Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisHard
A technician is diagnosing a vehicle with an intermittent 'Check Engine' light and a stored P0301 (Cylinder 1 Misfire Detected) diagnostic trouble code. The technician uses an oscilloscope to capture the secondary ignition waveform of cylinder 1. The waveform shows a short spark duration and a low peak firing voltage. All other cylinders show normal waveforms. What is the MOST likely cause of this abnormal waveform?
- AAn open circuit in the primary ignition coil winding.
- BExcessive resistance in the secondary ignition circuit (e.g., spark plug wire).
- CA rich air/fuel mixture in cylinder 1.
- DA lean air/fuel mixture in cylinder 1.
Show answer & explanationAnswer & explanation
Correct answer: D. A lean air/fuel mixture in cylinder 1.
A lean air/fuel mixture is harder to ignite, requiring higher voltage to jump the spark plug gap (higher peak firing voltage) and often resulting in a shorter spark duration as the combustion process is less stable. The waveform described (low peak firing voltage and short spark duration) is a classic indicator of a lean misfire. A short spark duration suggests the coil cannot sustain the spark, common with lean conditions.
Why the other options are wrong
- A. An open primary winding would prevent any spark, not just an abnormal one.
- B. Excessive resistance in the secondary circuit would cause a very high peak firing voltage, as the coil tries harder to jump the gap, and potentially a shorter duration.
- C. A rich mixture is also harder to ignite than stoichiometric, but typically results in a lower peak firing voltage and a longer, 'lazy' spark line on the waveform, which is not what's described.
Secondary Ignition Waveform Analysis (Lean Misfire)
Interpreting the characteristics of a secondary ignition waveform, specifically noting peak firing voltage and spark duration, to diagnose engine combustion issues like lean misfires.
- Peak firing voltage: Voltage required to jump the spark plug gap.
- Spark duration: How long the spark is sustained across the gap.
- Lean misfire: High peak firing voltage (harder to ignite), short spark duration (less stable combustion).
- Rich misfire: Lower peak firing voltage (easier to ionize), longer, 'lazy' spark duration.
Memory trick: Short and low? Lean's the flow. High and short? Resistance report.