Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a complaint of intermittent rough running and occasional stalling. A scan tool reveals a stored DTC P0301 (Cylinder 1 Misfire Detected). The technician swaps the spark plug and ignition coil with cylinder 2, but the misfire remains on cylinder 1. Next, the technician measures the fuel injector resistance for cylinder 1 and finds it to be within specification. Which of the following should be the technician's next logical diagnostic step?
- APerform a compression test on cylinder 1.
- BCheck for a vacuum leak around the intake manifold for cylinder 1.
- CReplace the Powertrain Control Module (PCM).
- DTest the fuel pressure and fuel pump volume.
Show answer & explanationAnswer & explanation
Correct answer: A. Perform a compression test on cylinder 1.
Since spark (coil and plug swapped) and fuel injector electrical integrity (resistance) have been checked and the misfire remains on cylinder 1, the next logical step is to investigate engine mechanical issues. A compression test will quickly identify if cylinder 1 has a mechanical problem (e.g., worn rings, bad valve) causing the misfire.
Why the other options are wrong
- B. While a vacuum leak can cause a lean misfire, it's less likely to be isolated to a single cylinder unless it's a very specific gasket leak, and mechanical integrity is a more fundamental check after verifying spark and injector function.
- C. Replacing the PCM is a last resort after all other diagnostic steps have been exhausted and verified, as it's an expensive component and rarely the cause of a single-cylinder misfire.
- D. Testing overall fuel pressure and volume is important for general fuel delivery issues, but a single-cylinder misfire after confirming injector resistance points to a more localized problem than a system-wide fuel pressure issue.
Misfire Diagnosis Steps
Diagnosing a single-cylinder misfire typically involves systematically checking the 'big three' elements: spark, fuel, and compression, in a logical order to isolate the root cause.
- Swap spark plug/coil to rule out ignition.
- Check injector pulse/resistance for fuel delivery.
- Perform compression test for mechanical integrity.
Memory trick: Spark, Fuel, Compression: The Misfire Trinity.