Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A technician is using an oscilloscope to analyze the secondary ignition waveform of a cylinder exhibiting a misfire. The waveform shows a normal firing line (kV) but the spark line (ignition duration) is significantly shorter than the other cylinders, followed by strong, rapid coil oscillations. What does this waveform MOST likely indicate?
- AAn excessive spark plug gap.
- BA lean air-fuel mixture.
- CA rich air-fuel mixture.
- DLow compression in that cylinder.
Show answer & explanationAnswer & explanation
Correct answer: C. A rich air-fuel mixture.
A rich air-fuel mixture contains more conductive particles (hydrocarbons) which can make the spark plug gap more conductive. This allows the spark to jump at a normal voltage but then extinguishes prematurely due to the increased conductivity, resulting in a shorter spark duration (spark line) and often followed by pronounced coil oscillations as the energy is quickly released.
Why the other options are wrong
- A. An excessive spark plug gap would cause a significantly higher firing kV, as more voltage is needed to jump the larger gap, with a potentially normal or slightly reduced duration.
- B. A lean mixture typically causes a higher firing kV and a longer, weaker spark duration, or even a misfire.
- D. Low compression typically leads to a lower firing kV and a longer spark duration due to reduced resistance in the cylinder.
Rich Mixture Waveform
A secondary ignition waveform indicating a rich air-fuel mixture typically shows a normal firing voltage (kV) but a significantly shorter spark line (ignition duration), often followed by pronounced coil oscillations.
- Normal firing kV.
- Short spark duration.
- Increased conductivity in gap.
- Often accompanied by strong coil oscillations.
Memory trick: Rich makes the spark 'Short & Strong Oscillation'.