Start / Calculations / Engine Calculations / Compression Test
The Compression Test page helps you analyze the results of an engine compression test.
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Start with a warm engine if possible.
Make sure you have a healthy battery.
Important: disable fuel and spark.
Remove all spark plugs.
Insert compression tester in one spark plug hole.
Crank engine until compression tester reaches its maximum reading.
Record the reading.
Repeat for each cylinder.
All cylinders should be reasonably similar and not a lot lower than the theoretical maximum. They don't have to be perfect.
In case of any anomalies, run the entire test again in case something went wrong the first time.
If some cylinders are significantly lower than others, add about one tablespoon of motor oil to each low cylinder through the spark plug hole and re-test. If this improves the compression then the piston rings may be the cause. If not, it could be the valves or the head gasket.
If two or more adjacent cylilnders are low, it could be the head gasket.
If all cylinders are low, normal engine wear is the probable cause.
If any cylinders are zero or very low, engine damage is the likely cause.