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Anyone know what would cause low compression other than worn piston rings?

I did a compression check and according to the service manual it should be between 139 - 215 PSI. I am getting 90 PSI on cylinders 1 and 2, 120 PSI on cylinder 3, and 180 PSI on cylinder 4.

I plan to pull the motor apart and change the ring since they are cheap, $40 a set, but I would like to know if any of the valve may contribute to this issue so that I can get it fixed while im in there.

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Whens the last time you've had a valve adjustment? Out of tolerance and valve/valve seat wear will drop compression.

One easy test to figure out if its rings- squirt some plain old motor oil in the cylinder that is low through the spark plug hole and re-test. The oil will get the rings to temporarily seal up and your compression will jump up. If it goes up alot then its rings... if only a little or none at all you've got valve issues.

I would do the above test on all cylinders before i attempted a re-ring job.

Also, a bad head gasket can cause low compression, usually it will be seen on two cylinders side by side that will read close together.
 
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