Compression Test: The Paper Test

An engine compression test is one of the fundamental engine diagnostic tests that can be performed.  For smooth engine operation, all cylinders must have equal compression.  An engine can lose compression by leakage of air through one or more of only three routes:

- Intake or exhaust valve
- Piston rings (or piston, if there is a hole)
- Cylinder head gasket

The paper test involves holding a piece of paper near the tailpipe of an idling engine. A good engine should produce even, outward puffs of exhaust. If the paper is sucked in toward the tailpipe, a burned valve is a possibility.