Car Floor Wet Under Mat (Top 5 Causes)


Fogged car window

I noticed my Audi’s windshield kept fogging up, even when parked. I suspected a water leak, and I was right… Arrgh!! wet carpets drive me crazy. We have to take care of this now, water leaks only get worse.

The top five causes of wet car mats include:

  1. Blocked windshield cowl drain
  2. Blocked air conditioning drain
  3. Leaking heating system
  4. Blocked sunroof drain
  5. Windshield seal fault

Apart from being really annoying, a leak has the potential to create lots of other very expensive problems. In this post, I’ll cover the top five causes of wet carpets and what you can do about it.

Wet car floor mats

The Problem With Water Leaks

Some water leaks go undetected for years, and that’s when water damage becomes irreversible. The water inside your car can cause all sorts of horrible problems, some of which can be a hazard to your health. Here’s a list of just some of the problems with undetected water leaks:

  • Bacteria and mold
  • Corrosion
  • Intermittent no starts
  • Pre-mature component failure
  • Wiring faults
  • Control module failure
  • Carpet rot

1 Blocked Windshield Cowl Drain

Blocked windshield drain

The windshield cowl drain is the cover at the bottom of the windshield. On most cars it’s plastic and on older cars, it may be louvered metal.

Anyway, water is designed to run down the windshield where it’s guttered into a cowl drain. The drain will serve to catch and prevent large debris from passing to the firewall drain.

The firewall drain or bulkhead drain is usually part of the metal structure of the vehicle and is formed in such a way as to channel the rainwater to a drain on either side of the car.

Blocked cowl drain

So what’s the problem?

The cowl unfortunately doesn’t catch all the debris, pine needles for instance will pass through and eventually block the firewall drains. When this happens as you can imagine the water backs up and submerges body seams and body grommets, and then you notice damp carpets.

Cowl drain

The fix here is easy, so long as the water inside the car hasn’t caused any wiring corrosion.

Most cars conveniently allow enough access to the cowl and bulkhead drains without too much difficulty.

Car leak

What can I check?

Pop your hood and check either side of the cowl, the drains will be pretty obvious. remove leaves and crap from the drain. Check for any standing water behind the cowl, you can usually peer through or use your phone light to find the firewall drains, they’re usually close to or below the cowl drains.

A metal coat hanger makes a good tool for poking through the firewall drains.

Air intake drain
HVAC assembly air intake drain

Some models may install a drain hose inside the cabin under the carpet. It’s designed to carry away any condensation or rainwater that may enter the HVAC air intake assembly. The drain exit is on the underside. Check if your vehicle has a drain and that it’s clear.

A workshop manual covers such details and you’ll find a link to the workshop manuals I use here on the Mechanics tools page.

2 Blocked Air Conditioning Drain

Blocked a/c drain

If your car has air conditioning, and most do today, then it will have an air conditioner drain. Just like a household fridge, your car’s air conditioner will cause condensation.

The evaporator (EVAP) is an important component of the system, it’s fitted just behind the dashboard and is responsible for removing the hot air from your car.

The EVAP is ice cold and as you know hot and cold air will cause moisture to form on a surface. The evaporator is designed to handle this, moisture is gutter from the evaporator through the firewall and released onto the ground.

So what’s the problem?

Air con drain
Aircon drain

The rubber EVAP drain on the engine side of the firewall blocks up debris and causes the EVAP moisture to build up and overflow inside the car. As the EVAP is positioned behind the dashboard it will cause the front carpets to become damp, usually the passenger side foot-well.

What can I check?

The problem may only be noticeable when the a/c system is running. But when your car is parked with the a/c on, a pool of water should be noticeable under the car to the rear of the engine. The liquid on the ground should be clear, any color in the fluid indicates a different leak.

While finding no condensation drip under the car isn’t conclusive of a blocked drain, it does warrant further investigation.

First check the location of your a/c drain, a quick google search for your model or it may be listed in your driver’s manual, or consult a workshop manual, see the Mechanics tools page for the manuals I use.

With the drain located, usually on the firewall (Metal body structure between engine and cabin), reach down and using your hand pinch the end of the drain tube, often that’s enough to cause the blockage to clear, if not use a suitable object to pass through the drain tube. (nothing sharp)

3 Leaking Heating System

Car heater hoses

Your car’s cooling system takes hot coolant (yes, I see the irony) and plumbs it into the cabin. The hot coolant (water and antifreeze) is sent through a heater core (like a mini radiator), where the heater fan blows the heat from the core into the cabin, the cold coolant returns to the engine to be heated again and so the cycle goes.

Heater core

So what’s the problem?

Coolant should be changed every three years, as it degrades over time. Old coolant allows corrosion to take hold and it’s acidic which attacks metal, rubber, and plastic components breaking them down.

A heater core unfortunately is one of the more fragile components, liable to be damaged by frost, corrosion, and plain old wear and tear. As they’re filled with coolant when they break they leak all over the cabin.

Drying car carpets

What can I check?

If you suspect a coolant leak, you may have a few of the following symptoms:

  • Chesty cough
  • Fogged windows
  • Low coolant level
  • Sweet smell inside the car
  • Damp carpets
  • Mold and stale smell

The coolant inside the car isn’t good for your health. It can lead to serious lung infections.

Strip out the carpets and check the color of any pooling water, it may have a pink, greenish or yellow look. Coolant will feel sticky and irritate the skin.

Heater core leak

Take a look under the dash at the center console, try and locate where the two coolant pipes travel through the firewall. They will often have a tell-tale leak or drip stain on the pipe fitting.

Unfortunately replacing the heater core is one of those difficult jobs. The dash will need to be removed and the heater assembly. More than one Saturday is needed.

