No. Mountain Dew doesn't contain embalming fluid in it. See a list of ingredients found in Mountain Dew here.
The idea could have risen from the preservatives added to the beverage to lock in freshness, the same characteristic found in "embalming fluid" (also known as formaldehyde) used to temporarily prevent the decay of a body after death.
Sodium benzoate is added to soft drinks as a preservative and a bacteria-destroying agent. Also known as E211, it's a major ingredient in diet soft drinks. Pepsi Max, as well as the diet versions of Mountain Dew, Sunkist Orange, Nestea and Nordic Mist, also continue to use the preservative throughout the world. Learn more about it at The Los Angeles Times.
You might also be thinking of the issue of aspartame, the artificial sweetener, breaking down to formaldehyde. Dr. Woody Monte, a toxicology expert, discusses this pesky issue here.
Just in! Mountain Dew has teamed up with Microsoft to offer an exclusive on-pack promotion for the eagerly anticipated new games console. They will give entrants the chance to win a new Xbox One, plus two games, every day! Know more at Connected Digital World.