A fuel filter is a filter in the fuel line that screens out dirt and rust particles from the fuel, normally made into cartridges containing a filte paper. They are found in most internal combustion engines.



Some filters, especially found on larger engines, are of a bowl-like design which collect water in the bottom (as water is more dense than fuel). The water can then be drained off by opening a valve in the bottom of the bowl and letting it run out, until the bowl contains only fuel. Many fuel filters contain a water sensor to signal to the engine control unit or directly to the driver (lamp on dashboard) if the water reach the warning level. It is especially undesirable for water in fuel to be drawn into a fuel based engine system, as the system relies on the fuel for lubrication of the moving parts, and if water gets into a moving part which requires constant lubrication (for example an injector valve), it will quickly cause overheating and unnecessary wear. This type of filter may also include a sensor, which will alert the operator when the filter needs to be drained. In proximity of the diesel fuel filter there might be a fuel heater to avoid the forming of paraffin wax (in case of low temperatures) inside the filtrating element which can stop the fuel flow to the engine.
