What Effect Does a Bad Coil Have on Engine Performance?

by Chris Stevenson
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Ignition coils come in a variety of configurations, depending upon the make, model and year of the vehicle. Single coils, popular on the older distributor ignition systems, and coil packs that furnish voltage to sets of cylinders or individual cylinders represent the different design types. Coils can also be located inside the spark plug, on top of it or near it. Whatever configuration, coils basically function the same by acting as step-up transformers; they increase 12-volt battery voltage through electromagnetic conduction and a switching device that produces anywhere from 30,000 to 60,000 volts. This high voltage arcs across the electrode in a spark plug, causing ignition. Bad or defective coils can adversely affect engine performance in a number of ways.

Idle Performance

A weak coil that has lost its ability to supply enough voltage to fire all of the spark plugs in a consistent manner can show itself in a rough or stumbling idle. A single coil system can affect all of the spark plugs but will particularly manifest itself with a plug that has a worn electrode and widened gap. The worn plug requires more voltage to jump an arc between the electrodes. Failing individual coils, as in one coil per cylinder, or coils for cylinder sets, will affect a specific single or set of cylinders.

Decreased Fuel Economy

A failing coil, unable to provide a high-voltage ignition spark, will cause one or several cylinders to miss intermediately or completely, providing less power and rpm normally seen with maximum cylinder combustion. The reduced engine power will give fewer miles per gallon. The decreased gas mileage might be accompanied by a noticeable miss or general lack of power through all driving ranges.

Stalling and No-Start

A failing coil can cause a stalling condition during idle or low-speed operation. If the coil or coil pack can not recharge fast enough to delivery the voltage demands to the plug, the engine can cut out when the accelerator pedal is applied. A totally failed coil that produces no spark, or multiple coils that have failed completely, can cause a no-start condition. The starter motor may turn, but the voltage required to fire the spark plugs is missing. With no ignition for combustion, the engine will not start. The no-start condition can be more prevalent when the temperature is cold and the humidity is high.

Catalytic Converter Performance

A weak-enough spark will cause a spark plug misfire, which allows raw fuel to pass through the exhaust manifold and into the catalytic converter. Raw fuel can saturate the interior catalysts of the converter, halting the oxidation and afterburner process. This creates a carbon buildup, overheating conditions and clogged flow. A clogged catalytic converter will create exhaust system back-pressure, disallowing engine "breathing." Clogged catalytic converters will give off a rotten egg smell, cause hard starting, no starting or cause very sluggish engine performance.

High Speed and Under-Load Performance

A coil that produces a weak spark can cause a high-speed miss when the voltage demands are greatest for spark plug ignition. A miss can also occur when the power demand is greatest under a load, like climbing a hill with a trailer. Unburned fuel that ignites in the exhaust system can cause an audible backfire, which can create a jerking or hesitation miss. One of two individual coils that fail can cause a persistent miss in one or multiple cylinders, making the engine run poorly all the time during all driving ranges.

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