What effect does oil have during a wet compression test?

During a wet compression test, oil is introduced into the cylinder to assess the condition of the piston rings and cylinder walls. The primary function of the oil in this context is to improve the sealing between the piston rings and the cylinder bore. When oil coats these surfaces, it fills any small gaps or imperfections that may exist due to wear or damage. This better seal can lead to increased compression readings compared to a dry compression test, where no oil is added.

This method is particularly useful because it can help differentiate between issues related to the piston rings (which seal the combustion chamber) and other potential problems such as valve sealing issues. If the compression improves significantly with the addition of oil, it often indicates that the piston rings may be worn or damaged, whereas if the compression does not change much, the problem might lie elsewhere.

The other options do not accurately describe the role of oil in a wet compression test, as they either refer to effects unrelated to compression testing or misinterpret the purpose of the test altogether.