Where is the water in my car's trunk coming from?

February 4, 2008 4:54 PM

Where is the water in my car's trunk coming from?

I drive a 97 Mazda 626LX. The trunk has a false bottom (just a piece of upholstered plywood really) and the spare tire is stored in a well under it. Twice in the last few months I have found a small pond in that well after a exceptionally heavy rain. However, the top half of the trunk (where stuff might be stored) is completely dry. In both cases I only knew about the water because I was accessing the spare tire area. There is no rust under there, so I don't believe this is a regular occurrence. Also, no moldy smell either.

- I've checked the underside of the car closely - cant find anyplace that water could enter.
- I don't see any sign that it is entering via the tail lights. I park on a slight incline so I think that also makes the tail lights less likely.
- There is no water around the trunk seal that would make me suspicious that it is coming in that way. Also, the trunk lining is dry.

As soon as I get a warm weekend day I plan to climb inside the trunk and have my son use the hose from the outside to help find the leak. In the meantime, does anybody have an educated guess about where the water is coming from?

posted by COD to travel & transportation (15 comments total)

Taillight seals or the trunk weatherstrip. Water is flowing down the sheet metal, bypassing the false bottom in your trunk and that's probably why it's not wet.
posted by IronLizard at 4:57 PM on February 4, 2008 [1 favorite]

One other thing, check the seal on your antenna (doubtful).
posted by IronLizard at 4:59 PM on February 4, 2008

There is every chance that IronLizard is right. The water doesn't have to drip into the boot and soak through. It can often run along the metal and run underneath the soundproofing and the carpet - perhaps along some of the grooves from the sheet metal strengthening process, or a weld line. Basically, where the carpet and soundproofing doesn't soak up the water.

If you try the hose pipe trick, here's an option for first try: Take out the boot carpet and the spare wheel well contents. Dust the boot floor around the well lightly with talcum powder and hose the boot lid for 5 minutes all over. Then you'll have somewhere to look in terms of direction. Then take off the relevant side panelling at the side/point of the suggested leak, re-talc and try again, working upwards to the leak until it becomes obvious. Talc is pretty easy to clean up afterwards.

Just sitting in there and looking is unlikely to work if it isn't apparent anyway. But cleaning off your boot seal may well sort it all out. If you do find it's the seal, and it looks fine, I'd take it off, clean it and re-fit it. Perhaps with some light applications of silicone sealant on the inside edge (where it clips onto the metal).
posted by Brockles at 5:10 PM on February 4, 2008

Oh, in case it wasn't clear, the Talcum powder is instead of you getting in the boot...
posted by Brockles at 5:11 PM on February 4, 2008

once upon a time i drove a 1990 mazda 626. it had the same problem, which i never found the root cause of. however it helped immensely when i took out the rubber plug in the bottom of the spare tire well. water would get in but it would drain, for the most part.
posted by noloveforned at 5:18 PM on February 4, 2008

I had the same problem with a car I used to own. It had been leaking probably since the car was manufactured, and seemed to be coming from the right taillight. I found the leak by removing the taillight assembly and shining a flashlight along the seams in that area. A small section of the seam had not been caulked at the factory, so I sealed it up by pushing a bit of painter's caulk. Never leaked again. Has the car been wrecked and repaired lately? Has any of the glass been replaced lately?
posted by Daddy-O at 5:18 PM on February 4, 2008

Taillight seals most likely, you can re-seal them easily with silicone or caulk.
posted by bizwank at 5:40 PM on February 4, 2008

Same thing happened to my car (Dodge Neon). Took it to the dealer, they shut a guy in the trunk with a flashlight (really), and by shining it around, a guy on the outside was able to find the hole, and it was repaired post-haste. So I think you should just take the car in. It happens, they'll know what to do.
posted by bibbit at 5:49 PM on February 4, 2008

be careful, tho, the guy in the trunk can make quite a mess if you leave him there longer than overnight!

if your car has a sunroof, another thing to check.

likeliest either a maple wingo thingie on the trunk seal, or the taillights.
posted by KenManiac at 6:34 PM on February 4, 2008

Another possibility is the rear window seal.
posted by Mitheral at 6:43 PM on February 4, 2008

Often, there is a small drain in the bottom of the spare tire well. It should be fitted with a rubber grommet that allows moisture to escape but not enter from outside. Often, this grommet gets removed or damaged, allowing an amazing amount of water to enter the wheel well.
posted by Thorzdad at 7:09 PM on February 4, 2008

Does the car have a sunroof? I see that it was an option in this model/year. My Integra has two little drain hoses that run from the sunroof tracks down to little elbow gaskets behind the plastic liner in the trunk. They allow water collecting in the sunroof track to drain outside the car. This hasn't happened to me, but a common source of water in Integra trunks is that one or both of the hoses become disconnected from said gaskets. Then when it rains, water in the sunroof drains right into the trunk. Easily fixed, just take out the plastic panels and re-connect the hoses. Use some tape to make them less likely to fall off next time.
posted by autojack at 7:44 PM on February 4, 2008

All cars leak, the seals are never perfect, check the drains and make sure they are operational see Thorzdad's comment. Good repair manuals show the location of the body drains perhaps your local library has one for the Mazda.
posted by flummox at 7:49 PM on February 4, 2008

No sunroof on the car, so I don't have to worry about that. It does have a drain plug in the bottom of the spare tire well. I suspected that last time it leaked so sealed around it. Obviously that wasn't it. It rained again yesterday but it doesn't look like any new water is in the truck. It really seems to be tied only to torrential downpours. I ordered the Haynes manual for the car over the weekend, we'll see if that offers any help when it gets here.

Thanks for all the ideas.
posted by COD at 6:49 AM on February 5, 2008

Just wanted to update this for future searchers as I think I found the source of the leak. If you pull off the plastic molding at the back edge of the trunk and pull the felt material out of the way you will find two holes that have no apparent use. I'm guessing maybe they were tool insertion points during manufacturing or something. Whatever Mazda used to seal them had turned to dust in the 10 years since the car was built. You can see the holes from the outside if you look under the rear bumper. They face straight back from the last piece of metal on the frame of the car. That also explains why I only saw water in the truck after a very heavy rain. The only way water can get in that hole is to splash up from the ground when driving.
posted by COD at 7:53 AM on March 9, 2008