Why Is My Engine Misfiring?

Picture the car juddering at traffic lights, hesitating as you pull away, or running as if it is on fewer cylinders than it should. That is a misfire. One or more cylinders have failed to burn their fuel and air mixture cleanly, so the engine loses smoothness and power.

The usual causes are ignition parts, fuel delivery or air supply, with occasional mechanical faults in the engine itself. Because the fix depends entirely on the cause, it is worth understanding the options and knowing which warning signs mean the car should stop.

What does an engine misfire feel like?

Symptoms vary with the severity. A mild misfire may only show as a slightly rough idle or a flat spot when accelerating. A stronger one makes the whole car shake, the revs flutter, and the engine management light come on. You might also notice a smell of unburnt fuel, popping from the exhaust, or worse fuel economy.

Sometimes it only happens at certain times, such as on a cold start, in damp weather, or when the engine is under load. Noticing the pattern and telling the garage about it is genuinely helpful, because intermittent faults are harder to find than constant ones.

What are the most common misfire causes?

On petrol engines, worn spark plugs and failing ignition coils are the most frequent culprits, followed by damaged leads on older cars. Other causes include a vacuum leak letting unmetered air in, a dirty or faulty fuel injector, a sensor reporting wrong information to the engine computer, or low fuel pressure.

Diesels do not use spark plugs, so the usual suspects are injectors, glow plugs on cold starts, air leaks or fuel supply problems. In rarer cases a misfire points to low compression, caused by a worn valve, damaged piston ring or a failing head gasket, which needs proper testing to confirm.

• Worn spark plugs or weak ignition coils
• Faulty or blocked fuel injectors
• Air leaks in the intake system
• Sensor faults affecting fuel mixture
• Low compression from internal engine wear

Why is my engine light flashing?

A steady engine light means the system has logged a fault. A flashing one usually means a misfire serious enough to risk damaging the catalytic converter, because unburnt fuel reaches the exhaust and overheats it. Replacing a damaged catalytic converter can be a major expense, so a flashing light is a signal to act promptly.

If it flashes while you are driving, reduce speed, avoid hard acceleration and find a safe place to stop. Check your handbook for what your warning lights mean, as symbols vary between makes.

How does a garage find and fix a misfire?

The starting point is reading stored fault codes, which often identify the cylinder concerned. The technician then works through the likely causes: checking the plugs and coils, testing injector operation, looking for air leaks and, if the simple items are fine, testing cylinder compression.

Swapping a suspect coil to another cylinder is a quick way of confirming whether the fault follows the part. Cost depends on the cause, the number of parts involved and how much has to be removed to reach them. At Automaster, a garage at Longcombe Cross near Totnes, we diagnose first so you only pay for work that is actually needed, and we will explain what we find before going ahead.

Can I drive with an engine misfire?

A light, occasional misfire might allow a short careful drive to a garage, but it is not something to live with. Continued misfiring can wash oil from the cylinder walls, overheat the catalytic converter and make the underlying fault worse.

Do not keep driving if the engine light is flashing, the car is shaking badly, power is badly reduced, or you can hear knocking or smell burning. A misfire at a junction or on a fast road, where the car might lose power, is a safety risk. Stop in a safe place and seek advice.