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.