What Causes Low Compression in an Engine?

Low compression in an engine may be caused by different factors such as bad exhaust valves, blown head gaskets or extensive ring and cylinder wear. Conducting an engine compression test can help narrow down the root cause of the issue.

A car engine is a vacuum system; in order for the engine to function properly, it must be able to create and maintain a proper level of compression. Most normal engines require 140 to 160 pounds per square inch of compression.

Check compression levels with a manual compression gauge, or by the use of an electronic engine analyzer. While the data generated by an engine analyzer is far superior to a handheld gauge, a manual compression test is sufficient for regular maintenance and troubleshooting.

A low compression reading in one cylinder normally indicates an exhaust valve issue. Low compression readings in adjacent cylinders are typically a sign of a blown head gasket. If every cylinder returns low compression levels, it could be an indication of worn-out rings and cylinders. If this is the case, you may need to have the entire engine rebuilt.

Sometimes, it is not clear whether the problem is the rings or the valves. In order to isolate the problem, squirt a small amount of motor oil into the cylinder through the spark plug hole. The oil temporarily seals the rings. If the compression readings do not change after introducing the oil, the problem is likely the valves. However, if the compression readings are higher after dropping the oil, the rings or cylinder may be the issue.