The interpretation of test results from a used oil analysis consists of two processes: comparison and diagnosis. In the comparative process the current sample is compared to previous samples and warning limits.
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Knowing whether a test result is normal is key to proper interpretation of used oil analysis. While most of the test results will indicate a normal condition, it is the results above some threshold or warning limit which triggers an action that must be identified. Setting warning limits too low will trigger false alarms, while setting limits too high will miss potential problems.
Warning limits should be specific to an engine or a family of engines within a fleet. The use of general warning limits published for single sample analysis are often used by oil analysis laboratories and are inappropriate for fleet monitoring. The process described here establishes limits based on unit or fleet history through the use of statistics and can be easily determined using a calculator or personal computer.
Oil samples should be taken at regular intervals. The accumulation of contaminants such as wear metals and soot are time dependent. That is, the longer the oil is in the engine, the higher the levels of contaminants will be. If sampling is too irregular, an adjustment to the data is necessary before it can be interpreted. The data needs to be prorated relative to oil mileage or operating time. Generally, no adjustment is necessary if the samples are taken within 10% of the sampling average.
Critical Contaminant Levels.
The most common problem encountered in interpreting used oil analysis is not missing an impending engine failure, but reaction to false alarms -- unnecessary actions taken based on a single oil analysis. Many times an engine is unnecessarily disassembled, only to find that no engine problem exists. For this reason, under no circumstances should an engine be disassembled based on a single oil analysis unless there is mechanical substantiation.
When a critical level oil analysis is received a resample at 1/2 the interval that the critical sample was taken or at 1/2 the normal sampling interval is recommended. If the critical level was prompted by fuel, coolant, dirt, soot, or other identified contaminant, a correction should be made as soon as possible. Reduced oil drains may be needed until the correction is made. Remember to prorate the test result of the 1/2 interval sample to obtain the correct action level.
If an obvious cause for the critical oil analysis report is not found, the unit should be removed from service and examined in detail. Running on a chassis dynamometer may be necessary. If the cause of the critical oil analysis cannot be found, return the unit into service and continue to monitor it closely. The engine manufacturer may not be responsible for costs incurred with an engine teardown which is not confirmed by a detailed mechanical inspection.
During the break-in period, usually through the first two oil changes, wear metals will be high and may generate a critical report. Oil samples taken during the first two oil drain intervals should only be monitored for contaminants such as fuel and coolant. Wear metals from these samples should not be used for averaging.
Many of the characteristics of a used oil analysis are interdependent. The objective of used oil analysis interpretation is to determine if a problem exists and then to identify the potential causes of the problem. All the characteristics must be examined just as a detective would examine clues to solve a case. The best way to do this is to understand the relationship between the characteristics. This understanding comes with experience and knowledge of the engine being monitored. Attending an engine service training class given by the engine manufacturer may be helpful in this regard. Although a complete list of interactions covering all the combinations would be impossible to create, the interactions listed in the chart below may provide a starting point. It is recommended that the customer establish a similar table based on experience gained with his specific fleet.