Spark Plugs, Glow Plugs and Injectors: Why Engines Misfire or Run Rough

What a misfire actually is

An engine needs air, fuel and a way to ignite or compress that mixture at exactly the right moment in every cylinder. A misfire happens when one cylinder does not burn its charge properly. You feel it as a stumble, vibration or loss of power.

A flashing engine management light during a misfire is a warning to ease off. Unburnt fuel can damage the catalytic converter, so avoid hard driving and have it checked promptly.

Petrol engines: plugs, coils and fuel

On a petrol engine, the usual suspects are worn spark plugs, failing ignition coils or leads, and air or fuel supply problems. Plugs wear with use, and when the gap grows or the electrode erodes, the spark weakens. A coil that breaks down under load can cause a misfire that comes and goes.

Other causes include vacuum leaks, dirty throttle components, fuel injectors that are not spraying properly and sensors feeding wrong information to the engine computer.

• Hesitation or juddering when accelerating
• Rough idle that smooths out as the engine warms
• Poorer fuel economy than usual
• Flashing or steady engine management light

Diesel engines: glow plugs, injectors and more

Diesels do not use spark plugs. They rely on compression, with glow plugs helping the engine to start when cold. Failing glow plugs often show up as hard starting in cold or damp weather, white smoke on start-up and rough running until it warms up.

Once running, injectors are a frequent cause of trouble. If an injector leaks, clogs or sprays unevenly, you may get rough running, black or grey smoke, a smell of fuel, knocking noises or a drop in economy. Turbo faults, blocked filters and exhaust-related problems can feel similar, which is why a proper check matters.

Hard starting, smoke and fuel economy

These are all clues. Hard starting may point to plugs, glow plugs, fuel delivery or a weak battery. Blue smoke suggests oil is burning, white smoke can mean unburnt fuel or coolant, and black smoke usually means too much fuel or too little air.

Rising fuel consumption with no change in how you drive is also worth investigating, since a misfiring engine wastes fuel. Do not ignore sweet-smelling white smoke or a rising temperature gauge; those can indicate cooling-system or head gasket trouble, and driving on can cause serious damage.

Why diagnostics come first

Swapping parts until the problem disappears is slow and often costly. Engine diagnostics begin with reading fault codes from the engine computer, but a code points to a symptom, not always a cause. A misfire code for cylinder three might mean a plug, a coil, an injector or a compression issue.

A technician then checks the likely causes in order, using live data, tests and a physical inspection. This is how we avoid replacing good parts and how we give you a clear explanation of what is wrong.

Repairs that may follow

Depending on the findings, the fix might be as simple as a set of plugs and an engine service, or it might involve injectors and fuel-system repairs, cooling system work or a look at the timing belt or chain. If compression is the problem, cylinder-head repairs or head-gasket replacement may need discussion. Anything bigger would be talked through with you first.

At Banks of the Tyne in Hebburn we handle engine diagnostics, servicing and repairs, including injectors and fuel-system repairs. Regular servicing is a good defence against most rough-running problems, because plugs, filters and fluids do wear.

Frequently asked questions