Low compression

1975 PLYMOUTH DUSTER
100,000 MILES • 5.2L • V8 • 2WD • AUTOMATIC
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KATBO
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Less than 100 on four different cylinders. How does a person know what exactly causes this? I know several differential equations things can cause, but how can a person test to see which is causing?
May 25, 2018 at 2:34 PM
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ASEMASTER6371
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Good afternoon.

To determine the failure, you need to remove the spark plugs and get an adapter to screw into the head where the spark plug goes. Then you inject shop air into the cylinder. Then you look for the air leak. It will show in either:

1. The carburetor, this means intake valve.

2. Exhaust pipe, this means exhaust valve.

3. Radiator, this means a head gasket.

4. Oil fill cap, that means either a piston or piston rings.

Roy
May 25, 2018 at 2:58 PM
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KATBO
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Do you do this for each cylinder? Car will not start, only turns over.
May 26, 2018 at 6:03 AM
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PATENTED_REPAIR_PRO
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Squirt a little motor oil into each cylinder before you repeat each test and if the psi goes up you have worn piston rings on all of them.
May 26, 2018 at 8:15 AM
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ASEMASTER6371
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Yes, you do it for each cylinder.

Have you checked for spark and fuel?

Roy
May 26, 2018 at 12:04 PM