A technician connects an ignition analyzer (oscilloscope) to the ignition harness of a reciprocating engine to troubleshoot rough running. One cylinder's firing pattern shows a noticeably higher peak firing voltage (kV) than the other cylinders. What condition does this most likely indicate?
- AA completely open ignition lead with no continuity to the spark plug
- BA shorted spark plug or ignition lead with very low resistance to ground
- CAn excessively wide spark plug gap or high resistance in that cylinder's lead, requiring more voltage to fire
- DA fouled spark plug with carbon bridging the electrode gap
Show answer & explanationAnswer & explanation
Correct answer: C. An excessively wide spark plug gap or high resistance in that cylinder's lead, requiring more voltage to fire
The firing voltage required to jump a spark plug gap increases with gap width and with resistance in the ignition circuit. A pattern showing abnormally high peak voltage on one cylinder points to a wider-than-normal gap or added resistance (such as a partially fouled connector or carbon-tracked lead), which forces the coil to generate more voltage before the spark can jump the gap.
Why the other options are wrong
- A. A completely open circuit generally produces no firing trace at all (infinite voltage attempt with no spark), not simply a higher voltage spike.
- B. A short to ground would typically show abnormally low firing voltage, not high, since voltage bleeds off before it can build up.
- D. Carbon fouling/bridging usually causes a low-voltage or erratic pattern from current leaking across the deposit rather than requiring higher voltage.
Ignition Analyzer (Oscilloscope) Pattern Interpretation
An ignition analyzer displays firing voltage patterns for each cylinder; abnormal peak voltages help diagnose plug gap, fouling, or circuit resistance problems.
- Higher-than-normal firing voltage suggests a wide gap or added circuit resistance.
- Lower-than-normal firing voltage suggests fouling or a partial short.
- No trace at all suggests an open circuit with no spark produced.
Memory trick: Bigger gap, bigger push — high voltage means the spark had to work harder.