Carbon deposit buildup on the intake valves is something that I remember being an often-discussed topic from my GTI and BMW ownership days. The general consensus I found was that carbon buildup certainly occurs and if you want to get your valves cleaned, do it about every ~60k miles (or more frequently for vehicles running aftermarket tunes). However, waiting longer doesn't necessarily hurt either. That, and as direct injection engines have continued to improve, the carbon buildup problem has become less of a concern.
I ran my last vehicle, a BMW 335i, up to ~75k miles and never felt the need to clean the intake valves. The car pulled strong up until the day I traded it in for the Mazda (and boy do I miss it!). Never a misfire or hiccup and that sweet N55 turbocharged inline-6 motor saw the upper rev range frequently. I admit that I'll probably get another once my CX-9 lease is up. I miss it too much, haha.
Anyway, As it relates to Mazda and the 2.5T specifically, here's some info from a Mazda engineering in an interview (
https://www.savagegeese.com/reviews/mazda-cx-9):
Carbon buildup on DI intake valves has two contributors. The oil itself getting on the valves, and the valves being at a temperature that promotes the formation of hard deposits.
We've dramatically reduced the oil in the intake stream compared to our last DI Turbo through several measures. First, oil blowing past turbo seals directly into the intake stream has been reduced with better turbo oil seals and lower oil pressure in the turbo itself. Actual blow-by has been reduced across the board with SKYACTIV engines by improving the roundness of the cylinder bores through improved analysis of casting, machining and assembly stresses that can slightly warp the bores. Taking this into account in the design process has given us much better blow-by performance.
Finally, the realization of the sensitivity to intake valve temperature has allowed us to design the cylinder head structure, cooling system and calibration to manage intake valve temperatures to prevent carbon buildup.
Otherwise, there is the theory (though unproven as far as I know) that a good ol' occasional "Italian tune-up" can knock and burn off some carbon buildup (or at least enough of it) on the intake valves if you feel the need to do so. That said, I think most research shows it doesn't actually do much. It's just fun to drive at higher revs from time to time.
). Other than that, everything is good.