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