What effect does a bad coil have on engine performance?

A bad coil is bad news. Your engine’s performance will definitely be affected by a bad coil. While you might be able to ignore some engine problems, albeit temporarily, the rough running that can be caused by a bad coil will be a constant and physical reminder that you need to get something done about it.

Ignition coil system

An ignition coil is really two coils working together. One creates the magnetic field and the other creates the voltage. This voltage creates the spark that jumps across the gap on the end of the spark plug. Sparks have to be timed precisely, in order to be effective at igniting the compressed air/fuel mixture. If any part of the system becomes compromised, sparks will be intermittent and there will be a knock-on effect.


According to automotive technologist Jack Erjavec, a bad coil can cause misfiring. An engine misfire indicates an interruption to the air/fuel ratio within the combustion chamber. This can cause one or more cylinders to misfire. A four-cylinder engine, moving a car along at a speed of 70 kilometres per hour, fires over 5,000 times per minute. Irregular sparking will lead to a rough ride, instead of the normal smooth running.

Running rough

When an engine is running rough, it may stutter, be very irregular and even stall, according to former automotive service technician Anthony E. Schwaller. Running rough makes the ride experience very unpredictable. You might find yourself trying to anticipate the engine’s behaviour rather than concentrating on the road and other road users. In this case, a bad coil doesn’t only have an effect on engine performance but on driver performance too.

Failure to start

A bad coil can cause ignition problems to the extent that it is impossible to start the car in the normal way, by turning the key in the ignition. You may be able to perform a jump start, using jump leads connected to the battery of another vehicle. However, for safety’s sake, the AA warns that some vehicles are not suitable for jump starting and recommends that you follow the specific procedure detailed by your car manufacturer.

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About the Author

Frank Luger had his first educational resources published in the early 1990s. He worked on a major reading system for Cambridge University Press, became an information-technology adviser and authored interactive whiteboard resources for "The Guardian." Luger studied English literature and holds a Bachelor of Education honors degree from Leeds University.