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.
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.
