Fungi and other microorganisms can survive and multiply in diesel fuel if water is present. The fungi can be present in any part of the fuel handling system. These fungi grow into long strings and will form into large globules. The growths appear slimy and are usually black, green, or brown. The fungi may grow anywhere in the fuel but are most plentiful where diesel fuel and water meet. As the fuel is agitated, when service station tanks are being filled, fungi are distributed throughout the tank and may be pumped into a vehicle.

Fungi use the fuel as their main energy supply and need only trace amounts of water and minerals. As they grow and multiply, they change fuel into water, sludge, acids, and products of metabolism. The most common symptom is fuel filter plugging; however, various metal components including the fuel sending unit, pipes, fuel injectors, and injection pump can corrode.

Warning: Avoid physical contact with the biocides in order to avoid personal injury.

If fungi have caused fuel system contamination, use a diesel fuel biocide to sterilize the fuel system. Do not exceed the dosage recommended on the label. Discontinue the use of a biocide when towing a trailer. It is permissible to have biocide in the fuel when starting to tow, but do not add any biocide while towing.

Steam cleaning may be necessary if most of the fungus growth cannot be removed with biocides.

The presence of water or gasoline in diesel fuel may also cause injection pump and nozzle damage.

This procedure checks for the presence of water and gasoline in diesel fuel that may cause injection pump and nozzle damage.

Remove and inspect the fuel filter element.

1. Disconnect the negative battery. Refer to Battery Negative Cable Disconnection and Connection.
2. Remove the sending unit. Refer to Fuel Sender Assembly Replacement.
3. Inspect the fuel tank and the fuel sender for rust, fungi or bacteria. If there is rust, replace the rusted components.
4. Clean the inside of the fuel tank and the fuel sender with hot water.
5. Use compressed air in order to dry the fuel tank and the fuel sender.
6. Disconnect the ends of the following lines:

•	The fuel filter inlet line (both ends)

•	The transfer pump pressure line and suction line (if applicable)

•	The fuel filter outlet line (both ends)

•	The fuel filter drain

•	The fuel return line (both ends)

7. Inspect each of the pipes and lines.
8. Replace any rusted pipes.
9. Clean the inside of the fuel filter housing.
10. Dry the fuel filter housing with compressed air.
11. Dry the inside of each line with low pressure air.
12. Disconnect the crankshaft position (CKP) sensor. Refer to Crankshaft Position Sensor Replacement.
13. Install a new fuel filter. Refer to Fuel Filter Replacement.
14. Install the sending unit. Refer to Fuel Sender Assembly Replacement.
15. Add clean diesel fuel to the primary tank until the tank is ¼ full.
16. Reconnect the following lines:

•	The fuel filter inlet line

•	The fuel filter outlet line

•	The transfer pump pressure and suction (both ends) lines

•	The fuel return line (tank end)

17. Connect the batteries.
18. Install a hose on the fuel return line near the glow plug relay, and insert other end into a 7.6 liters (2 gallon) metal container.
19. Turn ignition on with the engine OFF. Allow the fuel pump to run until the cycle ends or until 3.8 liters (1 gallon) of fuel has passed into the container.
20. Turn the ignition OFF.
21. Connect the fuel return line.
22. Clean any fuel spillage from the engine.
23. Reconnect the CKP sensor.
24. Start and run the engine.
25. Stop the engine.
26. Check for oil leaks.
27. Fill the fuel tank and add a biocide, if needed.

1. Drain the fuel tank.
2. Fill the fuel tank to ¼ full.
3. Disconnect the CKP sensor. Refer to Crankshaft Position Sensor Replacement.
4. Install a hose on the fuel return line near the glow plug relay, and insert the other end into a 7.6 liters (2 gallon) metal container.
5. Turn the ignition ON with the engine OFF. Allow the fuel pump to run until the cycle ends or until 3.8 liters (1 gallon) of fuel has passed into the container.
6. Reconnect the fuel return line.
7. Clean any fuel spillage from the engine.
8. Reconnect the CKP sensor.
9. Attempt to start and run the engine for 15 minutes. If the engine does not start, cycle the ignition and allow the fuel pump to run until the cycle ends.
10. Start the engine.
11. Stop the engine.
12. Check for fuel leaks.
13. Clear the engine of any diagnostic trouble codes (DTCs).