There are two times of year that usually have me changing the cabin filter on my cars: Pollen season and leaf season. But if you have an E39 like the BMW 525i, you should probably take the job one step further.
One of the E39’s known design weaknesses is the compartment that houses the brake booster. It can fill up with water if not properly maintained. This is an out-of-sight, out-of-mind problem waiting to happen. It can cause our brake booster fill with water, which causes unsafe braking (hard pedal travel) or pass water to your car’s engine via the vacuum system.
Parts
Cabin air filter kit (includes two filters)
BMW Cabin Air Filter (E39) - Meyle 64112182533
Tools
- Long screwdriver
- Shop vacuum
- Paper towels and window cleaner (optional)
Replacement
With the car’s hood open, you will be able to see the pair of housings for the cabin air filters.
The filter housings are mirrored images of each other. There is a long metal clip running along the forward edge of the housing. Pull it forward to detach.
Lift the forward edge of the housing upward. This will allow you to undo the hinges located at the back edge of the housing. Remove the housing cover.
Lift the old filter out of the housing.
The inside of the housing will likely be dusty and may include some leaf debris. Clean the inside of the housing. Make sure to remove any leaf debris from the air inlets around the outer edges of the housing (Red Arrows). Also this is a good time to clean the dust out of the housing with some window cleaner and a rag.
Reinstall the new air filter. Note that some air filters use a soft cardboard outer edge that simply slips into place, while others have a hard plastic edge with some tabs that may need to be aligned with the housing.
Reinstall the housing cover. Close the metal clip to secure the house and cover.
Repeat this process for the other cabin filter.
One Step Further: Save Your Brake Booster
While you are servicing the driver side cabin air filter, let's go one step deeper and make sure that your brake booster isn't about to get flooded in the next big rain. For some reason nobody can explain, BMW really dropped the ball well they designed the box around the brake booster. It resides underneath the cabin filter, and it has some woefully inadequate drain holes. These must be kept clear otherwise the box can fill up with water and eventually flood the brake booster. Make this a regular part of your cabin filter routine.
With the cabin air filter and cover removed, unplug the hood switch.
Use a small flat blade screwdriver to pry up the three tabs that connect the air filter housing to the air tube.
Rotate the air tube a few degrees counterclockwise. The airtube should be able to pull straight out.
There should be a spring clip (missing from this photo) attaching the front foot of the air housing to the cars body. Squeeze the clip and lift it out of the way.
Lift the air filter housing out of the engine bay. Note where it connects to the edge of the cowling near the base of the windshield.
There are three drain holes in the compartment surrounding the brake booster. All of them are small enough to become easily clogged: wiring pass-through, crescent-shaped hole directly under brake booster, circular drain hole (deep below and behind the brake booster).
Use long screwdrivers to loosen up the dead leaves. Then follow up with a shopvac with corner extension to clean up all the debris.
Reinstall the cabin filter housing, air tube, new air filter, housing cover, cover clip, and the wire leading to the hood switch.
