Bacteria and other microorganisms that live inside your BMW’s evaporator housing give off a musty odor when you start the car. Constant condensation of air humidity at the evaporator creates a favorable environment for bacteria. Together with dirt brought in by fresh air, this may cause microbiological growth, and consequently the smell we’d rather not think about.
Now that we established from where the smell comes, let’s have a look at how to get rid of it.
Replacing The Cabin Air Filter
Replacing the microfilter alone will not help you to get rid of the smell. However, regularly changing the microfilter may reduce the entry of dirt and therefore delay or, in rare cases, prevent the development of smelly cultures.
Another point worth mentioning is the microfilter’s proper installation. I’ve seen hundreds of crumpled and wrongheaded filters in BMWs. So, if the microfilter is twisted, the food for bacteria will simply pass by the filter right into the ventilation system. This will accelerate the development of smelly cultures.
Cleaning The AC Evaporator
Cleaning the evaporator with a disinfectant spray lance is the most effective and long-lasting method of removing the musty odor from the air conditioning. In this technique, the disinfectant is sprayed directly on the evaporator with high pressure. This is the official BMW Service procedure used by BMW dealers.
BMW F30 3 Series
I won’t bother you with complicated BMW procedures as they require some hard-to-get/expensive special tools. Instead, we’ll take a look at three different DIY methods that’ll get you results as close as it gets to professional equipment.
Note! Always start the engine and switch on the air conditioning before you begin. Check that the condensation is flowing from the drain hoses by letting it run at idle for a few minutes.
Method 1: Soaking the evaporator with AC foam cleaner.
When you bathe the evaporator in AC cleaner foam from both sides (front and back), the cleaner will go through the core of the evaporator, kill the stinky microorganisms, dissolve dirt, and then drain it through the drain ports after it liquifies.
In this example with E39 5 Series, you can see how it works.
Method 2: Blowing the AC cleaner foam through the evaporator.
The rationale behind this method is that the blower fan will push the AC cleaner foam deep into the evaporator.
Here’s the F30 3 Series example:
The product used in the video:
Method 3: Apply the AC cleaner foam through a microfilter drain hose.
Because you can apply AC cleaner foam through the microfilter drain line, this is the simplest technique. This is also the approach I take the majority of the time. This will work for most F and G Series BMWs.
Here’s the F30 3 Series example:
Which Method to Use?
Deciding which method is best for you mostly depends on the BMW model you own. For example, on E46 3 Series you can remove the blower motor resistor to access the evaporator and soak it with cleaning foam. On E39 5 Series you’ll need to drill a hole, as in the “method 1” video (this is actually official BMW procedure).
Drilling the hole for evaporator cleaning (E39)
On E7x X5 and X6 you can access the evaporator through the evaporator temperature sensor and soak it with foam.
Pull out the evaporator temperature sensor (E70, E71)
On E6x 5 and 6 Series you can access the evaporator from the front after removing the fresh air duct under the windscreen.
Cleaning the evaporator (E60, E61, E63, E64). When working on the E6x series, I’ll remove only the microfilter housings (left and right).
On F2x 1/2 Series and F3x 3/4 Series, you can access the evaporator through the microfilter housing by removing the microfilter, or through the microfilter drain hose.
Remove the microfilter cover and microfilter to access the evaporator (F2x, F3x).
On Fxx 5, 6, and 7 Series you can also access the evaporator the same way.
Remove the microfilter to access the evaporator or use the microfilter drain hose (F01, F02, F03, F04, F06, F07, F10, F11, F12, F13, F18).
The same procedures can be used for the G series.
Remove the microfilter or use the drain hose (3). G30 5 Series example.
On the UKL platform-based BMW and MINI models (front-wheel-drive based) just remove the microfilter to access the evaporator.
Open the microfilter cover and remove the microfilter (F45, F46, F48, F49, F54, F55, F56, F57, F60).
To remove the blower motor resistor on Z4 E85 and E86 models, you’ll need to do a little additional work. But then, you can always open the top.
And for the E8x 1 Series and E9x 3 Series you’ll need to remove the blower fan.
Spray lance schematic illustration on removed and cut-open AC unit (E81, E82, E84, E87, E88, E90, E91, E92, E93). By carefully rotating the spray lance back and forth in the direction of the arrow, evenly apply AC cleaning foam to the evaporator.
Liqu Moly AC System Cleaning Instructions
https://www.liqui-moly.com/en/us/products/product-groups/ac-system-instructions.html#tabItem58815
Finally some post after searching for about 1 hr. I called 2 dealers in my area (Chicago IL) and they do not offer anything to clean that funky smell from a/c. I used few things like “whole car re-fresher” but seems not work to well.. Last years I sprayed vents and outside and worked for a little. Now its smells again and I was thinking about just do the method you put there. I have 2014 BMW x6 M, I am a little worry that I might screw something up. Just pull that board near the pedals and remove evaporator temperature sensor and soak in with foam? The car should be off since I am removing sensor?
Hi there! Sorry to hear the BMW dealer can’t help. For E71 just pull the sensor out and soak the evaporator housing with foam. The sensor removal doesn’t affect anything, but you want the blower OFF while spraying inside. Here’s the step-by-step procedure:
– A/C system must be switched to air recirculation function
– Close all air vents manually
– Switch the car OFF
– Remove trim above pedals (3x torx 20 IRC) and disconnect the footwell light
– Pull out the temp sensor and put it aside
– Soak the evaporator and put the temp sensor back
– Wait for about 5-10 min.
– Start the engine
– Open all air vents
– Set the A/C to footwell ventilation and fresh-air mode and open side windows/doors
– Let the A/C system run for at least 5 minutes at maximum blower output
– Enjoy 🙂
Thank you Sir, (I am reading respond late 30JUL23, but here is my story)
Sounded complicated at the beginning but it was very easy. I did that 2 times. First I did it and it lasted for about a 2-3 weeks and smell came back. Especially first 2-3 second when I started A/C. I noticed that when I just put fan without a/c it was bad smell nonstop. I ended up repeating the process, and I bought this times 2 cans of nextzett 10oz. I sprayed evaporator, and then I put some foam far in via front middle vents. In my opinion that smell was somewhere in the middle/Top (those vents seems to have some kind auto close after few seconds). So far about a month no bad smell. A/C seems to perform really good at this time. For some reason I think those foam sprays should have sizes like 20oz.
Its my second BMW. I still love this car (X6M) even if its 9yrs old, drinks a lot of gas, but has only 50k miles on it.
Thank you for sharing your knowledge, you helped a lot of people. Your post been share throughout many forums (while I was searching for that steps). Most of us have love/hate relationship with BMW.