Fuel filter

The fuel filter protects the internal combustion engine against the ingress of dirt, particulate matter and water. Filtration is usually multi-stage. The first filtration occurs before the fuel pump inlet, followed by the main fuel filter and the filter screen before the high-pressure fuel pump or injector.

The purity and quality of fuels is closely monitored by manufacturers, suppliers and inspection authorities. The fuel sold must comply with the characteristics defined by the standard. Nevertheless, various impurities and deposits may enter the fuel during transport and storage. Therefore, no fuel is 100% pure, and the fuel needs to be filtered before entering the fuel line. In particular, modern internal combustion engines equipped with high pressure fuel injection systems are very sensitive to any impurities in the system.

In order to perform its role well, the filter must not be clogged or damaged! A clogged or damaged filter can even harm the vehicle and its engine. If the driver observes any of the following typical symptoms of a clogged fuel filter, he should not delay replacing the fuel filter.

How do I know if I have a clogged fuel filter?

  • the fuel filter can be checked visually, this is more likely to be the case with older vehicles
  • poor cold and hot engine starts, which may not be solely due to a weak car battery
  • the engine may stall at idle
  • irregular engine running may also be caused by a clogged fuel filter
  • noticeable loss of power when accelerating or driving uphill
  • a jump in fuel consumption, consumption can double in a vehicle with a clogged fuel filter
  • electronics report problems with the fuel pump

So, in general, a clogged fuel filter causes the engine to not get enough fuel at any given time. Therefore, in conditions requiring more fuel, the engine loses power and runs erratically.

If the driver ignores these symptoms of a clogged fuel filter, in extreme cases it can end up with a complete blockage of the fuel supply or with dirt entering the injectors and damaging them. The result of neglecting to change the fuel filter is therefore a non-drivable vehicle.

Fuel filter design:

The fuel filter system usually consists of a pre-set coarse filter, which is usually located in the tank upstream of the fuel pump. There is also a main filter, which is usually located in the engine compartment (sometimes on the underside of the vehicle). The last member, located in front of the high pressure pump, is usually a filter screen.

Generally, the main part of any fuel filter is the filter element, which is either housed in its own metal housing (called a spin-on) or, more commonly, integrated into the housing in the fuel line structure (called a cartridge). The filter cartridge must be capable of capturing particles of several microns in size.

The filter cartridge is made of special non-woven paper (Melt-blown) and synthetic fibres. Since diesel filters in particular must also be able to remove water, the filter material is hydrophobic. To separate water from the diesel, the fuel filter also contains a container for the separated water with a drain valve.


The diesel filter system is generally more maintenance and design intensive. Not only is the diesel fuel more contaminated and the water in the diesel fuel must be removed, but the diesel fuel also contains paraffins. The paraffins in diesel cause it to solidify at low temperatures. For this reason, some diesel fuel systems are equipped with fuel preheating.

How to choose the right fuel filter?

Choosing the right fuel filter is not difficult. You can find the type of filter in the service manual, you can also contact an authorized service center or use the Internet. If you have an AUDI TT with a 2.0 TDI engine from 2009, then the BOSCH fuel filter with a height of 136.1 mm and a diameter of 79.5 mm will suit it according to the manufacturer’s specifications.

Replacing the fuel filter

The fuel filter is most often located in the engine compartment, sometimes it can be found on the underside of the floor or in the worst case directly in the tank. When replacing the fuel filter, it is important to remember:

1/ the filter is filled with fuel that is toxic and flammable.
2/ the fuel system may be under high pressure, in which case the pressure should be relieved first
3/ pay attention to the correct orientation of the filter (IN/OUT)
4/ wait a while for the pump to prime the new filter with fuel before starting

What is the fuel filter change interval?

The fuel filter change interval is listed in the service book. Generally speaking, one to two years. However, we are talking about the filter so logically it depends on how much fuel and what quality the filter has filtered.