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How to trace interior water leaks in your car

How to trace interior water leaks in your car

There are few things more irritating that discovering that your car leaks water into the interior! But what causes them, and what are the most likely culprits?

Diagnosing a leak in your car

Finding water in your car isn't hard – It usually collects in the foot wells! But diagnosing where it came from is notoriously hard.

We all know that water obeys the laws of gravity, so will always pool in the lowest area – but it can run along wires, travel up material/fabric surfaces and generally do its best to disguise where it came from in the first place!

The first thing you need to identify is WHEN it happens? Is it after rain, when the car's parked on a slope? Identifying when can potentially narrow down the cause.

If it leaks even when it's not rained, then it's coming from somewhere within the car itself. If it only happens when the cars on a slope, it's probably a seal, or blocked drain tube...

Potential causes of leaks in your car

Door Membrane
Behind the door card there is a membrane – usually a sheet of either polythene or thin foam. Water can enter the body of the door when it rains, and generally drains out of holes in the bottom of the door.

If the membrane gets ruptured or torn, water can run down the membrane and soak into the door card. If you're getting a pool of water in the foot well, check to see if the bottom of the door is either damp, or has evidence of water staining. If it does, whip the door card off and check the condition of the membrane. Use waterproof tape to repair any tears.

Door Seal
The rubber seal around the edge of the door can sometimes be a cause of leaks. If the seal gets damaged or perished water will track down the front of the door and into the car. A visual inspection is usually sufficient to diagnose whether it's the culprit. If it's not 100% replace it.

Door Weatherstrip
This is the rubber seal that rests against the side windows (the bit the glass rubs against when you wind them up/down – If this isn't flush to the glass it can let too much water enter the door cavity (a little is to be expected) too much and it can end up soaking into the door cards and causing a leak.

As with the door seals a visual inspection should be able to identify is this is a likely cause.

Heater matrix
If the car leaks, and car smells like coolant when it gets warm, then check to see whether the liquid collecting in the car is anti freeze or water (Don't taste it! Smell it!). If it is, it could well be the heater matrix.

This is like a little radiator which water passes through to help heat the cabin. If the matrix corrodes, or one of the connections comes loose then it can leak into the car. They can be very difficult to access as they are generally buried away behind the dashboard, but if it's leaking a replacement is often the only fix.

Air conditioning
Have you ever noticed a pool of water under your car after the air conditioning has been on? This is perfectly normal.

Sometimes however the drain tube can become blocked and the water finds its way behind the dashboard and into the front carpets. Try not using the air con and seeing if the problem persists.

Sunroof
One of the biggest causes of in-car leaks is the sunroof! They are by design intended to leak – the seal is not usually totally waterproof – but there will be a channel around the sunroof aperture which allows water to flow down drain tubes either mounted in some, or all of the corners.

Often a leak that only occurs when the car is parked on a slope can be attributed to the sunroof drain tubes becoming blocked. To test - open the sunroof and poor a small amount of water into the drain tube.

Does it go down straight away, or does is drain very slowly or not at all? The water may even pool in the drain channel, run across the headlining and either drip out of the courtesy light, or even run down the A-pillars and into the carpets.

To fix it you need to find where the drain tube goes, (usually down the A pillar) and disconnect the end and blow through it with an air line – be aware that a surprising amount of filthy smelly water may come out!

Pollen filter
Some car's pollen filters are fitted in the engine compartment just in front of the windscreen – these are prone to leaking on some models unless fitted correctly, and the lid secured properly. If there is any sign of damage to the seal replace.

Windscreen rubber
This is applicable to both front and rear screens – If the rubber surround becomes perished, or is fitted incorrectly water will come in to the car. (If you've recently had glass fitted and that has coincided with a new leak this could well be the reason). Having the screen refitted with a new seal is the cure.

What to do once you've found a water leak in your car

There's not a huge amount of point in drying out the car until you identify the leak – but you really need to dry the car thoroughly – a damp car will go mouldy very quickly, and you could find yourself driving about in a car that smells like a compost bin.

So once you've found and fixed the leak - if it's wet and you can remove it, remove it. Mop up as much liquid as you can with something very absorbent such as a microfibre cloth, or kitchen roll.

When you've got it as dry as you can either use a plug in dehumidifier or one of the standalone units intended to prevent damp.

Some people swear by leaving a tray of cat litter in the car too! It may take time to dry the car completely so whenever you drive the car have the heater on and windows ajar to aid ventilation.  

About the author

Dan is an experienced motoring journalist who has more than 20 years of experience. He has been the editor of titles such as Fast Ford and Redline, and his latest project was converting an old Renault Trafic into a family campervan.

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