Why does the air conditioning not cool when idling but works normally when driving?

[McHarrison]

Here are the reasons why the air conditioning does not cool when idling but works normally when driving: 1. Expansion valve damage: This prevents the formation of high pressure in the system before the expansion valve, causing the high-pressure refrigerant to fail to reach 150Kpa. As a result, the evaporator after the expansion valve cannot cool down, leading to ineffective air conditioning. 2. Internal wear of the air conditioning compressor: This results in the pressure between the air conditioning compressor and the expansion valve failing to reach 150Kpa~200Kpa, preventing the refrigerant in the system from circulating normally and causing the air conditioning to not cool. The idle speed can be adjusted by modifying the throttle size, among other methods. Generally, the optimal idle speed is the lowest speed at which the engine does not vibrate.

[KiaraDella]

As someone who loves studying cars, I often hear friends complain about this issue: when the car is idling, the air from the AC isn't cold enough, but as soon as you start driving, it gets freezing. The key issue is that the car's AC relies on the engine to drive the compressor. At idle, the engine speed is low, so the compressor runs slower, and the refrigerant isn't compressed enough, leading to poor cooling performance. Additionally, when parked, there's no airflow, so the condenser at the front doesn't dissipate heat well, and the fan might not spin fast enough to keep up with cooling. This is especially noticeable on hot summer days. If your car is old or the refrigerant is a bit low, the problem becomes obvious at idle. I recommend first checking the AC system's pressure to see if it's low on Freon. Also, clean the condenser mesh—if it's clogged with dirt, heat dissipation suffers. Regular maintenance can prevent this, like getting an annual AC check-up. If you encounter this while driving, avoid prolonged idling—moving the car naturally helps.

[Phil]

As a veteran driver with twenty years of experience, I'm quite familiar with this issue. If the AC isn't cold at idle but works fine while driving, it's mostly due to an underperforming cooling system—the fan spins too slowly at idle, failing to cool the condenser, which reduces cooling efficiency. The compressor also has weaker output at low RPM, especially in older cars or aging systems. The solution isn't difficult: start the engine with the AC on and listen for the fan—if it's silent, it might need repair. Also, check the compressor clutch to see if it engages steadily at low RPM. Avoid prolonged idling with the AC on, as it can overheat and damage the entire system. Cleaning the external radiator or upgrading to a high-efficiency fan might help. Don't delay—visit a professional shop early to inspect the system; a small expense now can save major trouble later.

[LeChase]

I've encountered a similar situation where the air conditioning feels lukewarm when the car is idling but cools down once moving, which is quite common. The issue lies with the engine RPM—at idle, the speed is low, causing the AC compressor to rotate insufficiently, resulting in weaker cooling. Additionally, parked cars lack airflow, leading to poor heat dissipation from the condenser. It's advisable not to frequently idle with the AC on; instead, find a shaded spot in summer. Once driving, the moving air helps with cooling. Check if the AC filter is dirty, as it can obstruct airflow. A simple cleaning of the front compartment's cooling fins can improve things. If this happens often, consider topping up the refrigerant or inspecting the fan system, but avoid DIY repairs.