There are a number of ways that water can find its way into a car's fuel tank. For instance, if you accidentally leave the fuel cap off when it is raining, water can get inside. If you live in a very humid environment, the humidity can settle in the tank as water over time. In a rare situation, you may have the misfortune of putting gas in your tank that is already mixed with water. There are a few signs that you should look out for if you suspect that there is water in your fuel tank.
Car Not Starting or Jumping
The engine needs fuel to start up, so if it's being fed water or gas that is heavily mixed with water, it won't get the spark it needs to fire up. Also, if you're driving along and your car starts to jerk and jump as if you're about to run out of gas, even though your tank is on "F," then this could be a sign of water in the fuel. The car jumps because the engine is being fed too much water, and starts up again when the actual gas hits the system, allowing the car to continue on.
Bad Fuel Injection
The water in a tank of gas could corrupt your fuel injection system. Over time, the water will cause rust to form in the system (it could cause parts of the engine to rust over as well). If you take your car into a mechanic and see rust or another problem with the fuel injection system, then that is a sign that you may have water in the fuel.
Gas Runs Out Quickly
One clear sign that your gas tank might have water in it is if your gas runs out quicker than usual. Check your car's advertised mileage per gallon and compare that your actual miles per gallon. If the car is running out of gas too quickly (assuming all other possible issues are ruled out), there could be water in the tank.