Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A vehicle experiences intermittent misfires and hesitation, particularly during damp weather. The technician inspects the ignition coil and finds no cracks, but notices some corrosion on the coil tower terminals. The spark plug wires are relatively new. What is the MOST likely cause of the intermittent misfires?
- AHigh voltage leakage due to moisture and corrosion.
- BIncorrect spark plug gap.
- CA faulty engine control module (ECM).
- DInternal short in the ignition coil primary winding.
Show answer & explanationAnswer & explanation
Correct answer: A. High voltage leakage due to moisture and corrosion.
Corrosion on coil tower terminals, especially in damp conditions, provides a conductive path for high voltage to leak to ground instead of reaching the spark plugs. This 'tracking' or leakage reduces the voltage available at the plug, leading to intermittent misfires and hesitation.
Why the other options are wrong
- B. An incorrect spark plug gap would cause a more consistent misfire, not one specifically triggered by damp weather.
- C. An ECM fault would likely produce more consistent or widespread issues, and is less likely to be triggered specifically by damp weather.
- D. An internal primary winding short would typically cause a more consistent misfire or no spark at all, not usually intermittent misfires tied to dampness.
High Voltage Leakage (Ignition)
The unintended diversion of high voltage from the secondary ignition circuit (e.g., coil, wires) to an unintended ground path, often exacerbated by moisture or contamination.
- Leads to weak or absent spark at plug.
- Often causes intermittent misfires, especially in damp conditions.
- Caused by cracked insulators, corroded terminals, damaged wires.
Memory trick: Dampness and dirt make sparks short circuit.