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2021 MY vs. 2018 M3, cabin filter mods

I have a 2018 M3 and 2021 MY. Today I replaced the cabin filters and thought I would share some observations. There are good videos on how to replace (and clean the coils) but haven't seen what I've done.

TL;DR I JB-weld glued a bolt to my 2018 filter cover screw and I used a carbon filter and double sided take to try and keep moisture off the filters.

2021 Model Y

Same T20 screw and same stinky feet problem, but they moved the screw to the bottom of the lid making it MUCH easier to remove. After just 2400 miles the bottom cabin filter was already discolored on the bottom of the filter. This indicated to me that the primary source of the problem is water pooling around the bottom and soaking up through the filter. I thought about how I could keep the new filter from touching the bottom using what I have on hand. I didn't want to do something that might come off or apart and get stuck.

What I ended up doing was cutting a length of carbon filter that would wrap all the way around the filter outside edge, including the tab. I used double sided tape (between carbon strip and the cabin filter) to keep the carbon strip from moving around. No chance of losing it because I will be able to pull it up and out with the tab. It fit fine, the second filter was a bit tighter fit, but I think that is mostly because the carbon "caught" the filter and didn't let it slide in as easily.

My theory: The carbon filter will not soak up moisture like the paper/fuzzy filter does. The double sided tape will also create a moisture barrier between the carbon and cabin filter.

Only time will tell, but maybe others will have a better idea to test. I also thought about ways to try and keep some spacing/barrier between the metal coils and filter, but there seems to be very little space to add anything there, hope someone has something to try.

Edit: I didn't see a leaf inlet cover for the frunk, the 2018 and 2021 model 3 are different shapes and the 2021 Model 3 leaf filter didn't look like it would fit on my 7-seater 2021 MY. So I took the same carbon sheet and applied it to the bottom of the frunk cover (no tape) by cutting some holes to go over the clips.

Model 3

Took the opportunity to make the higgen T20 screw into a thumb screw. Borrowed ideas from other posts but kept it simple and just JB weld glued a bolt onto the end of the screw (no drilling / cutting, etc.).

Source of the problem?

It seems like some people don't have this same problem. I'm in Austin, TX and have an attached garage. My garage is a warm 85 degrees but not air conditioned. I believe the warm humid outdoor air when it comes into my garage cools down which raises the relative humidity. So for most of their lives the cars sit in relatively higher humidity than if they just sat outside.
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I didn't take pictures. But here are links (not affiliated) of what I had. I cut the carbon to about the width of the scotch tape, and the length to go the length and width of the filter outside edge (including tab). I then placed the same length of tape on the carbon, and then stuck it to the outside edge of the bottom filter (and tab). I didn't do the top filter because I don't think it is necessary and may be too tight. Hope that helps.

You'll have to wait until next summer to find out how well it has worked, and I'll be following this thread in case anyone else has good ideas :)
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@dduffy Just curious if you have started smelling anything bad yet from your filter mod since it's been a few months and the rain has come out. About to change my filters for the 5th time in 3 years because this issue is so bad.
Yes, unfortunately the 3 has but the Y has not. I will pull them both this weekend to inspect differences. The Y is using Tesla OEM filters and the 3 is using cheap Amazon filters.
Yes, unfortunately the 3 has
Darn- was hoping you found a solution. Let me recommend this video put out by SaraJAwesome, she uses an endoscope to look deep into the air system, as well as takes out the frunk to see drainage:

I'm about to put in one of those off-brand filters with less paper materials. I've also read some suggestions about spraying with a mold control product (Concrobium). May give it a try.

Let us know if you found anything new to try as well.
Okay, so my experiment didn't work.

First the 3. The filter still looked good, no staining, etc, but the stink was through both filters. Also it wasn't stinker around the edges. So I used a camera and the coils looks clean. At the bottom there was a triangular area I guess for drainage that looked okay too, but I could clearly see mineral deposits (or mold) a bit further away.

Could not get a good pic, but closer to the passenger side is the triangular drain area, and near the driver's side was a flat area with mold or mineral deposits.

So I don't think it was the coils or edge of filters. Rather that area of mold saturating the air that was getting sucked through the filters.

This time I used two cans of "Kilma" (used Kool it before). The Kilma cleaner came with a much longer straw so I pushed it all the way DOWN to start filling from the bottom up to make sure to saturate the moldy area.

That filled half way (or until the lower opening). Then I used a second can to fill the entire cavity.

I also used some $60 charcoal filters from Amazon this time (but I don't think it is a filter problem now).

Time will tell. I really liked the Kilma better due to its straw. It has also left a string chemical smell that hasn't cleared out right away. Hope that means it's working!

The Y was not stinky yet in the cabin, but I checked the filters and they too had a slight smell. Not bad enough to enter the cabin yet though. Using the scope I noticed the inside was slightly different than the 3. There was some white residue at the bottom as well, but not as much. Because I didn't have any Kilma or filters I just but them back in. They will definitely stink come spring/summer.

Hopefully the double shot of Kilma does the trick this year.

Also the 2021 Y is MUCH easier to replace. I stripped the speaker wire from the 2018 3 as the wire got caught on something :(
Good write-up. I have been wondering for some time if the charcoal filters work as well as a repellant to the mold smell a little longer. Unfortunately, they won't control the mold itself. Trying to figure out if a mold control spray will help in the long run, or if it will kill me.
Following up, the stink came back on both the 3 and Y.

On the 3, last fall I used two cans of Kilma and a "HEPA charcoal" filter. Now humidity is raising the stink is back. I was hoping that by doing it in the fall the dry winter air would have killed off anything lingering after the clean.

Someone recommended "breath easy" filters, so that's what I am going to use in both cars for this summer.

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