Ignition and mixture faults — likely causes of engine misfiring Which of the following conditions is most likely to cause engine misfiring during operation?

Introduction / Context:
Engine misfire arises when the charge in a cylinder fails to ignite or burns too weakly. Identifying common causes enhances diagnostic efficiency in service bays.

Given Data / Assumptions:

• Spark-ignition petrol engine.
• Normal operating temperature and fuel supply unless otherwise noted.
• Standard spark plugs with specified gaps.

Concept / Approach:
Sufficient spark energy and proper mixture are required for reliable ignition. Excessive spark plug gap raises the voltage required to jump the gap; under high load or at idle with reduced system voltage, the coil may not provide enough voltage margin, resulting in intermittent or consistent misfire.

Step-by-Step Solution:
Recognise that spark initiation depends on coil voltage, plug condition, and gap.If the gap is too wide, breakdown voltage rises beyond available ignition voltage.Result: misfire events, especially under high cylinder pressure or weak ignition conditions.

Verification / Alternative check:
Scope patterns and misfire counters often show increased misfire rates when plug gaps exceed specification; correcting the gap or replacing the plug typically resolves the issue.

Why Other Options Are Wrong:

• Spark plug gap too small: can cause weak spark and fouling but is less likely to cause outright misfire than an excessively wide gap.
• Vapour lock: occurs at high fuel temperature in certain systems; not a typical everyday misfire cause compared to ignition issues.
• Incorrect fuel–air mixture: can cause rough running, but modern closed-loop systems compensate; the question asks for a likely direct cause—an overly wide gap is a classic, specific trigger.

Common Pitfalls:
Replacing coils unnecessarily; always check plug gap and condition first, then coils, wires, and injectors.

Final Answer:
spark plug gap too wide