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The past few weeks, I have been hearing a water sloshing sound from inside the car. It sounds almost like there is a large-ish tub below the passenger seat filled halfway with water, which then sloshes as I drive the car. The sloshing is most audible when I accelerate and decelerate, and a bit less so when turning or going on slopes.

At first I wondered if it was gasoline moving inside the partially full tank, but that makes little sense. First, I've never heard it before, second, my gas tank is in the trunk and not below the passenger seat, third, the sloshing continues with a full tank.

There are no water bottles or similar containers in the car that would account for the noise.

I am now suspecting that the water is accumulating in the AC ducts:

  • It has been raining a lot lately and my AC has a mildew smell reminiscent of still water.
  • Earlier, when I had hot AC run for a while the sloshing lessened (water dried?). Unfortunately since then it rained even more and now the AC trick doesn't seem to work.
  • I noticed some dampness on the carpeting right in front of where the AC outlet for feet is, below the seat.
  • I've occasionally heard what sounds like splashing below the seat, which is consistent with a container that opens into the cabin.

Unfortunately, when I tried looking into the AC outlet and feeling with my fingers, I couldn't confirm nor deny this suspicion.

So there are three related questions here.

  1. How do I confirm it's really water in the AC?
  2. How do I get it out?
  3. Is this supposed to happen, or is something wrong with my car that led to this? The only thing that comes to mind is that weatherstrips at the bottom of my side windows have come off recently, but I couldn't say for sure if the sloshing is coming from the bottom of the car or from inside the door. The door doesn't seem to leak any water.
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    What is the year/make/model of the car? – Pᴀᴜʟsᴛᴇʀ2 Jan 5 '19 at 18:35
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    If the door drain holes are blocked, then they will hold water and quite a bit... have a look along the inside edge and you should see them if they are there, a small wire or plastic cable tie is usually sufficient to push the crud out... – Solar Mike Jan 5 '19 at 19:50
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    Have you tried removing the rubber plugs from under your sill they can also fill with water – Graham Green Nov 18 '19 at 14:52
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    Solar Mike's suggestion of checking door drains was my first thought...I've had this happen once and it was really hard to tell where the sloshing noise was coming from. Easy to check! – Tedwin Nov 18 '19 at 23:32
  • Have you checked to see if the AC condensate drain line makes a puddle of water on the pavement directly underneath it? It might be plugged. This test should be done when (1. ) the AC is running on max but (2. ) is drawing in outside air (and not recurculating the inside air), (3. ) the humidity is high enough (say, over 60% RH), and (4. ) the outdoor temperature is over 40° (preferably higher). – Mike Waters Nov 20 '19 at 0:00
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The issue turned out to be water accumulating under the passenger side floorboard.

On this model of car, the sunroof is designed in a foolish way - when it rains, water can get into the car. Why Volvo engineers decided to not simply seal it is beyond me - maybe so that the car is ruined faster and people buy more cars? But in any case, their solution is to add a shallow resevroir around the perimeter of the sunroof that collects a small amount of the water, and two tiny hoses that route the water through the A-pillars, past the engine, and down to the ground.

If you park your car outside like most normal people, these tiny hoses quickly get clogged. The water instead goes inside the car, and collects in the passenger side footwell. I removed around 2 buckets of water from there (!!!) and the sloshing stopped. I fixed the clogs and it didn't come back again.

I won't go into too much detail on fixing as there is plentiful information and videos online (eg. 1, 2 and 3) once you know what to search for.

Getting the water out

There are 2 ways to remove the water: You can open a plug at the bottom of the car, or simply remove the carpeted flooring. I did the latter as I had no easy access to the bottom.

You have to remove the carpeted bottom panel where the passenger puts his feet. This requires removing some plastic parts, and unscrewing some nuts under the seat, but you do not need to remove the seat. The carpeting is glued to about 12 inches of foam, which is heavy and unwieldy to begin with, but even worse when soaked with water, so prepare for a mess. The foam will likely tear as you try to wrench it out - oh well.

In my case, I decided not to remove the foam entirely, but simply lift one corner and prop it up with a hammer. This exposed painted metal bottom of the car, with several inches of water in it. I sucked it out with a vacuum cleaner - which broke the vacuum cleaner and probably could have electrocuted me. A pump or siphon is a much better choice.

After you get most of the water out, you'll have to leave the car for a few days to let the water in the foam to come out, and suck it out again. After that leave it for a bit more to let it dry. It probably won't fully dry, but it's still better than it being fully moist.

Fixing the clog

The videos explain this pretty well, but basically there are 2 major points of failure:

  • The front corners of the sunroof, where the hose begins
  • The rubber elbows that connect one hose to another, located behind the side mirrors

In my case, cleaning the latter was enough. I removed the guards covering the A-pillar (the ones with the handhold, the screws are hidden inside the handholds). This exposed the hose. I pulled out the rubber elbow and carefully cleaned all the mud out of it in a sink. I tested by pouring some water around the sunroof, and clearing the elbow seemed to fix the issue for me.

I then put the hose back in the elbow and reassembled everything. Note that the elbow is also very poor quality, and barely makes a seal - the hose can easily pop out of it.

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