- Engine:
- C4.4 (S/N: 4441-UP)
- C6.6 (S/N: 6661-UP)
There have been isolated reports of discoloration of fuel or fuel that turns black on certain C6.6 and C4.4 engines. There have been no associated reports of any operational problems, that would indicate electronic unit injector or other fuel systems related concerns.
Tests have been conducted on fresh fuel samples. Tests have been conducted on discolored fuel samples that have been obtained from engines. These tests have included the Accelerated Fuel Oil Stability Test in "ASTM D-6468". This test determines the relative instability of a fuel that is subject to thermal degradation. This test is significant because the fuel is exposed to an environment that is similar to the actual operating conditions. The fuel is delivered to the injectors and only a small portion of fuel is injected. The rest of the fuel returns to the fuel tank.
The test results of the discolored fuel samples meet the requirements in the Caterpillar Commercial Diesel Engine Fluids Recommendations.
Fuel samples that are analyzed also meet thermal stability requirements that indicate that the fuels are suitable for use despite discoloration.
Analysis indicated that discolored fuel showed slight presence of engine lubricating oil and additives. A small degree of leakage of lubricating oil from the fuel injection pump in the fuel system is a known occurrence. This is a normal part of engine operation.
The discoloration of the fuel has been attributed to an acceptable level of engine lubricating oil and carbon fuel deposits as well as the exposure of the fuel to high injection pressure and exposure to high temperatures.
The diesel fuel can degrade if the diesel fuel is thermally stressed or stored during long periods of time. Complex chemical changes can cause degradation and oxidation of the fuel. The changes can lead to deposits or sediment from certain hydrocarbons and compounds that contain naturally occurring nitrogen and sulfur. In addition to composition of the diesel fuel, environmental factors can influence the rate of degradation and oxidation of the fuel.
Correct maintenance and service intervals become critical due to the possibility of reactions that occur between oxygen and the components of the fuel system. The reactions can generate particles in the fuel that eventually result in the formation of gums, resins and sediments that can affect the performance of the fuel pump and the electronic unit injectors.
Refer to the Operation and Maintenance Manual, "Maintenance Interval Schedule" for the correct maintenance and service intervals.
Based on the test results, it is concluded that the issue of fuel discoloration without significant increase in oil consumption or other operational problems is considered a common industry occurrence. This does not affect the engine performance.