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Low compression

Tiny
KATBO
  • MEMBER
  • 1975 PLYMOUTH DUSTER
  • 5.2L
  • V8
  • 2WD
  • AUTOMATIC
  • 100,000 MILES
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?
Friday, May 25th, 2018 AT 2:34 PM

4 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
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
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Friday, May 25th, 2018 AT 2:58 PM
Tiny
KATBO
  • MEMBER
  • 2 POSTS
Do you do this for each cylinder? Car will not start, only turns over.
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Saturday, May 26th, 2018 AT 6:03 AM
Tiny
PATENTED_REPAIR_PRO
  • MECHANIC
  • 1,853 POSTS
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.
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Saturday, May 26th, 2018 AT 8:15 AM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Yes, you do it for each cylinder.

Have you checked for spark and fuel?

Roy
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Saturday, May 26th, 2018 AT 12:04 PM

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