to clean, but I was dreading having to do this cleaning every few days. After the first tank of gas was gone (first tank was compliments of the dealership), I started using Mobil 93 octane exclusively. Mobil is a Top Tier fuel, and I'm pleased to report that my tailpipe tips are no longer covered in carbon.
From what I've seen, fuel quality is important if you want clean tailpipes on a direct-injected engine.