Carbon Buildup
Accumulation of hardened carbon deposits on intake valves, pistons, and combustion chambers, common in direct-injection engines.
Carbon buildup is the accumulation of hardened, black carbon deposits on internal engine surfaces, particularly on the backs of intake valves, intake manifold ports, spark plugs, and piston tops. It is a natural byproduct of incomplete combustion, but it has become a prevalent issue in modern gasoline direct-injection (GDI) engines.
In GDI engines, fuel is sprayed directly into the cylinder, meaning fuel detergents never wash over the intake valves. Oil vapors from the PCV system and exhaust gases from the EGR system pass over the hot intake valves and cook, forming a crusty carbon layer. This buildup restricts and disrupts incoming airflow, leading to engine misfires, rough idle, poor throttle response, increased emissions, and a drop in fuel economy.
Diagnosing carbon buildup involves using a borescope to look down the intake runners or performing a compression test. To clean the valves, technicians must perform a specialized valve cleaning service, such as walnut shell blasting (spraying compressed air and crushed walnut shells to scour the valves) or manual scraping after applying chemical carbon solvents.