Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a persistent misfire on cylinder #2. They perform a power balance test and confirm the misfire. Next, they use a spark tester to check for spark at the cylinder #2 spark plug wire, and observe a strong, consistent blue spark. What should the technician's NEXT diagnostic step be, considering these findings?
- APerform a compression test on cylinder #2.
- BCheck fuel injector operation for cylinder #2.
- CReplace the ignition coil for cylinder #2.
- DInspect the crankshaft position sensor (CKP).
Show answer & explanationAnswer & explanation
Correct answer: B. Check fuel injector operation for cylinder #2.
Since a strong, consistent blue spark is confirmed at cylinder #2, the ignition system for that cylinder is likely functioning correctly. A misfire, in this case, points to other potential causes, such as a fuel delivery issue (e.g., a clogged or faulty injector) or a lack of compression. Checking the fuel injector operation is a logical next step to rule out a fuel-related misfire before moving to mechanical issues.
Why the other options are wrong
- A. While a compression test is important, fuel delivery is often easier and quicker to check than compression, making it a good intermediate step after confirming spark.
- C. Replacing the ignition coil is unnecessary, as a strong spark has been confirmed, indicating the coil is working.
- D. The CKP sensor affects all cylinders; if it were faulty, the misfire would likely be across multiple cylinders or a no-start condition, not isolated to one cylinder with good spark.
Misfire Diagnosis Flow (Spark Confirmed)
When a misfire is confirmed on a specific cylinder and a strong spark is present, the diagnostic focus shifts from ignition to fuel delivery and then to mechanical engine issues.
- Misfires are typically caused by ignition, fuel, or compression problems.
- Confirming spark eliminates the ignition system as the immediate cause.
- Next steps involve checking fuel delivery (injector) and then engine compression.
Memory trick: Spark, Fuel, Compression: the misfire trilogy.