A car usually runs smoothly, unfortunately, sometimes it does not. When a car runs rough, a variety of problems may be caused by a cracked ignition coil. Diagnosing a bad coil may appear daunting, but narrowing down the problem is easy, if you know what to look for. By understanding the effects of a bad ignition coil, you will be able to determine if the coil is bad, or if the problem is elsewhere.
A coil going bad causes hard starting. What happens is a cracked coil allows overnight moisture to seep in. Moisture reduces the internal transformer effect of converting low voltage to high voltage The result is insufficient electricity to the spark plugs, producing a weak spark. If fuel ignition is incomplete due to a weak spark, hard starting is the result. The problem is especially pronounced the first time the car is started.
The car is running smoothly, but all of a sudden it stalls. When the coil is cold, it's producing enough electricity. When a bad coil heats up, its internal resistance goes up, and it starts to produce less electricity. The end result is a weak spark, causing hot stalling.
A car is running smoothly, but as soon as it rains, it starts to run rough or stall completely. If the coil is cracked, it allows moisture to seep in, reducing the transformer effect. The end result is stalling or rough running during damp weather.
The car is running smoothly, but occasionally it "coughs" or hesitates for a moment. This is called misfiring. A cracked coil may have intermittent operation, causing reduced electricity to the plugs for just a second. Because the coil is 90 per cent good, this problem is hard to diagnose unless the car is being driven. Automotive Service Excellence, or ASE, mechanics state that misfiring is a common symptom of a coil going bad.