Bleach is one of the few substances that may ruin the engine when added to the gas tank. It causes the engine to stop running and rusts the fuel tank.
Although there are many substances that can damage parts of the engine when put in the fuel tank, most of them do not do any major damage to the engine as a whole. Sugar is commonly cited as an agent that ruins engines, but in reality it often does nothing. If it does have a negative effect, it is usually because it clogs the fuel filters or fuel injectors. In extreme cases, it may also require removal and thorough cleaning of the fuel tank.
If bleach is added directly to the oil, it may destroy the engine. Other substances, including sugar, may also cause serious damage if added to the oil. People who think their cars may have been sabotaged in this manner should not attempt to start the car and should instead contact a qualified mechanic with their concerns.
Water can also ruin engines. This usually occurs when the car is submerged in water due to a flood or accident. Most engine damage occurs because coolant, oil or other essential liquids are not kept full and changed regularly.