I tend to disagree, I think that it is the relatively cold exhaust pipe that lowers the water vapour in the exhaust to below it's dewpoint. As the pipe heats up, so the condensation reduces. I don't think that there is enough water in the exhaust pipe to generate the amount of condensation that we see on cold mornings as there is insufficient volume in the exhaust pipe. But going back to original question - what about the white trails we leave behind in the sky? Yes, water is produced as part as a by-product of burning hydro-carbons.