Does idling with the AC on cause carbon buildup?
2 Answers
Idling with the AC on can cause carbon buildup because the engine remains in idle for extended periods, preventing the complete combustion of the air-fuel mixture inside the engine, which leads to increased carbon deposits. Do not let your car's engine idle for long periods. Carbon buildup has many negative effects on the engine. If carbon deposits accumulate on the piston top, it can increase the engine's compression ratio, leading to engine knocking. Knocking can result in higher fuel consumption, reduced power, and in severe cases, even cause connecting rod deformation or piston cracking. It is not recommended for car owners to sleep in the car with the engine and AC running continuously during summer. If the engine and AC are left on for prolonged periods while sleeping inside, it can lead to carbon monoxide poisoning, which is extremely dangerous.
As a tech-savvy professional who works with cars year-round, I believe this issue depends on specific circumstances. During hot weather, many people habitually idle their engines with the AC on while waiting in the car, which indeed makes carbon buildup more likely than during normal driving. When the engine idles at low speed, gasoline atomization becomes less effective, and the air-fuel mixture doesn't burn completely. Those unburned sticky residues gradually form deposits on valves and piston tops. I've observed that vehicles idling continuously for over 20 minutes show significantly dirtier throttle bodies. Occasional short-term use isn't a big problem, but prolonged idling is harmful—it wastes fuel, damages the engine, and produces unpleasant exhaust fumes. If you notice your car struggling during acceleration or a sudden increase in fuel consumption, carbon deposits are likely the culprit.
