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There are a number of potential causes for this problem. These include:
- Engine mechanical problem (low compression in one cylinder)
- Plugged or defective fuel injector
- Ignition system fault
One of the most common reasons is an ignition system that is leaking high voltage.
It may only misbehave on damp days. There are a few checks that one can do.
1. Open the hood and listen while the engine is running. Listen for a sharp cracking/arcing noise that the sparks make as they jump from the ignition wires or cap. If you do this at dusk, you may see the sparks jump.
2. On a relatively dry day, when the car has been running ok (assuming that it intermittently runs poorly) lightly mist the ignition wires with water from a hand sprayer. If the engine idle speed suddenly changes, and you start to hear the sparks jumping, you have confirmed the culprit.
3. Physically inspect the parts for black burn marks or pitting. A black/grey rough burned appearance on the tip of some rotors is normal, it is a coating that is applied for RF interference suppression reasons.
The most important thing to do when replacing ignition system parts is to only use high quality parts. There are many substandard suppliers out there. In some cases, a set of 10 year old OEM wires is better than a set of cheapo new wires. Generally, you get what you pay for.
Having said that, don't expect 10 HP more than stock because you spent $250 on a set of purple 10mm ignition wires.
Don't forget that a cracked, oil fouled or otherwise defective spark plug can easily be the source of an ignition system problem too.
A faulty ignition system will damage the catalytic converter if it is not fixed.
Each time a misfire occurs (if due to an ignition system fault) air and fuel are dumped straight in to the exhaust. This can cause a catalytic converter to be damaged, and in severe cases can cause a fire when the catalytic converter glows red.
Newer vehicles may no longer have ignition wires, you might find ignition coils mounted directly on the spark plugs.
This does not cover every conceivable ignition system problem. There are a multitude of other ignition system problems that can be difficult to diagnose, and difficult to include in a basic guide.