Signs of a Clogged Fuel Filter. Every vehicle has a fuel filter, whether it uses gasoline or diesel as fuel. As the name suggests, it is the fuel filter which cleans the fuel before it makes its way into the engine. At some time or the other, fuel filters show signs of wearing out. Advertisement. If you feel that your fuel filter has a problem, remove the fuel filter and blow into it. If air doesn't pass through, then it is a sign of a clogged fuel filter. It signifies that the fuel filter needs to be replaced. Some of the other symptoms are.