Water primarily gets into a fuel filter because it forms inside the fuel tank and is then drawn through the fuel system towards the filter.
The most common way water enters the fuel system is through condensation inside the fuel tank. This process is significantly influenced by temperature changes and humidity.
How Condensation Forms in the Fuel Tank
- Air Exchange: As a vehicle operates and consumes fuel, air enters the tank to replace the volume of fuel used, preventing a vacuum. When the tank cools, the fuel contracts, also drawing in air.
- Humidity and Temperature: Often, the air is drawn into the tank while operating in warmer and more humid conditions. This air contains water vapor. Later, when the temperature inside the fuel tank becomes colder than the ambient air, condensation forms inside the tank. This happens as the warm, humid air cools down, causing the water vapor to turn into liquid water.
- Partially Empty Tanks: Tanks that are frequently run with low fuel levels are more susceptible to condensation because they contain a larger volume of air relative to fuel.
The Path to the Filter
Once condensation forms, the liquid water collects at the bottom of the fuel tank because water is denser than most fuels (gasoline or diesel). The fuel pump draws fuel from the bottom of the tank. As the fuel level drops or the vehicle moves, stirring the contents, this accumulated water is drawn up the fuel line along with the fuel.
The fuel filter, located in the fuel line before the engine's fuel injectors or carburetor, is designed to trap contaminants like dirt, rust, and often, water.
Why Water in Fuel is a Problem
Water is detrimental to fuel systems and engines:
- Corrosion: It can cause rust and corrosion in the fuel tank, lines, and critical engine components.
- Fuel System Damage: Water can freeze in lines or filters in cold weather, blocking fuel flow.
- Engine Performance: Water does not burn like fuel and can cause misfires, poor performance, and even engine damage over time by interfering with lubrication and combustion.
- Microbial Growth: Especially in diesel fuel, water can promote the growth of bacteria and fungi (often called "diesel bug") which clog filters and damage components.
Protecting Your Fuel System
Modern vehicles, particularly diesel engines, often have fuel filter/water separator systems. These filters are specifically designed to separate water from the fuel.
- Separation Mechanism: They typically contain a medium that repels water or allows it to coalesce into larger droplets that fall to the bottom of a collection bowl.
- Draining: These systems usually have a drain valve or plug to periodically remove the accumulated water.
Regular maintenance, including draining water separators (if equipped) and replacing the fuel filter according to the manufacturer's recommended schedule, is crucial to prevent damage from water in the fuel.
