Diesel fuel is chemically similar to the lubricants that are used in diesel engines. A slow fuel leak will blend the fuel into the oil. Normal operating temperatures may cause volatile parts of the fuel to vaporize. The fuel that remains in the oil is less volatile.
A closed cup flash test can be performed in order to detect fuel dilution. The flash test is designed to measure the volatile parts of the fuel that are remaining in the oil. Detecting less volatile fuel is difficult. The lack of volatility reduces the accuracy of the flash test.
Since the flash test does not accurately detect fuel dilution, do not use the flash test as the only measure of fuel dilution. Instead, verify the dilution by the following methods:
- Gas chromatograph fuel dilution test
- Oil viscosity
The test that uses a gas chromatograph is designed to measure fuel dilution in crankcase oils. The gas chromatograph can identify the small chemical differences between diesel fuel and lubricating oil. A gas chromatograph is used to measure fuel dilution at all Cat® regional SOS laboratories. Some Cat dealers also use a gas chromatograph. Even though the gas chromatograph provides a more accurate measure of fuel dilution, always verify the results with the viscosity test.
A significant level of fuel dilution reduces oil viscosity. If an unacceptable level of fuel dilution is suspected, the kinematic viscosity of the oil must be measured.
Fuel dilution that is greater than 4 percent will usually cause viscosity that is less than the specified viscosity grade. If the oil is still within the specified viscosity grade, fuel dilution is unlikely to have reached an unacceptable level. Use the following chart to determine if viscosity has reached the minimum acceptable level. The guidelines of viscosity in the chart are slightly less than the limits of the SAE viscosity grades. However, these guidelines still provide adequate engine protection.
| Viscosity Grade | Minimum Oil Viscosity at 100 °C with Fuel Dilution Greater Than 4% as Measured by a Gas Chromatograph | Action |
| 0W-40
5W-40 10W-40 15W-40 |
12.0 cSt | Investigate the cause of fuel dilution or reduce the engine oil change interval. |
| 0W-30
5W-30 10W-30 |
9.0 cSt |
Always verify fuel dilution by the combination of a viscosity test and a gas chromatograph test that gives a result in excess of 4 percent. If a fuel dilution test cannot be performed locally, submit an oil sample to a Cat dealer for analysis.
- Fuel injector tip
- Shaft seal for the high-pressure fuel pump
Look for signs of damage to the fuel injectors. Check the fuel injector tip for cracks or breakage. If necessary, replace the unit injectors.
Shaft Seal for the High Pressure Fuel Pump
Check for fuel leakage around the shaft seal for the high-pressure fuel pump.
If the fault is still present, contact the Technical Communicator at a Cat® dealer.