... internal headgasket leak. the coolant can leak from the coolant ports into the combustionn chamber and just be burned out the tailpipe. If this is happening you aught to see good amounts of white smoke on a regular basis. Ignore white puffs of smoke upon startup though cause that can be just the water/condensation that builds up in the exhaust. if it burns white smoke after 20 minutes then maybe thats it. Doubt it though. confirmation of burning coolant though is to check the spark plugs for heavy white soot.
It is something on the verge of a fairy tale to think that white smoke would necessarily signal a coolant leak to the cylinders. NOrmal combustion products will yield 1.3 gallon of water (exiting as vapour ) for each gallon of fuel burned and this water condenses in the exhaust system and drips down until the pipe is hot enough to carry the vapour as such to the tail of the exhaust system.
fuel + air
C7H16 + 11O2 + 44 N2 = 7CO2 + 8H2O + 44 N2
by molecular weight:
100 + 352 + 44 N2= 308 + 144 + 44 N2 (452 + 44 N2 on bothsides, nothing gained, nothing lost)
thus 1.00 pound of fuel C7H16 requires 3.52 pound of O2 (or 15.179 pound of air, the rest being Nitrogen carried through almost unchanged) and yields roughly 3 pounds of CO2 and 1.4 pound of water as combustion by products.
at 60 mph, when your car burns 3 gallons of fuel per hour, you have like 4 gallons of water per hour coming out as vapour....that is a lot of vapour...so even emptying the entire cooling system in one hour would not generate as much "white smoke" as what already goes out normally....invisibly.
The only case where this near urban legend holds true is when the leak occurs at rest and water accumulates in cylinder or exhaust manifold and on a sudden, at start-up, all of it suddenly evaporates and would procure a heavy white cloud, really out of the ordinary, and this, for a
limited time. But a slow leak that occurs only when engine runs is not detectable at the exhaust.
WHite smoke seen on a continuous basis is rather from oil....pumped from the oil pan through damaged piston rings.
But.....I will admit that chemistry is "fail" if I can see it with my own eyes...
BUT...BUt...BUt....to be fully honest and consequent....I must however add a detail. The above discussion is considering the cooling liquid is water. However, ethylene-glycol used as coolant has a boiling point of nearly 400?F....hence a good chance of NOT vaporising in the exhaust....it then could probably be visible as a fume.....and this does not contradict chemistry....
