Sometimes while driving in the traffic, I come across a car or two which would be dripping water-like drops from its exhaust steadily in 4-5 second intervals. I tried to ask a couple of people at the local workshops; they say, and I quote, "The car is giving an amazing mileage". And I am like, what does that water dripping mean even then?

Question is: Why does the water drip? What is the source of it? And what does it signify?

  • That's water. Condensation is a byproduct of combustion.
    – geoO
    Commented May 10, 2019 at 15:15

3 Answers 3


It signifies that the car is running absolutely correct. Here is the reason why:

A gasoline (petrol) molecule is made up as such:

C8H18 (or 8 Carbon atoms and 18 Hydrogen atoms)

Energy is obtained from the combustion of it by the conversion of a hydrocarbon to carbon dioxide and water. The combustion of octane follows this reaction:

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O

Or better said, you have two of the hydrocarbon molecules along with 25 oxygen molecules, they swirl together into a mix, the spark plug ignites them, boom, and out the tail pipe comes 16 carbon dioxide molecules and 18 water molecules ... at least in a perfect world. Some cars don't put out exactly that ratio. There may be a little bit of carbon monoxide (CO), unburnt hydrocarbons (C8H18), and/or nitrogen oxide (NO2) coming out of the engine exhaust port along with the CO2 and H2O. In this case, the catalytic convertor's job is to help clean these up so you can get closer to the perfect ratio described above.

As described, the water coming out of the tail pipe is a natural occurrence of the combustion process. You will usually see it coming out of the tail pipe when the engine and exhaust system of the vehicle is not completely warmed up. When it does become completely warmed up, you won't see it any more, because it comes out as steam (well, you'll see it in the winter time if it's cold enough, but you get the idea).

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    "The spark plug ignites them, boom" - Haha, perfect. Thankyou for the explanation, sir.
    – Sandman
    Commented Jul 13, 2014 at 10:11
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    It isn't that the engine needs to warm up, it's that the exhaust system needs to warm up. The water vapor condenses when it comes in contact with a (relatively) cold exhaust pipe. Commented Jul 17, 2014 at 20:14
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    @hamstar - I'm sure not exactly mainly due to the chemical makeup of diesel being slightly different, but the same basic premise applies ... You're going to get a lot of water vapor coming out of the tail pipe. Commented Nov 12, 2015 at 2:54
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    @hamstar - Diesel can range from C10H20 to C15H28 ... the most common fuel makeup is supposedly C12H23. If you burn two of these and 71 O2, your exhaust should be 48 x CO2 and 23 x H2O (as long as my math is right). So a lot of water getting pumped out here as well. Remember also, this is the "perfect world" we're talking about. Commented Nov 12, 2015 at 23:53
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    @purpleACR - More than likely there is a lot of water build up in the system. But, also smell the exhaust and see if it's sweet smelling. If so, there might be a coolant leak from the engine. Commented Aug 4, 2016 at 23:06

Every gallon of fuel your vehicle burns produces a gallon of water out of the exhaust. If the weather is cold you will see it as steam. If the rear of the exhaust system is still cold even in warm weather it will be for a short time after start-up, you will see the drips. On a hot engine/exhaust you will not see any drips from the tail-pipe. If there are drips, or water, coming out the exhaust on a hot system you are proberbly seeing an engine water leak - most likely head gasket.

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    How did you get from a gallon of fuel to a gallon of water? I would expect a different ratio.
    – Hennes
    Commented Jul 13, 2014 at 11:52
  • Hiya. If you bear in mind that the energy used to propel the vehicle is really the energy released from the conversion of the fuel and air rather then 'burning' of it, you can see that oxygen in the air, hydrogen in the fuel combine to create water-H2O. It really does mount up. Commented Jul 13, 2014 at 12:09
  • I expected a lot more water. Taking the C8H18 example from Paulster2's answer that would be 9 water molecules per single carbohydrate molecule. More (but lighter) water molecules. Now water may be more dense than petrol. (gasoline ranges from 0.71–0.77 kg/L, while water is at 1kg/l). (All of that assumes that C8H18 is average. No partial burned products. It ignore temperature differences etc etc. None of which I am qualified to properly calculate the results of). My intuition however expects way more water (in volume) then gas in.
    – Hennes
    Commented Jul 13, 2014 at 12:29
  • The water content in automotive exhaust from car engines is 12% for petrol and 11% for diesel. There are so many variables, too many to go into here, that you can turn the corner of a street and make incremental differances. Commented Jul 13, 2014 at 12:48
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    You can't do a 1/1 volumetric conversion (gallon to gallon). You have to a do a molar conversion. Commented Aug 8, 2014 at 7:08

The car has not been driving long. As the others have said, the water comes from the combustion of the fuel. When the exhaust system is cold, it cools the exhaust enough that the water can condense. After a while of operation, the exhaust system heats up. The same water vapor is present in the exhaust, but it stays vapor and doesn't drip.

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