See this image someone shared in an STI/WRX forum section.
So, in the FA24DIT, Subaru has used a number of different methods to mitigate buildup. I am over the 50,000 mile mark, and may consider having mine checked, but, from what I’ve seen in groups discussing the WRX and FA20DIT, buildup is considerably less than one would expect.
Here’s Subaru’s response when I asked. the FA24DIT is supposed to be their pinnacle of this technology, and improved even over the FA20DIT, btw.
“Thank you for contacting Subaru. I appreciate the opportunity to be of assistance.
Older direct injection systems that experienced carbon build up were so called “lean burn” systems. Unfortunately these systems suffered many times from uneven combustion due to variations in the fuel and air mixture throughout the combustion chamber. This created a mix of lean and rich conditions resulting in variations in combustion temperature and uneven burning of the gasoline. This uneven burning lead to the condensing of unburned and partially burned hydrocarbons which over time resulted in carbon buildup.
Subaru engineers understood this when they created our current direct injection system. They developed innovative yet simple systems to manage and control the combustion variations inside the combustion chamber. These include features such as specially shaped piston crowns, multi-patterned injection spray, and tumble generator valves. All of which promote the precise swirling and intermix of fuel and air under the variable conditions required across the full range of power requirements. When combined with very high fuel pressures and extremely short injection duration of the Subaru direct injection system, stoichiometric combustion is maintained across the combustion chamber resulting in very even and complete burning. As a result abnormal residue and carbon deposits are not created and engine performance and fuel economy are maximized.
Please note that our research does show that routine maintenance, per the warranty & maintenance booklet, is another important factor when avoiding any potential carbon buildup concerns.”
