For a compression check, what percentage of compression loss is considered excessive?

For a compression check, a loss of 25% of compression is generally regarded as excessive. This percentage indicates that the engine may be experiencing significant wear or damage, potentially leading to reduced performance, increased oil consumption, or other serious issues.

To perform a compression check, a mechanic typically measures the cylinder pressure during the compression stroke and compares it with the manufacturer's specified values. A loss of compression can result from various factors, such as worn piston rings, damaged cylinder walls, or leaking valves. When compression loss exceeds 25%, it often signifies the need for further investigation or potential corrective actions, such as repairs or engine overhaul.

In summary, a 25% compression loss threshold is a commonly accepted benchmark for determining whether an engine's condition warrants concern, making it crucial for maintaining engine integrity and performance.