[TripleBlack]

If you are driving an E93 or F83 (the two models I have experience with), make sure that the rear quarter panel drain plug is not blocked or just completely removed from the car.

I'm in the process of preparing my E93 for sale and the cable in the rear passenger window mechanism snapped out of no where leaving me with a window that wouldn't drop on its own.

Got back there and found that the lower part of the system was permanently in an 11 inch deep pool of water that it rusted over. The wire that pulls the window down snapped.

I always knew the water collection was a thing but I didn't know it was in the same place as important components and just how much I was carrying on regular journeys. Almost like having a permanent passenger everywhere I went.

(FYI, the water draining in the first photo isn't the plug I am referring to. That plug is plastic and can be reached from underneath the car. This particular drainage started because I removed a screw that kept the window mechanism in place and the water just found another way out.)

[micdev42]

Hi, thanks for posting this, but I'm struggling to see where these parts / areas are. I have a 2007 pre-LCI E93. Any chance you can post some photos taken further back with some arrows on them to show where you are talking about? Thanks

[TripleBlack]

Soz.

The middle of the video in the below link should show you what needs to be done to avoid the issue.

[TripleBlack]

I'm back again to reiterate the same thing for my F83 now.

Its cold outside lately, probably freezing at night FYI if you are reading this in the far future.

STORY:
I recently carried out a brake fluid change and decided to do a final test on the brakes by doing an emergency stop. I don't think I went any faster than accelerating to 20mph before slamming the brakes.
....a wave of water washed over my right shoulder and thigh covering the driver side window, door panel and part of the dash and carpet all the way to the accelerator pedal!
I immediately knew what it was but I was surprised because that would mean the water level in the rear quarter would have had to have been up to the very top of the window frame.

I got under the car, removed the side sill and gained access to the drainage plug.
[Bear in mind that this plug is covered by two separate panels so I am unsure how it is meant to drain effectively. Water would have to fall on one panel, either flow back and forth
within that panel until it escapes or overflow on that same side until it falls on the next panel to then find another escape route again.

I had to use a screw driver to dig out the plug which wasn't that hard.
Out came a popsicle!
Take a look at the photos to see how what came out compared to how it is meant to look.

By this time it was 3pm and the sun had been out for 5 hours. A trickle of water came out, so I stabbed at the inside of the hole breaking the ice to allow the rest of the water to come out.
I think about 3-4 litres poured out of that side of the car.

I made a decision to not re-insert the plug this time around. I may put it back in in a couple of months, made easy by the fact that I have cut out an opening in both panels to allow the water to just fall straight out.
I think the low location of the plug results in it being the last part of the body work to receive some warmth which doesn't help when water and dirty accumulates and freezes very easily there.

My next task is to spray some lubricant up the hole and hope for the best.

TLDR:
Remove the drain plug from underneath your rear quarter panel. It is retaining rain water that is mucking up your fuel economy and destroying the metal mechanisms back there.
Also when water freezes it expands right? Imagine what that is doing to micro holes and tears in the body work.