Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician performs an engine compression test on a vehicle with a misfire on cylinder #4. The results are: Cylinder #1: 175 psi, Cylinder #2: 170 psi, Cylinder #3: 172 psi, Cylinder #4: 90 psi, Cylinder #5: 170 psi, Cylinder #6: 175 psi. What is the most likely initial step to further diagnose cylinder #4?
- ACheck the spark plug on cylinder #4 for fouling.
- BAdd a small amount of oil to cylinder #4 and re-test compression.
- CInspect the fuel injector on cylinder #4.
- DPerform a cylinder power balance test on cylinder #4.
Show answer & explanationAnswer & explanation
Correct answer: B. Add a small amount of oil to cylinder #4 and re-test compression.
A significantly low compression reading (90 psi compared to ~170-175 psi in other cylinders) indicates an issue with cylinder sealing. Adding oil and re-testing compression helps differentiate between worn piston rings (compression increases) and valve/head gasket issues (compression remains low).
Why the other options are wrong
- A. While a fouled spark plug can cause misfire, it would not directly cause low compression.
- C. A faulty fuel injector would not directly cause low compression, though it could cause a misfire.
- D. A power balance test would only confirm it's not contributing, but not why compression is low.
Wet Compression Test
A diagnostic procedure where a small amount of oil is added to a cylinder with low compression, then the compression test is repeated to help identify the source of the leak.
- If compression increases significantly, piston rings are likely worn.
- If compression remains low, valve or head gasket issues are more likely.
- Performed after a 'dry' compression test shows low results.
Memory trick: Compression tells the seal's tale, low pressure means a likely fail.