Depending on the source, some say that sugar in a gas tank will not cause any engine problems. Others say you'll need to replace the engine. That is quite a difference of opinion. I guess a why would be in order! Apparently since sugar will not dissolve in gas, the sugar will simply plug up the fuel filters. However, once the sugar has dissolved in water (condensation in the tank) or it is introduced into the tank as a liquid (syrup or Coke for example) won't the sugar solution pass through the filters and eventually into the combustion chambers. If so wouldn’t the heat crystallize the sugar, leaving deposits on the valves causing problems. If so, what happens if Diet Coke or Diet Pepsi is used? If I knew a chemist I'd ask them also.

They had this on mythbusters some time ago
MythBusters (a Titles & Air Dates Guide)

There was little reaction to sugar in a gas tank. Of course a person doing this, normally to someone else's car is breaking the law and could be liable for having the effect checked out.

The other issue we are talking about 16 oz of coke, to 20 plus gallons of gas, if you put in 3 or 4 gallons of coke it would have more of an effect