The only way to know if a filter needs to be replaced is to remove it and blow through it. If there is little blockage then the filter is fine and does not need any replacement. In case if there is more than minimal blockage then it is dirty and needs to be replaced.
WARNING: Gasoline is dangerous and contains poison; it should never be tasted even by mistake. The filter must not be held to the mouth for blowing through it. The best way is to attach a short piece of clean rubber pipe to the filter and then blow through the pipe for testing the filter.
A fuel filter that is completely plugged will choke the fuel flow to the injectors or carburetor and will stop the engine cold. This can also stop the engine from starting; it can also stall or die.
A spring loaded bypass is present in some filters although it helps the fuel to bypass the filter elements if it is blocked. Fuel flows continuously but it can carry dirt in to the injectors or carburetor. This can create more problems.
Normally enough fuel will be passed through a partly restricted fuel filter for keeping the engine running at low speed or idle position but the engine may starve for fuel at high speeds of over loads. The engine may work fine and run around the city but it can lack power if driven with speed at highway.
A screen is located inside the fuel tank that acts as a pre-filter for keeping the rust and big dirt pieces from getting sucked into the fuel pump.
If some dirt blocks the screen, the effect can be the same as a dirty or plugged fuel filter. If any problem of fuel starvation is being experienced and it was also replaced which did not solve the problem, it means that the screen inside the tank may be the real reason of the problem. The fuel tank usually has to be removed for cleaning or replacing it.
CAUTION: Proper sealed container must be used for storing the fuel. To prevent any risk of sparks, the battery also has to be disconnected. One should not smoke while working on the filter, fuel tank or fuel lines. Any flammable objects like lighters, heaters or plot lights must be kept away from the work place. It is also important to empty the fuel tank before removing it.
The fuel filter has to be occasionally replaced once a year for reducing heavy maintenance costs and risk of driving problems related to filter. However, many vehicle manufacturers do not indicate a replacement time interval for the fuel filter. If they indicate it, then it is a long interval like once after every five years or 50,000 miles. Although this is very unrealistic for many mechanics because changing the fuel filter after a very long time interval means putting oneself in trouble especially if the car is driven on roads which are muddy, dirty and have gravels. The cheapest gas can be bought from stations that offer less price products. Gas containing alcohol can be used or the vehicle is more than seven years old and maybe having a rusty tank.
The carbureted engine with fuel filter is normally located on the carburetor’s inlet fitting.
One has to be very careful not to over tight the filter when replacing its screws into the carburetor’s inlet fitting.
The carburetor has fairly soft threads that can be easily stripped. It also has to make sure that the filter is not leaking. It’s good to apply gasket sealer to the filter threads to guarantee that the connection has no leaks. RTV silicone sealer dissolves with gasoline while Teflon tape can end up in the carburetor, therefore they should not be used.
For replacing an in-line filter, the filters that have two new rubber pipes going on either side f the filter must be used. The old pipes must not be used again because they get worse with the passage of time and can result in leaks or shed small flakes going into the carburetor or filter. The pipe clamps must also be tight and properly positioned.
NOTE: Many in-line filters show an arrow indicating the direction in which the fuel should flow through the filter. The arrow should point towards the carburetor.
Fuel Injection Filters
Fuel injected engines have fuel filters that are typically larger having a finer filter element than the ones present on carbureted engines. As a result, they are typically more expensive.
The fuel tank, throttle body or injector fuel supply rail are the places where the filter maybe located. The filter is located under the vehicle along a frame rail in many vehicles. The filter in some vehicles is a part of electric fuel pump assembly inside the fuel tank. A shop manual can be consulted for finding the location of the fuel filter.
WARNING: There is normally a great amount of left over pressure in the fuel line of fuel injected engines even when the vehicle has not run overnight. The manufacturer’s recommended procedure should be followed to relieve the pressure in the line before removing the filter or a rag can be wrapped around the pipe to slowly loose the connections.
An arrow on the filter indicates the direction of the flow. The filter should be installed with the arrow pointing in the direction of the engine and away from the fuel tank.
The tank would have to be removed if the filter is located inside the tank. Same precautions can be followed as described before for replacing pickup screen that is plugged.