Fuel filtration

When the temperature falls below 0°C, there are often problems with starting a car. How to prepare for such surprises? It is recommended to start by checking the fuel filter, which is very sensitive to temperature changes.


With currently popular Diesel engines with direct fuel injection system for cylinders (unit injectors or common rail), the injection pressure reaches 1600-2500 bar. The cooperating parts of the injection pump and injectors are precisely made and very accurately fitted components.   The quality of fuel and its effective cleaning are decisive for the work and durability of these elements of the fuel system. Apart from the function of energy supply during combustion of the mixture, the fuel circulating in the system meets the important function of lubricating the moving parts of the injection system. Long-life and trouble-free operation of the entire diesel fuel supply system are guaranteed by fuel filters.

A properly designed and manufactured fuel filter for self-ignition engines must be highly effective. In addition to purifying the fuel from solid inorganic contaminants, such as: tank corrosion products, quartz sand, sludge and fouling arising during abrasion of metal elements, the filter must also entrap organic pollutants remaining in the fuel after the refining process (paraffin, asphaltenes, etc.)

The ability to separate water is another important task met by the diesel fuel filter. The presence of water in fuel is an undesirable but natural phenomenon. As a result of condensation of water vapour and during refuelling, water enters the power supply system. It is therefore recommended to use fuel filters with the ability to separate more than 90% of water.

The growing requirements of the automotive market force the use of appropriate, very accurate filter materials in fuel filters. Currently, the use of a composite material made of 2 layers has become the standard.

  • a layer consisting of thin, synthetic polymer fibres that provide high efficiency, but their main task is high separation ability, i.e. separation of water from fuel,
  • a load-bearing layer, with a high weight, which, in addition to guaranteeing high cleaning efficiency, ensures high dirt absorption. High filtering surface ensures high dirt absorption.

Materials of this type have a filtration efficiency of above 92% for particles of 3-5 μm size. They meet the high demands of diesel engine manufacturers.

Table: Requirements of filtering efficiency for fuel filters
1. Cover with connections 2. Bimetallic valve 3. Filter cartridge 4. Housing 5. Drain plug Fig. Cross-section through Renault Clio II 1.5 dCi fuel filter (PZL Sędziszów FPP12). In the upper part – the filter cartridge, below – the collection chamber for separated water

The installation of a filter that uses unsuitable filter material results in damage to the injection pump, injectors or the unit injector system. Fuel filters in cars with diesel engines are usually installed in the low pressure system between the electric feed pump and the high pressure pump.  Filters are installed vertically, so that water separated from the fuel by gravitation settles in the tank at the bottom of the filter. The use of effective filtration material, with the ability to separate water, enforces periodic checking of the level and possible removal of water by unscrewing the screw of the steam trap. Some cars use sensors that use the conduction phenomenon, and monitor the presence of water in the filter.

The fuel filter is very sensitive in autumn and winter. During this time, the temperature falls below 0°C, there are often problems with starting a car. This is due to the phenomenon of accumulation of contaminants on the filter. The negative temperature causes water to crystallize and freeze, and organic impurities accumulated in the filter (e.g. paraffin) form an emulsion with water molecules that will clog the filter material. This results in a lack of fuel flow through the filter and problems with ignition and engine operation. Therefore, it is recommended to replace the fuel filter with a new one during this period.


Worn or faulty fuel filters can reduce the performance and smoothness of the engine, as well as interfere the functioning of the starting system.

When choosing a fuel filter, check:

  • filter tightness – a leaky filter can cause a fuel leak
  • filtration paper parameters – too low permeability can deform the core and affect the excessive increase of fuel flow resistance
  • steam trap screw – deformation and unsealing can occur when using fragile and faulty material

Source: Paweł Piotrowski, Świat Filtrów