Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisEasy
A technician performs a wet compression test on cylinder #3 of an engine that had a dry reading of 80 psi (specification 150 psi). After adding a small amount of oil, the compression reading for cylinder #3 increases to 130 psi. What does this result primarily indicate?
- AWorn piston rings
- BLeaking intake valve
- CCracked cylinder head
- DBlown head gasket
Show answer & explanationAnswer & explanation
Correct answer: A. Worn piston rings
If a wet compression test significantly increases the compression reading compared to the dry test, it indicates that the added oil temporarily sealed worn piston rings, confirming them as the primary cause of low compression.
Why the other options are wrong
- B. A leaking intake valve would not be significantly affected by adding oil to the cylinder, so the compression would remain low.
- C. A cracked cylinder head would not be sealed by oil and would likely cause coolant loss or mixing, with no improvement in compression from a wet test.
- D. A blown head gasket would not show improvement with a wet compression test; compression would remain low, possibly affecting adjacent cylinders or causing coolant issues.
Wet Compression Test - Worn Rings
A diagnostic procedure where a small amount of oil is added to a cylinder with low dry compression. A significant increase in pressure indicates worn piston rings.
- Follows a low dry compression reading.
- Oil temporarily seals piston ring gaps.
- Significant pressure increase confirms worn rings.
Memory trick: Oil boosts compression? Rings are the mission!