Changing the filter correctly is half the battle

Removing the old filter and putting the new one in is a simple task. After all, oil, air, cabin, and fuel filter replacements are a day-to-day task at every workshop! But be careful. There are a couple of things you should keep in mind, particularly when changing the fuel filter. Modern common rail diesel engines react in a highly sensitive manner—today’s compression-ignition engines are extremely susceptible systems, especially when air or dirt gets into the fuel. This is due to the steadily increasing injection pressures and component tolerances in the micrometer range— there is a good reason why components of modern injection systems are manufactured in clean room conditions!

Carrying out the procedure on the open engine

If the vehicle comes to the workshop for a service, it is no longer necessary to observe clean room conditions, of course. Nevertheless, you should ensure a certain degree of cleanliness if you are going to work on the injection system. Regardless of whether it is a diesel or gasoline engine, there are a couple of important rules you should follow. The engine and injection system will thank you for it at the next service!

The MAHLE purity requirements

Clamping the lines prevents fuel from leaking out and air getting into the system
  • Safety first: protective glasses and gloves are mandatory.
  • Under the magnifying glass: before assembly, it is worth comparing the old and new filters with each other. Things that look similar on the outside may be different on the inside!
  • No pressure: often there is residual pressure in the system, which should be eliminated, of course.
  • All clean: a spotless environment prevents dirt from getting into the system.
  • Clamp down: line filters (KL model) on the underbody are particularly prone to running dry when disconnecting the fuel line. Therefore, clamps should be used to prevent air from entering the system.
Gasket damaged as a result of dry assembly on a KC filter (left) and a KL line filter (right)
  • A tight seal: open lines should be sealed with plugs. Additionally, the transport plugs on the filter should not be removed until immediately before it is connected.
  • Everything must go: in the case of filter elements (KX model), the fuel must be drained from the housing.
  • Good preparation is everything: prefilling the spin-on filter (KC model) with clean fuel avoids the tedious procedure of ventilating it later.
  • In with the new: always use fresh gaskets.
  • Lubricating well is half the job done: always coat the gasket with clean fuel before assembling it.
  • Don’t get in a twist: rubber gaskets must not be twisted when fitted. Always check that they are positioned correctly, otherwise the new gasket will be squashed.
  • After replacing the filter, press the housing cover on carefully by hand. If the cover is pressed down with the screws, the gasket may become squashed.
  • Not like that! The housing cover must be carefully lifted open at the recess provided.
  • A small but highly effective gasket. If the fuel leaks at the filter housing, it can spray onto the belt drive and damage the V-ribbed and timing belts. This can cause the belts to tear and the piston to impact on the valves.
  • No bending: fuel lines and pipes must also not be twisted or bent during assembly.
  • Get the air out: the injection system must be ventilated cleanly—otherwise, it gets expensive. Injection pumps and air do not mix, and so the pumps must always be ventilated in accordance with the specifications. However, please do not fiddle with the starter until the engine starts—you can read about what can happen in that situation in our TM 02/2017.
  • Completely sealed? A final performance and leakage test must always be carried out.
  • So long! Naturally, old fuel must be disposed of in an environmentally sound manner.

By the way: you can also find further information on the fitting of fuel filters here:
Instruction: Fuel filter installation
All technical messengers on fuel filters


We regularly provide technical tips relating to the powertrain, thermal management, and mechatronics in automobiles.


MPULSE - The magazine

The print issue of MPULSE will come out twice a year.