I have a 2001 Jetta Wagon (Golf Variant is caller in my country). Petrol engine. Usually rides school and back (15 miles) with engine between 2000 and 3000 rpm. Just did a 300 mile trip at 3500-4000rpm. Next morning I noticed smoke and dark water coming out of the exhaust. Coolant is not dirty, smoke has decreased after coming back another 300 miles, but when I start the car it "spits" black soot/water.

What could be the cause?

I know some water out of the exhaust while it's cold is normal, but in this case, it happens at every start, even with a hot engine, and the soot is considerable. I've just seen a Mazda drip a few drops, which is normal, but if you see the video linked there is a lot of water and soot.

The electronics in the car are not working very well either.

Edit: There is no overheating, No oil in the cooling fluid and no other symptoms.

Video of the actual problem

  • 3
    Duplicate question? mechanics.stackexchange.com/questions/11315/…
    – JPhi1618
    Commented Oct 13, 2015 at 14:27
  • @jphi1618 doesn't explain black soot nor smoke. I will upload a video soon
    – marianov
    Commented Oct 13, 2015 at 14:40
  • How many miles does you vehicle have on it? Have you had any work done on it recently? Have you ever replaced the O2 sensor(s) or spark plugs? Did you notice a loss in power and/or mileage? Any new noises like an air hiss or something?
    – JaredW82
    Commented Feb 25, 2016 at 23:38

4 Answers 4


There is nothing wrong, water is a byproduct of gasoline combustion.

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O

where C8H18 is the gasoline reacting with oxygen (O2) and giving Carbon-dioxide (CO2) and water (H2O) as the byproduct.

So, this simply is:

2 * Gas molecules + 25 * Oxygen Molecules → 16 * Carbon Dioxide Molecules + 18 * Water Molecules

So the black soot is just water mixed with carbon in exhaust pipe.

Water usually comes out when engine is not hot enough like when starting it. So the liquid water drips out of exhaust with the carbon mixed in it making it black. But after driving a little the engine becomes hot and so thus the water and it comes out as steam. So you don't need to worry.


In some cases the cause of water leaking from exhaust is a blown up head gasket, or cracked head, but that also results in overheating of engine. Since your engine is not overheating so i would say that it's just water getting accumulated in our silencer and coming out when you start the engine.

I can't say much about the electronics, maybe just some wires got loose. You should probably get that checked by someone in the garage.

Hope it helped.

  • 3
    Equation does not help the answer without proper explanation of what the elements are , nice try but i would say explaining the equation rather than putting the whole thing might be helpful to people with less chemistry knowledge.
    – Shobin P
    Commented Oct 14, 2015 at 14:41
  • Added a video of the exhaust, is that considered "normal"? Just today I've seen a Mazda drip one or two drops of water when started. This is much more water.
    – marianov
    Commented Oct 14, 2015 at 14:48
  • Is your engine overheating @marianov
    – Deiknymi
    Commented Oct 14, 2015 at 15:58
  • @Anarach added a little description - will that be enough
    – Deiknymi
    Commented Oct 14, 2015 at 16:00

A little water is normal especially in colder weather. It is not likely that it is coolant coming out of the exhust, if it was you'd have a lot of other symptoms.

If you vehicle is leaving a noticeable black dusty or dark damp patch behind then this may indicate an issue. Along with that if your mileage has taken a slight dip as well as your vehicles power is a definite sign of a problem.

This can happen when your engine is running too rich. Most likely causes for this either relate to ignition system issues, like one cylinder not firing correctly, or O2 sensors. If your O2 sensors are providing incorrect information to the ECM your engine will run to rich. This should problem should not go long unchecked as it will cause your catalytic converter, muffler, and other exhaust components to become caked with soot. Also, your O2 sensors will become coated with soot and give off even worse readings further aggravating the issue.

See this link for troubleshooting 02 sensors. This link is good too.

Most of the time you can verify this with a code-reader. Most of time this will not throw up a engine light, but the ECM will notice inconstant readings. See diagram below.

o2 diagram

Before you have it serviced or replace parts check the codes for any indications of it running too rich. If you have the other symptoms and are getting error codes, I would suggest inspect/replace your spark plugs and, depending on the mileage on them, then your O2 sensors. If you are a DIY-er then you may need a special socket to remove the sensor. This is what my socket looks like:

o2 Socket.

They can be a real pain to remove. You may have to heat it with a torch and use a breaker bar in conjunction with a cheater pipe.

Most likely it is not more serious than that but sometimes if your exhaust manifold has a hole or crack in it or you have a hole in your exhaust pipe upstream from the O2 sensor then this can also cause it to run too rich as the sensors report too much air and not enough fuel in the exhaust. You should be able to hear this as well as see the breach. Inspect your pipe.

Cracked Manifold


These systems are setup to operate with a balance of fuel/air to complete the combustion process. An excess amount of soot would be an indication that excess amounts of carbon have built up creating an irregular combustion or potentially oil is slipping by the piston rings. Rather than do an expensive repair I would recommend you put a can of BG 44K pn#208 in a full tank of gas. Run until the tank is near empty to maximize the chemistry. To find a shop near you go to www.bgfindashop.com. This is a professional use product offered by majority of repair shops and dealerships. They offer another product BG EPR pn#109 that can be used to clean the cylinders of any sludge building up over time.


Another possible cause that was suggested by the technician is water condensation. After driving for a few more days the car no longer comes out of the exhaust.

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