Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is performing a comprehensive diagnostic on a vehicle with a persistent P0301 (Cylinder 1 Misfire) code. After verifying fuel and compression are good, they connect an oscilloscope to observe the secondary ignition waveform for cylinder #1. They notice that the spark line is very short in duration and has a very high firing voltage (kV). What does this waveform characteristic most likely indicate?
- AA fouled spark plug with excessive carbon buildup.
- BAn open circuit in the spark plug wire.
- CA shorted spark plug or excessive current in the secondary circuit.
- DAn excessively wide spark plug gap or high resistance in the secondary circuit.
Show answer & explanationAnswer & explanation
Correct answer: D. An excessively wide spark plug gap or high resistance in the secondary circuit.
A very short spark duration with a high firing voltage typically indicates that the coil is working hard to overcome excessive resistance or a large gap in the secondary circuit. An excessively wide spark plug gap or high resistance in the plug wire or coil boot would cause this, demanding more voltage to jump the gap and shortening the available burn time.
Why the other options are wrong
- A. A fouled spark plug (carbon) would typically lower the firing voltage and shorten the spark line, but primarily by providing an easier path to ground, not necessarily demanding a very high firing voltage initially.
- B. An open circuit in the spark plug wire would likely result in no spark at all, or a very high, erratic firing voltage with no burn line, not just a short duration.
- C. A shorted spark plug or excessive current would result in a very low firing voltage, as the voltage would quickly find an easy path to ground, and a very short or absent spark line.
Secondary Ignition Waveform Interpretation
A secondary ignition waveform showing high firing voltage and short spark duration often indicates excessive resistance or an overly wide gap in the secondary circuit, forcing the coil to work harder.
- High kV = high resistance or large gap.
- Short duration = energy quickly depleted.
- Indicates difficulty in spark discharge.
Memory trick: High climb, quick fall – resistance makes the spark stall.