An old fuel filter can often create intermittent and mysterious problems and make it difficult to detect engine problems. All gasoline contains a small amount of dirt and contaminates that your fuel filter collects and this gradually reduces the flow of gas to your engine. When the filter is completely blocked, your car stops, or perhaps it fails to start. Usually, while there is only a partial flow of gas you may have a hard time starting, starting and stalling, or everything will seem fine at slow speed but sputter and run badly on the highway. Change it every 30,000 miles to get better gas mileage and reduce the stress of a potential breakdown.