Engine Misfiring?
Find Out What’s Wrong

Spark plugs are the most common cause of engine misfires on petrol engines. As the electrode wears the spark gap increases — requiring more voltage to fire. Eventually the coilpack cannot bridge the gap reliably and the cylinder misfires. Standard copper plugs should be replaced every 20,000-30,000 miles. Iridium plugs last up to 60,000 miles. Always replace the full set.

Each cylinder on modern engines has its own ignition coil that generates the high voltage needed to fire the spark plug. When a coilpack fails the corresponding cylinder stops firing — creating a misfire that is felt as a rough idle, loss of power and hesitation. The engine management light typically illuminates with a specific cylinder misfire code (P0300-P0308). Worn spark plugs are the leading cause of premature coilpack failure.

A partially blocked or leaking fuel injector delivers incorrect fuel quantities to the cylinder — causing misfires. A completely failed injector leaves the cylinder with no fuel at all. Injector faults typically produce a misfire on one specific cylinder that persists regardless of which coil or plug is fitted. Professional flow testing confirms injector performance.

Compression loss from worn piston rings, a blown head gasket or damaged valves prevents the air-fuel mixture from reaching the pressure needed for ignition — causing a persistent misfire on the affected cylinder. A compression test quickly identifies this. Low compression repairs are more significant and costly than ignition system repairs.

A split or disconnected vacuum hose allows unmetered air into the intake — leaning out the mixture and causing misfires, rough idle and hesitation. Vacuum leaks are more common on older vehicles and those with high mileage rubber hoses. Smoke testing is the most reliable method of finding a vacuum leak.