Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with an intermittent misfire that primarily occurs during hard acceleration. They connect an oscilloscope to the secondary ignition system and observe a waveform with a very short spark line (ignition duration) and a high firing voltage (kV). Which of the following is the MOST likely cause?
- AInsufficient coil saturation time.
- BA fouled spark plug with excessive carbon buildup.
- CAn open in the secondary ignition circuit.
- DExcessive resistance in the primary ignition circuit.
Show answer & explanationAnswer & explanation
Correct answer: B. A fouled spark plug with excessive carbon buildup.
A fouled spark plug, particularly with carbon, can provide an easier path for spark, leading to a high firing voltage (as the coil struggles to jump the gap) and a very short spark duration (as the energy is quickly dissipated through the conductive fouling). This condition is often exacerbated under load.
Why the other options are wrong
- A. Insufficient coil saturation time would result in a lower firing voltage and a shorter spark duration, as the coil hasn't built up enough energy.
- C. An open in the secondary circuit would result in an extremely high firing voltage (often exceeding scope limits) or no spark at all, not necessarily a short spark line at a high but measurable kV.
- D. Excessive primary resistance would typically lead to a lower firing voltage and a shorter spark duration due to reduced coil saturation.
Fouled Spark Plug Waveform
A secondary ignition waveform showing a high firing voltage (kV) and a very short spark line (ignition duration) often indicates a fouled spark plug, especially with carbon buildup.
- High firing kV due to increased resistance or conductive path.
- Short spark duration as energy dissipates quickly.
- Often seen under load or acceleration.
- Carbon fouling is a common cause.
Memory trick: Fouled plugs make a 'High-Short' spark.