Expert advice: water inside your car? How to find where the leak is

One of the most frustrating elements of car ownership is when you discover a leak. You might be alerted to it by a stale musty smell. Or perhaps the carpet feels damp. Or you may notice that the car steams up of its own accord when there’s no one in it.

All are depressing because cars are supposed to be like a home from home; something that will keep you dry and warm whatever the weather. Finding water on the inside of a car is as unsettling as having a leak in your home. But leaks in cars are trickier to find than those in a building. Cars have lots of hidden pipe work and virtually every wall is an outside one. But it is possible and here’s how.

Where is the water gathering?

Just as when you have a water leak in your house, unintended liquid inside your car will pool in the lowest possible place. That makes finding the source tricky because it might be nowhere near where the water ends up. But there are certain assumptions you can make. If the car is parked on a flat surface, water leaking into the boot is unlikely to make it all the way to the front passenger footwell. And if it’s coming through the windscreen, it won’t end up in the boot.

The trouble is, cars are full of insulating material used for sound deadening. This will soak up a lot of water before it makes the carpet wet from the underside. If your car does have that damp smell or is steaming up randomly, you’ll probably have to lift the carpet and feel if the sound deadening material beneath is damp.

What is the fluid?

To establish where the leak is coming from, first identify what the liquid is. Your senses of touch and smell should be able to detect whether the fluid is oil or water based. If it is water, does it smell of screen wash? It’s not unheard of for the tube leading from the reservoir under the bonnet to the rear wash-wipe to spring a leak. But the most likely source is rain water from the outside.

Dry it out

If it’s not blindingly obvious where the leak is coming from, you need to find out when water is getting in. And to do that, you need to dry it out. If you can, remove the damp sound deadening and hang it out in the sun or in a warm room to dry. Then put some newspaper where the moisture was accumulating. This should show you exactly where the water is gathering.

Time to turn detective

You know where the liquid is, what it is, and when it gets in. Now it’s time to work out how it’s gaining access. If it gets in after rain, it’s fair to assume you’ve got a leak somewhere. But do you park on the flat or on a slope? And if it’s on a slope, does the water end up in the same place whichever direction the car faces?

Think about where the water is gathering and what’s above it. For example, if the water is in the passenger footwell and you don’t see it after rain, it might be coming from the ventilation system. Then think about your car. Has it been resprayed or had repairs to any doors or windows recently, or possibly had a new windscreen put in? How old is the car and is it conceivable that the rubber seals around the doors and windows might have perished?

Locating the leak

Internet forums can be a useful way of doing this. Chances are you’re not alone. Someone else is bound to have experienced the same problem with your type of car (and hopefully shared how they solved it!) Attempt to find your car’s drainage holes, which are an outlet for any rain water that’s collected. Leaks frequently happen because these become blocked. Water then either gets in and can’t get out again, or backs up and finds its way in through somewhere that isn’t designed to get wet. If you’re confident these are all as they should be, look at the rubber seals and make sure they haven’t perished and are seated properly.

As with many problems to do with cars, you may find where the leak is in minutes. Or it might take you months. Whichever it may be, good luck!