The engine fuel system consists of a fuel tank, filter, pressure regulator, fuel distribution pipe, injectors and pipes that transfer fuel to these parts.

When the fuel tank is used for a long time with empty or less than half fuel, water accumulates as a result of condensation and causes malfunctions in the fuel system (detail - in the section on the cause of malfunctions). This process can be followed in the following way:

- Injector location in engine
- 3rd stroke high temperature (Suitable for 4 stroke engines)
- Relatively low temperature fuel return (pink color)
- Pink color is the part that enters the fuel tank (low temperature)
- The fuel entering the fuel tank from the return of the injector is at a higher temperature than the fuel in the tank
- At this time, there is a temperature difference and steam is formed (for example, steam from the mouth and nose of people in cold weather)
- Considering that the upper layer of the fuel tank is in direct contact with the atmosphere, the atmospheric temperature has a direct effect on this process
- In order to exhaust the vapor generated in the fuel tank, it is released to the atmosphere or to the air intake manifold of the engine (in diesel engines to the atmosphere, in gasoline engines to the engine) through the exhaust pipe located in the upper part of the tank, from the fuel filler pipe
- If there is any malfunction in the steam discharge pipe or line, then the steam generated at this time cannot be discharged, and as a result of the change in atmospheric temperature (night, day), the process of passing steam into liquid, i.e., condensation occurs
- Condensation is the transition of steam to liquid (the steam coming out of the kettle in the form of steam turns into liquid on the wall surface). In this case, steam turns into water
- It is known from physics that the density of water is greater than the density of fuel. So the water sinks to the bottom of the fuel tank and enters the fuel system. The fuel filter reflects the water entering the driver information panel (in some cases, the water trap sensor on the fuel filter is faulty and drivers do not notice it)

The fuel filter retains the incoming water, so if it is not changed in time, it is gradually transferred to the fuel pump and injectors. At this time, serious malfunctions occur in the fuel system. In the picture below, it is possible to visually see the passage of water into the fuel line.