Originally Posted by SD Z4MR
So how did this stuff get past the cabin air filter? Was it not installed or not installed correctly? When I replaced mine there were a few leaves on the inlet side, but the outlet side was clean. That cabin air filter is a PITA, it's much easier to replace on my wife's 335is Coupe.
The cabin air filter is after the outlet flap in the air flow cycle. The filter is actually part of the problem as it is within 2mm of the open flap and the leaves rest up against the filter and accumulate. If fresh air flap is left open for a long period of time, leaves and debris accumulate and once they hit the filter, they have no where to go. On my car the leaves had accumulated about halfway between the cabin filter and the inlet under the hood. Bottom line, check or change air filter regularly.