Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A vehicle with a distributor-based ignition system exhibits a consistent misfire under acceleration. The technician notes that the misfire becomes more pronounced as engine RPM increases. Upon inspection, the distributor cap and rotor appear worn, with visible carbon tracking. What is the MOST likely cause of this symptom?
- AIncorrect ignition timing.
- BA clogged fuel filter.
- CA faulty crankshaft position sensor.
- DExcessive resistance in the spark plug wires.
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive resistance in the spark plug wires.
Worn distributor cap and rotor, combined with carbon tracking, indicate high resistance in the secondary ignition circuit. This resistance, especially in spark plug wires, can lead to voltage breakdown and misfires, which worsen under higher loads and RPMs.
Why the other options are wrong
- A. Incorrect ignition timing would cause driveability issues across the RPM range, not necessarily a misfire that worsens with RPM due to secondary circuit resistance.
- B. A clogged fuel filter would cause a lean condition, affecting all cylinders and typically causing a general loss of power, not specifically an ignition misfire.
- C. A faulty crankshaft position sensor typically causes no-start or widespread misfires, not usually isolated to acceleration.
Secondary Ignition Resistance
The total electrical resistance in the high-voltage circuit from the ignition coil secondary winding to the spark plug, including spark plug wires, distributor cap, and rotor.
- High resistance impedes spark delivery.
- Can cause misfires, especially under load.
- Worn components (cap, rotor, wires) increase resistance.
Memory trick: Distributor issues often lead to secondary circuit problems.