ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is performing a wet compression test on a cylinder that previously showed low dry compression. After adding a small amount of oil to the cylinder and retesting, the compression reading significantly increases. What does this result indicate?
- AWorn piston rings or cylinder walls
- BA cracked cylinder head
- CA leaking head gasket
- DA burnt exhaust valve
Show answer & explanationAnswer & explanation
Correct answer: A. Worn piston rings or cylinder walls
When oil is added during a wet compression test, it temporarily seals worn piston rings or cylinder walls, leading to an increase in compression. This confirms that the problem lies in the sealing ability of the piston assembly.
Why the other options are wrong
- B. A cracked cylinder head would not be sealed by oil and would likely show coolant loss, overheating, or specific misfires.
- C. A leaking head gasket would not be sealed by oil and would likely show coolant contamination or misfires in adjacent cylinders.
- D. A burnt exhaust valve would not be sealed by oil in the cylinder and would still show low compression.
Wet Compression Test Interpretation
A wet compression test helps determine if low compression is due to worn piston rings/cylinder walls or valve/head gasket issues by adding oil to the cylinder.
- If compression significantly increases, piston rings/cylinder walls are worn.
- If compression remains low, the issue is likely with valves or head gasket.
- Always perform a dry test first for comparison.
Memory trick: Oil boosts compression, rings are the problem.