Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairEasy
A client complains of an engine misfire that primarily occurs under heavy acceleration. A technician performs a resistance test on the spark plug wires and finds that one wire has a resistance of 35,000 ohms, while the others are all within the 5,000-15,000 ohm range. What is the most appropriate next step for the technician?
- ACheck the fuel injector for that cylinder.
- BReplace the ignition coil for that cylinder.
- CReplace the spark plug for that cylinder.
- DReplace the high-resistance spark plug wire.
Show answer & explanationAnswer & explanation
Correct answer: D. Replace the high-resistance spark plug wire.
The resistance of 35,000 ohms is significantly higher than the acceptable range for spark plug wires (typically 5,000-15,000 ohms per foot, or a specified maximum). This excessive resistance will impede high voltage delivery, leading to a misfire, especially under load when more voltage is required.
Why the other options are wrong
- A. While a fuel injector issue can cause a misfire, the diagnostic test has already identified a clear ignition system fault that should be addressed first.
- B. The high resistance is in the wire, not the coil. Replacing the coil would not address the root cause.
- C. While a bad spark plug can cause a misfire, the high resistance in the wire is a clear and direct cause identified by testing.
Spark Plug Wire Resistance
Spark plug wires contain a resistive core to suppress RFI (Radio Frequency Interference) and should have resistance within a specified range; excessive resistance leads to misfires.
- Typical range: 5,000-15,000 ohms per foot.
- High resistance impedes high voltage flow.
- Causes misfires, especially under load.
Memory trick: Resistance too high, the spark can't fly.