Does idling with the AC on cause carbon buildup?

[VanMila]

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.

[McPatrick]

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.

[DamianLynn]

I think it depends on how you use the air conditioning. My old car suffered before—during summer, I'd often idle for half an hour while waiting to pick up my kids from tutoring by the roadside. Half a year later, it started shaking during cold starts. A veteran mechanic later explained that running the AC while stationary puts extra load on the engine because the cooling fan spins aggressively. But since the car isn't moving, the engine stays relatively cold, and the fuel combustion can't keep up with the RPM demand, leading to carbon buildup. Older cars, especially, are more prone to this issue. Now, I use remote start and time it to activate the AC just five minutes beforehand. If I have to wait, I turn off the engine and stand in the shade. The engine runs much quieter now, and my refueling intervals have even extended by four days.

[SerenityRose]

Idling with the AC on does cause carbon buildup, but the key lies in time control. I conducted a test myself: within the first 10 minutes after turning on the AC compressor, the increase in carbon deposits was negligible, but exceeding 25 minutes resulted in visible black soot mixed with dripping water from the exhaust pipe. Modern fuel-injected vehicles perform much better than the carburetor era, but they still can't completely avoid carbon buildup. There are several situations particularly prone to carbon deposits: revving the engine while the AC is on and listening to music, vehicles with modified intake systems, or using engine oil with excessively high viscosity. My advice to car owners: avoid running the AC while parked for more than 15 minutes, take a highway drive monthly to flush out some carbon deposits, and using 95-octane gasoline with detergent additives from reputable gas stations is also very effective.

[McCatherine]

A veteran taxi driver with ten years of experience shared this insight: Idling with the AC on in summer is a major source of carbon buildup, especially in turbocharged models. He installed an OBD monitor in his car, and the data showed that the air-fuel mixture concentration was 11% higher when idling with the AC on compared to driving, leading to a sharp drop in combustion efficiency. Naturally aspirated engines fare slightly better. Nowadays, when waiting for passengers, he turns off the AC and opens the windows for ventilation. If he must use the AC, he switches to external air circulation and cracks the windows slightly. Based on his experience, using two bottles of fuel additive during each maintenance session keeps carbon buildup at bay for up to 50,000 km without needing specialized cleaning. His car has clocked 300,000 km, and the engine still runs strong—this tip is worth considering.