Is it harmful to the car to idle with the air conditioning on?
3 Answers
Idling with the air conditioning on for a short period of time is not harmful to the car, but it will consume gasoline, leading to carbon buildup in the engine, which can damage the engine over time. Here are the specific details: 1. Idling can easily cause oil burning: During idling, the throttle opening is small, and the vacuum inside the intake manifold is high, making it easy for oil vapor to be drawn into the intake manifold. Some of it deposits inside the intake manifold, forming carbon-like substances or coking. Prolonged low-speed operation or idling, especially in turbocharged models, can result in a pool of oil inside the intake manifold behind the throttle. 2. Idling can easily cause carbon buildup: During idling, the air-fuel mixture tends to be richer, so prolonged idling with a richer mixture can easily lead to carbon buildup in the engine.
With ten years of car repair experience, I've seen too many cases of idling with the AC on. Prolonged low-speed engine operation leads to incomplete fuel combustion, which easily generates carbon deposits, especially on the back of intake valves in direct injection models—it looks like a layer of black sesame paste. It's normal for the exhaust pipe to drip water, but if the exhaust temperature isn't high enough, it can corrode the inner walls of the muffler. Older cars need close monitoring of the coolant temperature gauge; if the cooling fan is spinning like crazy, it indicates high radiator cooling pressure. The most damaging issue is battery drain. My 2005 Passat once idled with the AC on for three hours, and the battery died the next day—the electric fan + AC compressor consumes too much power. Cars with start-stop systems need extra caution.
Last time I used an infrared thermometer to measure, the engine compartment temperature was about 15°C higher when idling with the AC on compared to driving. Rubber hoses age faster in this stuffy environment, especially for cars over ten years old—the surface of coolant hoses starts to crack. The belt slipping noise is particularly noticeable in quiet underground garages, caused by the sudden high load when the AC compressor kicks in. There was a modified Civic with low idle speed that actually displayed a transmission overheating warning on the dashboard after idling for half an hour—turns out it was because the radiator wasn't getting any airflow.
