What Causes the Sour Smell When Turning on the Air Conditioning in a BMW?
4 Answers
The sour smell when turning on the air conditioning in a BMW is caused by a dirty air conditioning filter that has been used for too long. The functions of the air conditioning system are: 1. To cool, heat, ventilate, and purify the air inside the car; 2. To provide a comfortable environment for passengers, reduce driver fatigue, and improve driving safety. Taking the 2021 BMW 5 Series as an example, it belongs to the mid-to-large-sized car category, with body dimensions of: length 5106mm, width 1868mm, height 1500mm, wheelbase 3105mm, fuel tank capacity 68l, and curb weight 1705kg. The 2021 BMW 5 Series features a double-wishbone independent front suspension and a multi-link independent rear suspension. It is equipped with a 2.0l turbocharged engine, delivering a maximum horsepower of 184ps, maximum power of 135kw, maximum torque of 290nm, paired with an 8-speed automatic transmission.
My BMW also had the issue of the AC blowing out a sour smell, which is mainly caused by mold inside the AC system. When the evaporator cools, it condenses moisture, and if the dampness isn’t properly vented after parking, it becomes a breeding ground for mold. Especially on rainy days, turning off the engine right after using the AC leaves residual moisture in the ducts, fostering bacteria that produce a musty, stale odor. I later learned a trick: turn off the AC a few minutes before reaching the destination and let the fan run for another three minutes to dry out the moisture. Additionally, regularly replacing the cabin air filter is crucial—a dirty filter acts like a bacteria breeding ground, and failing to replace it promptly will cause the smell to return.
Recently, I always smell a sour odor when turning on the AC. After some research, I found a minor design flaw in BMW's air conditioning system: the air intake is prone to accumulating leaves and dirt. Those small particles pile up on the evaporator, and combined with a humid environment, they ferment and produce the sour smell. I specifically cleaned the drainage channel under the windshield and indeed removed a lot of rotten leaves. Another often overlooked spot is the blower wheel. During the last maintenance, the mechanic showed me that the gaps between the fan blades were full of black grime. After spraying it with a specialized foam cleaner, the odor disappeared immediately. It's recommended to do a simple cleaning every quarter to prevent this issue.
Most air conditioning odors originate from mold growth in the evaporator core. When the AC is turned on, the evaporator temperature drops sharply, causing condensation when encountering humid air. After parking, this moisture remains trapped in the metal ducts, combining with dust to breed large amounts of mold. The most effective method I've used is purchasing automotive-specific disinfectant spray. After starting the vehicle, turn on the external air circulation at maximum fan speed, then spray the cleaning foam into the AC air intake. Allow the foam to sit in the ducts for twenty minutes before ventilating to dry it out - this can eliminate about 90% of odors. Remember to wear a mask during operation to avoid inhaling the chemical fumes.
