What clogged up the drain?

2014 Subaru Legacy, 122k miles.

My AC drain backed up and flooded the passenger side floor. I opened the hood and saw that I could reach the tube from above. I tried a small drain brush and a zip tie, neither worked. So I pulled back the carpet, removed the glove box and tray that was underneath it. Then I unplugged the hose from the AC coil compartment.

The hose had a thick tar like substance clogging the hose. It was black and a bit fibrous and sticky but not greasy. It was exactly like that non hardening sealer made of tar and fibers. I eventually got enough of it out so that water can flow through the hose, but I didn’t get it all out.

I wonder what it is and where it came from. BTW, it was not at the mouth of the hose but about 2" from the top and I estimate that it was about 3" long based on squeezing the tube before I finally got some of it out. I got some out using hot water, a 36 tie staip and some detergent. The dish detergent was a lubricant to help push the tie strap down through the hose.

It might be a combination of dust/dirt/& mold. Sometimes debris from the road, esp if you have to drive on newly paved surfacs, will coat the end of the tube, but doesn’t usually go up into it. AC drain tube clogs are a common thing reported here. Use the forum search feature for more info, upper right this page. It sounds like you are on the right track. Shops often used compressed air to clean AC drain tubes.

Some manufacturers put a foam gasket on the mode/blend doors so they’re quite during operation.


This foam breaks down over time and can clog the drain.


Mastic patches are sometimes used to seal the HVAC case where the evaporator tubes enter.

The substance is similar to butyl tape, if not manufacture to the proper constancy it car soften and drip in the heat. In the late 1990’s we had butyl tape dripping from behind the door panels on Mitsubishi cars.

If u can, put the extracted plug into some dilute bleach and see if it breaks down a a couple minutes. If so it is black mold based and u NEED to do something about that as that is causing severe lung issues in India right now.

My AC drains at home are often plugged with a translucent sludge which is a mold. When servicing I inject some pool based un-stabilized chlorine granules into the water trap reservoir. Works real well, cuts down service cycles by 50%!.

Mastic, that was the word I was looking for. I don’t think it is mold, we have a lot of that around here but it gets slimy when wet, this does not. The rubber tube is molded with several curves in it and over a foot long when straightened out. The plug was about two inches from the top and stretching down another three inches. No evidence of it entering the tube at the top or bottom. There is a 90 degree bend down about two inches from the top and the plug started at that bend. The rubber was perfectly clean above the plug.

This issue is so frequent that it’s darn near a maintenance item, not really evidence of anything that you can “fix”. Maybe it’s more common in areas with high humidity and a better environment for the growth of molds. Finding a way to force a solution of water and chlorine beach down through would be helpful, but it will come back.

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Yes it is an issue and I have dealt with it many many times in the past, but I have never seen a clog like this.

this has been driving me nuts since you posted. what the mystery object is.
just a thought. have you ever had the windshield replaced? if so maybe a piece of the windshield sealant fell through the leaf guard and made its way to the tube.

Strips of windshield urethane are hard and usually found on the cabin filter after a windshield replacement, this should not find its way to the evaporator housing.

Yes I did have the windshield replaced, but that was about 5 years ago. But I agree with @Nevada_545 that it could not have gotten through the cabin filter and into the drain tube without a lot of evidence left behind, of which there is none.

I had thought that it might have been some kind of a filter to keep critters from getting into the car that molded but it was not slimy, it was sticky like a non hardening mastic. But if/when it clogs up again, I will try the bleach to see what happens.

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