It’s common to see a small pool of water beneath your car on a hot day. The fluid comes from the car’s air conditioning and it’s entirely normal to see water under a car, particularly in warmer weather to see a car’s AC dripping water.
This water pools beneath the car as part of the air conditioning process. And it will continue for a few minutes, even after the car’s engine has been turned off.
How your car’s air con works
A car’s AC cools the air by taking it either from the cockpit or outside. Refrigerant is the key ingredient here. This can evaporate at a lower temperature, condense again at higher pressure. The air con’s compressor pumps this refrigerant as a vapour under high pressure to the condenser.
As its name suggests, the condenser condenses the refrigerant to a liquid. The cold, low pressure refrigerant moves into the evaporator. Here it vaporises and in so doing absorbs the heat from the air. The moisture from the air is drawn off at this point. The refrigerant then goes back to the compressor to start its journey again.
The warmer the air, the more moisture it usually contains. This moisture drawn off in the evaporator has to go somewhere. The car obviously can’t keep it or over time you’d be driving around in a swimming pool so it gets rid of it.
The system does this by allowing water to escape from the car through a drain hole. Hence the water you might see dripping from the car on a hot day.
Water dripping from cars isn’t always the AC
Some people see the water dripping from their car and are concerned that it’s sprung a leak. Frequently, their first thought probably isn’t that it’s from the AC. They probably – and often quite rightly ‑ think it’s from the cooling system.
The first thing to know is that if the AC system does have a leak, it’s likely to be gas that escapes rather than liquid, so you probably won’t be able to see it.
It’s also worth pointing out if it’s coolant that’s leaking from beneath the car, the liquid will be coloured because of the anti-freeze component that’s added.
Identifying if the water dripping is normal
The first thing is that if it’s a normal AC drip, it will leave a saucer-sized patch of water on the ground beneath the engine bay. This will probably evaporate pretty quickly too as for it to happen in the first place, the ambient temperature will be quite warm.
If there’s more than this saucer-sized patch, your car might well be leaking another kind of fluid. This is where your touch and smell come in. If it’s a normal AC water drip, the fluid will be clear and odourless.
If the puddle beneath your car is oily, it’ll be coming from elsewhere in the engine bay. It could be oil – that isn’t unheard of, particularly in older cars – or it might be brake fluid. If you touch brake fluid, it will feel very slippery and it might smell a bit fishy, kind of like cod liver oil. It may also be dripping near the wheels.
This is likely to be caused by corrosion in the brake pipes or possibly a leak from a seal somewhere.
If the fluid is coloured, as we’ve seen it’s probably coming from the coolant system. This could be a leak from a seal or there might be a hole in the radiator.
Tips to prevent your car’s AC dripping water
It’s impossible to prevent your car’s AC dripping water. In fact, as we’ve seen, you actively want it to happen because it shows that the air con is working. Having your AC regularly serviced is one way to ensure that it will continue to function properly.
By doing so, the technician should ensure that any drainage channels in the AC system remain unblocked.
If you’re confident the fluid that’s dripping isn’t from the AC, take the steps above to identify it. Then you’ll know how to solve the problem.
If the fluid is coloured, the first thing to do is to see how much coolant you have in the expansion tank. You don’t want to be running without sufficient coolant: it could cause permanent damage to the engine.
If the coolant is low, top it up, then drive the car a short distance and try to spot where the water is coming from.
Is my car leaking fluid a safety problem?
Most fluid leaks will simply result in a breakdown, leaving you stranded roadside. If your brake fluid runs out, it could cause your brakes to fail. Running out of brake fluid will mean you don’t have any hydraulic pressure in the brake system and the pedal could go to the floor.
If that does happen, try pumping the pedal. This might help to build pressure back up. And at the very least this might warn the cars behind that you’re in trouble. Then you need to use the gears and parking brake to bring the car to a halt.
Will this affect my car’s performance?
For most of us, car air conditioning is a must-have. But it’s important to remember that the system uses the car’s engine or electric motor to power it. The more we use the air con, the more energy we’ll consume.
And if your AC system isn’t working properly or needs a re-gas, you’ll be using all that fuel for no apparent gain.
Using and maintaining your AC is important
So your car’s AC dripping water isn’t a problem. It is entirely normal and doesn’t mean your AC is going to pack up at any moment.
But if you see your car leaking water abnormally, it’s important to find out what the problem is and address it as soon as you can. We recommend taking it to a professional to solve this. Chances are, it’s a problem technicians will have seen hundreds of times before and should know how to address, even if it isn’t a car air conditioning repair.
It’s also important to maintain your car’s AC properly. The better maintained it is, the less likely it is to pack up and the more likely you are to benefit from it when you need it.
And remember don’t be tempted to turn the air con off for long periods of time. The seals inside the system rely on the refrigerant to stay supple and do their job. If you don’t run the air conditioning, the seals can dry out and the system will start to leak its precious gas.
I’ve been writing about cars and motoring for more than 25 years. My career started on a long-departed classic car weekly magazine called AutoClassic. I’ve since pitched up at Autosport, Auto Express, the News of the World, Sunday Times and most recently the Daily Telegraph. When I’m not writing about cars and motoring, I’m probably doing some kind of sport or working in my garden.