Diesel engines need to have their fuel filtered for contaminants before it is used. However, unlike gasoline engines, diesels depend on the fuel itself for internal lubrication, so it is critical for water to be filtered out as well.
The function of water and other contaminant filtration will typically be combined in a single filter. The filter will have a bowl or other reservoir at its bottom, since water, being heavier than diesel, will sink to the bottom.
Diesel fuel filters will have a valve at the bottom where water can be drained. This is a simple matter of opening the valve until all the water has drained away, leaving only diesel fuel.
Many cars are equipped with sensors and electronic valves that perform this function automatically.
The typical sign of fuel contamination is when the engine starts to run rough, idle rough, or lose power. This can be a symptom of either solid or water contamination, and the first step is to drain the accumulated water from the filter, if it is a manual-drain system; otherwise, the best thing to do is to replace the fuel filter.
If you live in a humid environment, particularly one with large daily temperature changes, water may condense on the inside of the fuel tank, and contaminate the fuel. This in turn will overload the fuel filter. Other causes of contamination are when the fuel has been sitting for a long time, either inside a storage tank, or in a car that has not been driven for some time.
Replacement fuel filters are relatively inexpensive, and replacing them is within the scope of the home mechanic, but care must be taken to not allow air into the fuel system, When in doubt, the task should be entrusted to a professional mechanic.
