Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is performing a compression test on an engine. All cylinders show readings between 140-150 PSI, except for cylinder #3 which reads 90 PSI. A small amount of oil is added to cylinder #3, and the compression test is repeated, resulting in a reading of 135 PSI for cylinder #3. What does this indicate?
- AA cracked cylinder head on cylinder #3.
- BA leaking intake valve on cylinder #3.
- CWorn piston rings on cylinder #3.
- DA blown head gasket between cylinder #3 and an adjacent cylinder.
Show answer & explanationAnswer & explanation
Correct answer: C. Worn piston rings on cylinder #3.
The 'wet' compression test (adding oil) helps differentiate between worn piston rings and valve/head gasket issues. If adding oil significantly increases compression, it means the oil temporarily sealed worn rings, indicating they were the cause of the low compression.
Why the other options are wrong
- A. A cracked cylinder head would not be sealed by adding oil, and compression would remain low.
- B. A leaking intake valve would not be sealed by adding oil; the compression would remain low.
- D. A blown head gasket would not be sealed by adding oil, and compression would remain low or show other symptoms.
Wet Compression Test Interpretation
A diagnostic technique where a small amount of oil is added to a low-compression cylinder to help determine if the issue is worn piston rings or valve/head gasket problems.
- Significant increase in compression after adding oil indicates worn piston rings.
- Little to no increase indicates valve or head gasket issues.
- Oil temporarily seals gaps around worn rings.
Memory trick: Wet test: if oil boosts compression, rings are the problem's source, no boost, valve's course!