Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairMedium
A vehicle exhibits a rough idle, misfires under load, and a consistent P0301 DTC (Cylinder 1 Misfire Detected). The technician tests the ignition coil for cylinder 1 and finds it produces a strong spark. Next, the fuel injector for cylinder 1 is tested and found to be operating correctly. What is the most logical next step in diagnosing this misfire?
- AInspect the wiring harness to the ignition coil.
- BCheck the compression in cylinder 1.
- CReplace the spark plug for cylinder 1.
- DCheck for vacuum leaks near cylinder 1.
Show answer & explanationAnswer & explanation
Correct answer: B. Check the compression in cylinder 1.
After verifying spark and fuel, the next critical component for combustion is compression. A lack of compression in cylinder 1 would directly cause a misfire and should be checked before suspecting other less direct causes.
Why the other options are wrong
- A. The ignition coil was tested and found to produce a strong spark, indicating the wiring harness to the coil is likely functioning correctly at least for power and signal.
- C. While a spark plug could be faulty, the coil was confirmed to produce a strong spark, which suggests the plug might be okay or at least not the primary issue if there's sufficient voltage.
- D. Vacuum leaks can cause misfires, but typically affect multiple cylinders or cause general lean conditions. A specific P0301 points to an issue within cylinder 1 itself, making compression a more direct check after spark and fuel are confirmed.
Three Elements of Combustion
For an internal combustion engine to operate correctly, three primary elements must be present and correctly timed: sufficient spark, adequate fuel, and proper compression.
- Spark: Ignites the air/fuel mixture.
- Fuel: Provides the combustible material.
- Compression: Increases temperature and density for efficient combustion.
Memory trick: Spark, Fuel, Compression: The S-F-C of engine life.