Car exhausts - leaking fluid

[kshuker]

When I'm driving at slow speeds behind other cars, for example pulling away from traffic lights, I sometimes notice a fluid leaking out of the exhaust tail pipe. It looks clear but surely can't be water and I doubt if it's unburnt fuel. It doesn't seem to matter if the car is new or old or which make. Any ideas or should I get my eyes tested?

[Kev_UK]

I have seen the same thing myself. Generally it is seen on a coldish day in rush hour when people are going to work.. they've travelled a short journey and their exhaust isn't at full operating temperature so emmits condensation (water). Unburned fuel may be seen in a few cars though.

[Jenny Tools]

What you are seeing is definitely condensation and not unburnt fuel.

[sddsddean]

Definately water.As a rough guide, for every gallon of fuel you burn, the engine will produce a gallon of water as a by product of the combustion process. This is why so many people need new exhaust boxes and say 'but I only use the car to go to the shops' or whatever. Using a vehicle on lots of short journeys, the whole exhaust system hardly ever gets warm enough to evaporate all the water, so it ends up sitting in the boxes and causing rust from the inside out.

[keltik]

if anyone here knows their chemistry the reaction going on in your engine is this: C8H18 (petrol) ------->H2O (water) + CO2 (carbon dioxide) if the car is spitting water out of the tailpipe it simply means youre engine is working good and the fuel is burning well. after long periods motorway driving the exhaust gets hot enough to boil water and due to the speed the gasses are escaping you wont see the steam.

[kshuker]

Thanks gus and gals, that's great.

[kshuker]

guys even. D'oh!

[Sgally]

Its the cooling system venting the vapour that then turns into water as it cools. The only place the water can go is thru the exhaust. Save your money on getting your eyes tested!

[New Forester]

Keltic -- your equation is missing the oxygen on the left hand side. With the numbers in too: 2(C8H18) + 25(O2) becomes 16(CO2) + 9(H2O). I think that's right -- anyway, it's petrol and oxygen in, carbon dioxide and water out, and as you say, it's about as much steam out as petrol vapour in (petrol is a much bigger molecule, so 2 petrol is close to 9 water). Starting from cold the steam condenses on the inside of the pipe, and the exhaust is also cooled. You'll notice on cold days that the cars with dripping exhausts also tend to have visibly steamy exhaust gases. Once warmed up properly the steam is hotter, and it disperses before making a visible cloud. Sgally -- an engine should not be venting coolant into the exhaust -- and air-cooled engines produce water vapour too. If you had a leaking head gasket or core plug in a water-cooled engine, that would allow the cooling system and the oil system to contact one another. You'd then get brown mayonnaise under the oil filler, a shortage of coolant water -- and probably a very sick engine. However, even then the water vapour would mostly come out of the oil breather. Otherwise, normal venting of coolant (if needed at all) comes out of the pressure cap, which is usually the radiator or expansion-tank cap. It's tiny in amount compared with what we are talking about.