That refers to how hard each piston is squeezing the air it sucks in. Even 120 pounds is low. The rest are real low. That can be expected with such high mileage. Piston rings seal the air in and prevent it from leaking out into the lower part of the engine where the oil is. Those rings normally wear with age. There's also valves that let the air in and the exhaust out. Over time they too will not seal as well as when they were new.
A compression test is just a fast way of determining the quality of each cylinder and its ability to make power equal to all the other cylinders. Your mechanic can also do a more involved "cylinder leakage" test, or "cylinder leak down" test. That will identify exactly what is responsible for the low compression. They will likely elect to not do that test with the mileage you listed because everything is going to be worn. No professional will do a valve job, for example, and leave all the other worn parts in the engine. This is likely a case where the engine served you well but it's time for a total rebuild.
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Thursday, January 24th, 2013 AT 12:32 AM