ASE A1 Engine RepairGeneral Engine DiagnosisHard
A technician is diagnosing an engine with a persistent misfire in Cylinder 2. After performing a compression test (which showed good compression) and swapping the spark plug and coil with an adjacent cylinder (without changing the misfire location), what should the technician investigate NEXT?
- AThe fuel injector for Cylinder 2
- BA restricted exhaust system
- CThe engine control module (ECM)
- DThe camshaft position sensor
Show answer & explanationAnswer & explanation
Correct answer: A. The fuel injector for Cylinder 2
With good compression and confirmed good spark (by swapping components), the next logical step is to investigate fuel delivery to Cylinder 2. A faulty fuel injector (clogged, leaking, or electrically open) is a common cause of cylinder-specific misfires.
Why the other options are wrong
- B. A restricted exhaust system would generally affect all cylinders or cause a general lack of power, not a misfire isolated to a single cylinder.
- C. While the ECM could be faulty, it's less likely to cause a misfire in only one cylinder after other common causes are ruled out, and requires more advanced diagnostics.
- D. A camshaft position sensor issue would typically affect overall engine timing or multiple cylinders, not just one specific cylinder's misfire.
Misfire Diagnosis (Spark, Fuel, Compression)
Diagnosing a misfire involves systematically checking the three essentials for combustion: spark, fuel, and compression.
- Check compression first (mechanical integrity).
- Swap spark plugs/coils (isolate ignition issues).
- Check fuel injector operation (fuel delivery).
- Use a scan tool to identify misfire count and fuel trims.
Memory trick: Spark, Fuel, Compression – check them in succession.