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How to fix a water leak in your car

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.

How to trace interior water leaks in your car

Few things more irritating than finding a puddle of water in the car. But what caused it, and what are the likely culprits? Haynes has the answers

Diagnosing a leak in your car

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

Water 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 rain hasn't fallen, 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

Pollen filter
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Some cars’ 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.

Door membrane
There is a membrane behind the door card – 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. But 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 footwell, 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 (it'll be stuck on with strong glue) 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) – if this isn't flush to the glass it can let too much water enter the door cavity, 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 smells like coolant when it gets warm, then check to see whether the liquid collecting in the car is antifreeze or water (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.

Rear screen washer
Cars with rear screen wipers also have a washer jet, and this is usually fed from the bottle under the bonnet via rubber tubing. Sometimes it'll split or becomes detached, and because it runs along the inside of the headlining, you'll be able to see and feel damp spots on the ceiling. The headlining will usually need to be removed to access the pipe.

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 pour 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!

Windscreen rubber
This applies to the front and rear screens. If the rubber surround becomes perished, or is fitted incorrectly, water will come into the car. Have you recently had new glass fitted? Having the screen refitted with a new seal should cure it.

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, although 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, remove as much of the affected trim as you can and mop up the water with a microfibre cloth, or kitchen roll. When you've got it as dry as you can either use a plugin dehumidifier or one of the standalone units intended to prevent damp.

Some people swear by leaving a tray of cat litter in the car. It may take time to dry the car completely, though, so whenever you drive the car have the air-con on (with the heat turned up) to help.  

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