Understanding Compression Testing and Oil Addition in Engines

When performing compression tests on an engine, adding a minimal amount of oil to the cylinders can be crucial for obtaining accurate results. This article explores the implications of using increased compression test numbers after adding oil, and how this relates to the condition of piston rings and cylinder bores. We will also discuss the importance of maintaining proper oil levels and the significance of systematic testing procedures.

Proper Oil Addition for Compression Testing

In a wet or dry compression test, the amount of oil added to the cylinder is a critical factor in achieving accurate results. For a small engine with a displacement up to 500cc per cylinder, a typical value is around 10 psi. For larger engines with greater areas, adding more oil may result in a wet test pressure of up to 20 psi. Typically, just a few squirts of oil (around 5 squirts from an oil can) per cylinder for a wet test is sufficient.

It is essential to be mindful of the amount of oil added, as adding too much can lead to exagerrated results. Ensuring that only the minimum amount of oil is used helps to avoid false readings, whether they are high or low. In a worn engine, cylinder-to-cylinder pressure can vary significantly, making it necessary to use controlled amounts of added oil to get accurate measurements and avoid false positives.

Implications of Increased Compression Numbers After Adding Oil

When a substantial increase in compression numbers is observed after adding oil to the cylinder, it is not necessarily an indication of a problem with the piston rings or the cylinder bores. However, if the oil addition does not significantly improve the readings, it could indicate wear on the compression rings or scratches on the cylinder walls.

Adding oil to a cylinder can be messy and can get the oil on the tester and hoses. Moreover, it does not provide a definitive test of the actual condition of the engine. Even if the compression test shows an improvement after adding oil, it is crucial to consider whether the overall condition of the engine necessitates a teardown and inspection. Engine repairs based solely on compression readings without further investigation may not be reliable.

Important Considerations During Compression Testing

Engine mileage and sitting for a while can affect the compression readings. If the engine has been sitting for a while, the rings may temporarily stick and not seal as effectively. This temporary issue can lead to lower compression readings, which can improve once the engine is run. In general, a minimal improvement in compression numbers after adding oil suggests that the rings are functioning adequately. A significant improvement, on the other hand, could indicate severe wear on the rings.

A more substantial improvement in each cylinder indicates a potential need for engine rebuild. It is important to note that using oil to improve compression readings should not be the basis for quoting services for the engine. Instead, a more comprehensive approach, such as conducting a leak-down test, introducing smoke, and using a bore scope, can provide a more accurate assessment of the engine's condition.

Conclusion

Properly understanding the implications of increased compression test numbers after adding oil to the cylinder can help in diagnosing engine issues accurately. While this method can be useful, it is best combined with other diagnostic techniques to ensure that the engine is serviced correctly. By maintaining proper oil levels, using controlled amounts of oil for testing, and adhering to systematic procedures, you can achieve reliable and meaningful compression test results.