Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A technician is performing an engine compression test on a vehicle with a complaint of reduced power and rough idle. The dry compression readings for cylinders 1-4 are 145 psi, 150 psi, 100 psi, and 148 psi, respectively. What is the MOST likely cause for the reading on cylinder #3?
- AWorn or damaged piston rings.
- BRestricted exhaust manifold.
- CA clogged catalytic converter.
- DA faulty fuel injector.
Show answer & explanationAnswer & explanation
Correct answer: A. Worn or damaged piston rings.
The significantly lower compression reading in cylinder #3 compared to the others indicates a mechanical issue within that cylinder. Worn or damaged piston rings are a common cause of low compression.
Why the other options are wrong
- B. A restricted exhaust manifold would impact engine performance generally, causing back pressure for all cylinders, not a specific compression drop in one cylinder.
- C. A clogged catalytic converter would affect all cylinders and lead to a general loss of power, not an isolated low compression reading in one cylinder.
- D. A faulty fuel injector would primarily affect combustion and air-fuel mixture, potentially causing a misfire, but not directly reducing mechanical compression pressure.
Low Compression - Single Cylinder
A significantly lower compression reading in one cylinder compared to others, indicating a mechanical fault specific to that cylinder.
- Suggests a localized mechanical problem.
- Common causes include worn piston rings, burnt valves, or head gasket leaks.
- Requires further diagnostic steps like a wet compression test or cylinder leakage test.
Memory trick: Compression's Drop: Rings, Valves, or Head's Flop.