If you need tools to nail this repair, check out the “Mechanics tools page” and if you need a heater core or any parts, check out the Parts Geek link below.

Get up to 75% Off Dealer Prices on Replacement Auto Parts! Shop Now at Partsgeek.com!

4 Blocked Sunroof Drain

Sunroof drains

If your car doesn’t have a sunroof you can skip this one. The sunroof is a fantastic feature, who doesn’t like the wind in your hair and your tongue flapping about, I know our dog Sammy loves it.

If your sunroof is a tilt and slide type, it’ll have roof guttering and drains. One at each corner carries any rainwater that enters the sunroof assembly. Attached to each of the four assembly drains is a flexible hose fitted inside the roof pillars, carrying the water to the ground.

Sunroof drain

So what’s the problem?

Three problems are common. Blockages, disconnected pipework, and corrosion at the roof assembly drain spout. All as you can imagine leading to a water leak inside the car. Gravity does its job and carries it to the carpet.

Leaking sunroof assembly seals aren’t common but do happen. The sunroof assembly is fitted as a complete unit and sealed to the underside of the roof before the headliners and windshield are fitted.

The seal-like sealer will break down with age and can cause a leak. If this happens, the assembly will need to be removed and resealed.

Blocked sunroof drain

What can I check?

Open your sunroof, park on level dry ground and prepare a jug of water. Pour water into each of the four corners of the sunroof and check that the water emerges pretty quickly on the ground.

Note, you won’t be able to see either of the rear corner drains, you’ll have to make do with pouring the water into the gutter, in the direction on the drain.

Sunroof drain

If you find a blockage, you’ll need a coat hanger to have a poke around, poke and testing can work well.

However if you find there’s a problem with the rear drains, access can be difficult. Often the headcloth may need to be removed to inspect the rear drain outlets, but do a google search for your particular model first. Often you’ll find your car is prone to a particular leak.

5 Windshield Seal Fault

Windshield seal

With the exception of classic cars, most windshields are bonded in. A liquid bead of sealer is laid around the window frame and the windshield is pressed into it. The sealer cures within twenty-four hours and is weatherproof. It’s a fast and efficient way to fit windshields.

So what’s the problem?

The sealer breaks down after a number of years, temperature extremes will shorten its life. Generally, factory-fitted windshields don’t give much trouble.

However, I’ve found problems with after-market windshields. The old sealer can be difficult to remove which can cause problems with new windshield alignment, missing beads of sealer, incorrect window trim, and cowl fitting.

Windshield test

What can I check?

Remove the wet carpet and underlay to gain access to the suspected leak area. Ideally, this is a two-person job. Using a hosepipe, start at the windshield pillar and leave the hose run, leaks often take a while to present themselves.

Have your helper hold the hose while you sit inside the car, checking for the leak, a hand light will make it easier. Working in a systematic way from the bottom up will help isolate the leak.

Windshield leaks will often trickle from the kick panel area. Finding water leaks takes patience and common sense. If you find a window seal is a root cause you’ll need to reseal the windshield. Check out the Amazon link below for quality windshield sealers perfect for the job.

Amazon Windshield Sealer Repair

Body Seam Leak

Broken seam sealer

When car body panels are fitted to the shell, they’re spot welded. That means the joint isn’t weatherproof until they run a bead of sealer across the joint. You’ll recognize the sealer, it’s often most visible inside trunk and door jams, but it’s used extensively all over the car. The sealer is applied by machine and is over-painted.

So what’s the problem?

The missing sealer allows water to leak inside the cabin. After twenty-plus years as a mechanic, I’m often surprised at the volume of water, even the teeniest hole can cause.

Missing body sealers can be difficult to find. In my experience, this can happen to cars that were in an accident, even a small impact can cause the sealer to break and a seam to open.

Leaking body sealer

But brand new cars aren’t immune either, I’ve had to fix water leaks on cars without a mile on the clock.

So what can I check?

First off, ask yourself what’s new, any small accidents lately even a bumper nudge can do it. Any accessories fitted or repair work done?

Body seam leaks can be notoriously difficult to find, mostly because they’re not visible. Even if you could see the seam, the fault isn’t visible. In addition, rainwater runs across different panel surfaces, depending on the angle of the terrain. I’ve had cars that don’t leak when tested on the level workshop floor, but when parked on a hill, leak like a creek.

Mostly this is systematic checking of different areas of the car at different angles, you may already have some personal experience of when it’s worst. That’s the kind of info that’s really useful with this kind of detective work.

Who needs luck when you have patience and tenacity. I’ve had success by stripping the interior and using a shop jack to raise the car to simulate different types of terrain slopes.

Condensation Sweat

Condensation sweat

This one is a rarity, but worth checking. You already know condensation forms on a surface where hot and cold air meet.

Exhaust catalytic converters get extremely hot and where they pass close to the underside of the chassis, manufacturers place insulation to reflect the heat.

So what’s the problem?

Cars age and the insulation falls off. Seems trivial at the time and you may even have said “Naa, doesn’t need it”. Fast forward a few months and you find your carpets wet. You’ll never find the water leak, obviously.

What can I check?

Just take a look under the car where the carpet is wet. If you’ve got a hot exhaust right there and maybe no insulation, you may have found your problem.

Fitting new insulation over the exhaust or even under the carpet inside the car will help prevent sweat.

Using a workshop manual can really speed up the repair process, they only cost a few dollars for a digital copy and they’ll save you a ton of cash.

Related Questions

How do you dry out wet carpet? Wet auto carpet will need to be removed and hungover and fence to drip dry. The heavy insulation may take several days to dry completely.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty-five years, and I've worked for GM, Volvo, Volkswagen, Landrover, and Jaguar dealerships. My passion is cars. I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of car ownership, including buying advice, maintenance, and troubleshooting.

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