Warm up the engine fully. Remove the spark plugs. Block open the throttle - don't forget to close it when you are done testing! Screw the compression gauge fitting into the spark plug hole. Crank over the engine with the starter. I like to go at least 4 complete engine revolutions, but you can just stop once the needle on the compression gauge stops rising. Record the compression value. Now take a reading in each of the other cylinders. The lowest should be 75% of the highest. If it is good! If not, you need to look for a blown headgasket, bad valves, bad cam lobe.......
If you put some oil in your lowest cylinder and the compression goes up significantly, then it is the rings. If it remains the same, it's one of the other things previously mentioned.
also, before you start the car, make sure to disable the ignition. get the readys. the amount should be 75% of the highest amount. then you can do a wet test which is putting oil in to the cylinder than getting a reading. if the number is higher then what is was when its dry, then its your piston rings. if the number is lower or stays the same, it could be your head gasket or your valves may need adjustment.
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