ASE A1 Engine RepairCylinder Head and Valve TrainHard
A technician is diagnosing an engine with a persistent misfire in cylinder #3. A spark plug inspection shows the plug is fouled with oil. A compression test on cylinder #3 yields a reading of 90 psi, while all other cylinders are around 150 psi. A wet compression test on cylinder #3 brings the reading up to 145 psi. What is the MOST likely cause of the misfire?
- AA cracked cylinder head.
- BA blown head gasket.
- CWorn piston rings.
- DA worn exhaust valve.
Show answer & explanationAnswer & explanation
Correct answer: C. Worn piston rings.
The key indicators here are the low dry compression (90 psi vs. 150 psi) and the significant increase in compression during the wet compression test (up to 145 psi). This indicates that the problem is with the piston rings, as the added oil temporarily seals the gaps in the worn rings, improving compression. Oil-fouled spark plugs further support oil entering the combustion chamber past the rings.
Why the other options are wrong
- A. A cracked cylinder head might cause low compression, but a wet test would generally not improve it significantly, and other symptoms like coolant in oil or overheating would be present.
- B. A blown head gasket would likely not show a significant increase with a wet test, and coolant contamination or adjacent cylinder issues would be more prominent.
- D. A worn exhaust valve would typically not show such a dramatic increase in compression with a wet test, and oil fouling would be less common unless valve stem seals were also bad.
Wet Compression Test
A diagnostic test performed after a dry compression test, where a small amount of oil is added to the cylinder through the spark plug hole to temporarily seal worn piston rings, indicating their condition.
- Significant improvement in compression indicates worn piston rings.
- Little or no improvement points to valve or head gasket issues.
- Helps differentiate between ring and valve/gasket problems.
Memory trick: Dry low, wet high, rings are the guy.