What is the most likely cause of a cylinder starting at 30 psi and building up to 120 psi over multiple cranks during a compression test?

A cylinder that starts with a low psi, such as 30 psi, and gradually builds up to a higher level like 120 psi over several cranks typically indicates issues related to sealing within the combustion chamber. Worn piston rings are a common culprit in such scenarios.

When piston rings are worn, they do not maintain a proper seal against the cylinder walls, which can lead to low initial compression readings. As the engine is cranked over multiple times, the engine oil may temporarily help seal or improve the compression due to the oil coating the worn surfaces, which then leads to an increase in pressure. This phenomenon is especially noticeable in tests where the rings might not fully compress under initial test conditions but improve after a few cycles when oil is distributed better.

In contrast, a cracked cylinder head or damaged cylinder wall would generally result in consistent low compression readings, as these issues tend to cause immediate loss of compression rather than a gradual increase. A faulty head gasket typically leads to one or more cylinders exhibiting similar low pressures across the board, rather than allowing any cylinder to build pressure significantly during cranking.