Can a Cup of Water in a 10 Gallon Tank Hurt the Engine?

by Leslie Nierste
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A full cup of water or less can hurt any car's engine. While some very small amounts of water may find its way into fuel tanks naturally, any more water than this will cause a car serious problems. Should water get into a fuel tank, it is important to get it out again so the car's engine will operate properly.

Water in Fuel Tanks

There are several reasons water could have gotten into a fuel tank. Some practical jokers may have poured water into the gas tank deliberately to cause mischief or the fill cap could be loose on the tank. A vehicle could also encounter heavy condensation from a quick change in temperature or humidity or a driver may have pumped watered-down gas into the car from a sub-par gas station.


Water sinks to the bottom of fuel tanks because it is heavier than gasoline. The water is in prime position to be sucked into the pump and pushed through the pipes to the engine. This water will cause the pipes, hoses and injectors to rust, which can complicate the workings of the car. The car may smoke, start with difficulty, or not start at all.

Reversing the Damage

Methods for getting the water out depend upon how much is in it. Some commercial fuel additives will soak up very small amounts of water in the fuel. However, if a large amount of water is suspected to be in the car, a competent mechanic will be needed to remove it, and reverse the damage. A mechanic may even need to replace fuel filters and reinstall the fuel tank. Some swear by adding ethanol to a fuel tank; however, ethanol is an erosive agent and may cause more damage to a fuel tank.

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