Contaminated gas keeps your car from functioning properly. The bad gas makes the car run less efficiently and may even lead to a knocking sound or a sputtering problem. You pick up bad gas when gas stations are down to the last bit of gas and replace it with fresh gas. The fresh gas causes the debris from the tank to rise to the top, which you then put in your own tank. Not using your car for several months may also lead to bad gas in the tank. The only way to get rid of bad gas is to flush the gas out of the tank.1 Insert a long clear tube, known as a siphon hose, into the gas tank. Blow through the tube, listening for the air to push through the tank and make a small popping sound. This indicates that the tube is below the level of the gas in the tank. 2 Set a chair near the gas tank and pull the hose over the top, making sure that it’s higher than the gas level in the tank. Twist the hose once over the top of the chair. Place a plastic bucket near the tank to hold the bad gas when you remove it from the tank. 3 Suck on the end of the tube, watching for the bad gas to slowly begin coming up through the hose. When you see the gas reaching the top of the tube, place one hand over the end and push the hose into the bucket. The gas should keep coming out of the tank and drain into the bucket. 4 Pour one can of octane boost into the fuel tank. The octane boost is designed to keep your car running at optimum levels and maximize the effects of your gas. Add enough fresh gas to fill the tank at least halfway. Turn the engine on and let it idle for several minutes. 5 Drive the car around the block, listening for any sputtering sounds and watching for any signs of a problem. Fill the car with more gas, using a higher octane than you typically use in your car. If you use regular, opt for premium gas. It will typically take several days for the fuel system to completely flush out the bad gas.