Every time a petrol engine fires, a spark plug ignites the fuel and air in the cylinder, and an ignition coil supplies the high voltage that makes the spark. When either part weakens, the result is a misfire: lost power, rough running and wasted fuel.
Spark plug intervals
Plug life depends heavily on the type. Older copper plugs needed frequent replacement; most modern engines use iridium or platinum plugs that are specified for much longer intervals. Your service schedule lists the exact interval for your engine. Turbocharged engines often call for earlier replacement than naturally aspirated ones.
Symptoms of worn plugs or a failing coil
- Rough or shaky idle, especially with the air conditioning on.
- Hesitation or jerking when accelerating.
- A flashing check-engine light, which usually signals an active misfire. Reduce speed and get it checked; a sustained misfire can damage the catalytic converter.
- Higher fuel consumption than usual.
- Hard starting.
Why coils fail
Ignition coils sit on top of the engine, where under-bonnet temperatures in a Qatari summer are severe. Heat cycling breaks down the coil's insulation over time. Worn plugs also make coils work harder because a larger gap needs a higher voltage. That is why many technicians inspect the coils whenever plugs are changed.
How the fault is diagnosed
A diagnostic scan will usually store a code identifying which cylinder is misfiring. A common test is to swap the coil from the misfiring cylinder with a neighbouring one: if the misfire moves, the coil is at fault; if it stays, the plug, injector or compression need checking.
Related cooling-system work
Engines that run hot age ignition parts faster. If you are seeing temperature warnings as well as misfires, read why engines overheat in Qatar and check the condition of your coolant hoses.