Why would a car have water under the passenger seat?

Answer 1

There are a lot of possibilities. It could be anything from a bad door seal which lets in water while driving when it's wet outside to a leaking heater core.

To check for door leakage, simply open the door and look around the edges and underneath. If the rubber seal is damaged or missing, it may be the cause of the water intrusion. (Remember that the water may be coming from behind the seat too, depending on how level your floor is. So check both doors for seal damage.)

If this is the problem, you can get a replacement or you may be able to buy some self adhesive foam weather stripping and, after thoroughly cleaning the surface around the missing rubber seal, use the weather stripping to fill the gaps. This should solve the problem.

If this is not the case, you can look under the car and see if there is any panel damage to the floor. A puncture or rust holes under the seat may let in water from wet roadways.

To fix this, you can find some kind of filler (liquid electrician's tape, a spray-on rubber coating, expanding insulation foam, fiberglass, etc) to fill the damage. Thoroughly clean the area and apply the filler (if the hole is large, you will probably use the insulating foam and/or the fiberglass to make sure it is sealed properly.) After it dries, everything should be fine.

This is the harder of my suggestions, but is often the actual culprit. If the water is in the carpet in front of the passenger seat too, you may have a leaky heater core or hoses.

Sniff the rug. Is there an odd, almost fishy smell or is the water a rusty color? These are other signs that the heater core may be the problem.

You can often see the core under the dashboard without having to take anything off of the car. If the carpet is dry in front of the seat it could still be the core because most carpets have a plastic backing which could hide the water until it pools under the seat.

Wad up some paper and put it under the core (if you can see it). Check it after while a while, maybe even drive the car for a few minutes. If there is water anywhere on the wad of paper, you know that your core or hoses are leaking.

You might be able to fix the core yourself, depending on how it is installed. Hoses might be removable without removing the core if they are the problem, but it is too difficult to explain any further the repair of a heater core since there are so many styles and installations.

Another, yet much less likely possibility is that there may be damage to the door frame, door post, or roof of your car which may be allowing water to leak down from above and get under the seat.