The worst one I've seen so far - Skyactiv D carbon build up : r/Justrolledintotheshop
Cleaning intake valves - Mazda Forum - Mazda Enthusiast Forums
Mazda Cx-5 Walnut Blasting Intake Valves Carbon Cleaning. *Carbon Build Up Mazda SkyActiv engines can have issues with carbon deposits. #Mazda... | By CAR. MAN Garage KEDAHFacebook