Why does the idle speed increase when the air conditioner is turned on?
3 Answers
When the air conditioner is turned on, the engine requires additional power to drive the compressor, increasing fuel injection to supplement power, which results in a higher idle speed. Below is an introduction to car air conditioner maintenance: 1. Do not arbitrarily adjust the air outlet direction: When using the air conditioner, not paying attention to adjusting the direction of the air flow is not conducive to achieving the best effect of the air conditioner. According to the principle that cold air sinks and hot air rises, the correct approach should be to point the air outlet upward when cooling and downward when heating. 2. Regularly start the air conditioner in winter: Not using the air conditioner for a long time can cause its rubber seals to age, and the lubricating oil on the internal components will dry out. Regularly starting the air conditioner allows the components to be lubricated, maintaining good condition. Therefore, it is best to start the air conditioner once a week in winter. 3. Avoid using the air conditioner for too long each time: Keeping the air conditioner on for a long time after getting in the car can cause excessive pressure on the condenser, which can damage the cooling system. Therefore, it is advisable not to use the air conditioner for too long each time. If the temperature inside the car has reached a comfortable level, you can turn off the air conditioner and turn it on again after a while. 4. Ventilate and cool down before driving in summer, then switch to internal circulation: When you first enter the car, you should open the windows for ventilation and turn on the external circulation to expel the hot air. After the temperature inside the car drops, switch to internal circulation.
I've been driving for over a decade and have seen plenty of cases where the idle speed increases when the AC is on. Generally, this is normal because once the AC compressor kicks in, the engine needs to work harder to drive it. The ECU automatically raises the idle speed slightly to prevent stalling—an increase of 100-200 RPM is nothing to worry about. However, if the idle speed shoots up to 2,000-3,000 RPM and doesn’t stabilize, then there’s likely an issue, such as a stuck or dirty idle control valve or an overloaded AC system. I suspect that a vacuum leak in the engine bay or an inaccurate throttle position sensor signal could also cause the ECU to make incorrect adjustments. It’s a good idea to regularly check if the belts are loose and clean out carbon buildup. If the idle speed stays too high, get it fixed promptly to avoid wasting fuel and wearing out parts. Checking it yourself is simple: turn off the AC and see if the RPM drops back down, and listen for any abnormal sounds from the compressor. Don’t underestimate this issue—safe driving should always come first.
I've studied automotive control systems, and the high idle speed when the AC is on is essentially the engine ECU compensating for the load. When the AC is turned on, the compressor adds a burden, and the ECU fine-tunes the throttle opening to maintain stable RPM, with a normal increase of 100-300 RPM. However, abnormally high idle speed is often caused by a faulty idle actuator, intake leaks, or issues with the AC pressure sensor. I keep an OBD scanner at home—just plug it in to read fault codes, as sometimes a drifting MAP sensor can lead to misjudgment. Regular maintenance includes changing the AC filter to prevent clogging and increased load, and checking if the refrigerant pressure is balanced. Prolonged high idle speed wastes fuel and accelerates engine wear, so timely detection saves money and hassle.
