No. Mountain Dew doesn't contain embalming fluid in it.
Here are the Ingredients of Mountain Dew:
HIGH FRUCTOSE CORN SYRUP
CONCENTRATED ORANGE JUICE
SODIUM BENZOATE (PRESERVES FRESHNESS)
ERYTHORBIC ACID (PRESERVES FRESHNESS)
CALCIUM DISODIUM EDTA (TO PROTECT FLAVOR),
BROMINATED VEGETABLE OIL
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, as well as Erythorbic acid, are added to soft drinks as a preservatives 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 at Mercola.com.
It was also reported in 2014 that PepsiCo is working to remove a controversial ingredient from all their drinks, including Mountain Dew.
The ingredient, called brominated vegetable oil, had been the target of petitions on Change.org by a Mississippi teenager who wanted it out of PepsiCo's Gatorade and Coca-Cola's Powerade. In her petitions, Sarah Kavanagh noted that the ingredient has been patented as a flame retardant and isn't approved for use in Japan and the European Union. Read more of the news at USA Today.
Check out 20 Healthy Drink Options, shared by Prevention.