The importance of using clean diesel fuel is essential and often over looked. Old, dirty, and contaminated diesel fuel can do severe damage to a diesel engines fuel injection system. There are very tight tolerances in a diesel engine that can get fouled and plugged. Water, even in the smallest amount can cause rust inside the pump and injectors. Filters can get plugged and starve the engine for fuel. If the fuel tank is over fifteen years old there will most likely be sludge and water in the bottom of the tank that must be cleaned out.
Figure 1- https://www.gotpower.com/compliance/diesel-fuel-polishing/
When water contaminates diesel fuel, microbiological growth develops inside the fuel tank. This growth turns the fuel black in color and if left in a tank long enough it will turn into a mud like substance (sludge). Water gets into the tank in many ways. The most likely place of entry is the deck fill. If the cap is loose or missing its rubber seal, water will enter the tank. Condensation in the deck fill can also accumulate and enter the tank. The primary filter (first filter in line) will separate the water from the fuel and protect the engine. When water is found in the primary filter the fuel tank must be cleaned to ensure all water has been removed.
Old diesel fuel can be found on small sailboats that don’t use a lot of fuel and are stored every winter. The fuel inside these sailboats are most likely over three years old, unlike our cars, trucks and power boats that burn a lot of fuel. There are additives that we use when storing diesel fuel but over-treating can cause other problems. The best protection is using new clean filters to clean (polish) the fuel. All marine diesel engines have primary and secondary filters that must be maintained to protect the engine.
The best solution to the problem of old diesel fuel is to have the tanks cleaned and fuel polished ever three years. This is done by using a remote filter and fuel pump to remove the sludge and water. This can be expensive due to the amount of time and equipment it takes to do this task. Another solution would be to install an on-board fuel polishing system so the owner/operator can do this themselves. By recirculating the fuel in the tank through a filter it will break up and remove the sludge and water in the tank. All diesel fuel can be cleaned and made back into good useable fuel.
Figure 2 - http://dieselfueldoctor.com/cause.html
Do not use fuel that is not clear in color. It should be amber or red in color. You should be able to see thru it. If your fuel is black in color it needs to be polished. Check that the deck fill is tight and the O-ring seal is in place. Check primary filters often for water and sludge. Replace filters that are black and restricted. Always prime filters with good clean diesel fuel. Most boat owners know to change the engine oil every season, but neglect to check the fuel quality. Diesel engines are extremely reliable if we always feed them clean fuel and oil. Take care of the small things and the big things will take care of themselves.