coke
To add deleterious carbon deposits as a byproduct of combustion.
coke: when fuel leaves sticky carbon behind
To coke is to form hard, carbonaceous deposits inside an engine or fuel system as a result of heat and incomplete combustion. These deposits, called coke, accumulate on fuel injectors, intake valves, piston crowns, and combustion chamber walls. Over time they restrict fuel flow, increase friction, and degrade engine performance.
Coke forms when gasoline or diesel fuel is exposed to sustained high temperatures, typically above 200 degrees Celsius, before it can burn completely. Low-quality fuel with high aromatic content, fuel contaminated with water or sediment, and engines running lean or at abnormally high combustion temperatures are all prone to coking. Direct injection gasoline engines are particularly susceptible because fuel contacts hot metal surfaces before mixing with air.
Symptoms and consequences
A coking engine exhibits rough idle, hesitation on acceleration, decreased fuel economy, and increased emissions. Severe coking can cause the intake valves to stick open, leading to loss of compression and misfires. Carbon buildup on piston rings restricts their movement, increasing blow-by and oil consumption. In extreme cases, deposits on the piston crown create hot spots that promote detonation and engine knock.
Prevention relies on fuel quality, proper engine operating temperatures, and fuel system cleaners. Detergent additives in premium gasoline help suspend and remove deposits before they harden. Regular oil changes and proper ignition timing reduce the conditions that favor coking. Some engines require periodic intake valve cleaning or top-end decarbonizing to remove heavy deposits.