A bad coil can cause the engine to idle and run rough. It might feel weak with a tendency to stall, and may start to backfire. When the coil fails completely it will cause a non-running condition, or in the case of a coil-over-plug application, a dead miss in that particular cylinder. Bucking may accompany backfiring when the car's in gear, similar to the symptoms produced by bad timing. Some intermittent problems are heat related, and aren't apparent until the engine and coil are at operating temperature.
Testing the Coil
Remove the coil wires. Check continuity using an ohmmeter or a multimeter set to measure resistance. You should have continuity between the two primary-side terminals on the coil. You should not have continuity between either primary-side terminal and the secondary terminal or between either primary terminal and the case. If any of these checks fail, replace the ignition coil.
Loose wires at the coil, distributor or spark plugs can cause similar symptoms, so check the connections for damage, cleanliness and tightness. Incorrectly gapped or damaged spark plugs can also mimic a bad distributor. Be sure to check the plugs and gap or replace as needed. Low power due to a weak battery or failing alternator can also cause similar symptoms. Ideally, you should make sure that your battery and charging system is up to specs before beginning to troubleshoot any electrical problems.
- Photo Credit Zoran Zeremski/iStock/Getty Images
Signs of Bad Coil Springs & Shocks
Bad vehicle coil springs and shocks negatively impact a vehicle's stability and can cause a host of driving-related symptoms. Designed to stabilize...
How to Diagnose a Faulty Ignition Coil
If your engine is backfiring, stalling or failing to start and you have noticed a significant decrease in fuel mileage, these could...
How to Test a Faulty Ignition Coil Wire
Many electrical issues can be traced to loose or damaged wiring. Often however, the problem cannot be located through visual inspection, as...
Signs of a Weak Coil Pack
A coil pack is a pack of ignition coil generally located in the top area under the hood, above the engine. Part...
Signs of a Bad Pickup Coil
Pickup coils work with a vehicle's ignition system to regulate spark and communicate to fuel injectors. When one fails, it can have...
Signs of a Bad Coil Pack
A coil pack is an automobile component that works to help the fuel ignite in the cylinders by giving the spark plugs...
Ignition Coil Types
An ignition coil is actually two coils of wire wrapped around an iron core. The primary coil is made of heavy wire...