Symptoms of a Bad Distributor Cap
The distributor cap doesn't sound like such an important part, but it is integral to the performance of your car's engine. If the distributor cap is faulty then the ignition sequence inside your engine will be out of order. Here are a few tell-tale signs that your distributor cap needs to be replaced.
If you're idling at a stop sign or red light and your car seems to be shaking more than normal, that could be a sign that the rotor inside the distributor cap is not spinning properly, and therefore the timing to start the firing process correctly is off.
Starting in the Cold
If you live in a cold weather area, unfortunately your distributor cap will be more likely to break than if you live in warm weather. This is because the cap is covered in plastic and when it goes from cold to hot instantaneously, it can crack and break. If you notice that your car has trouble starting on a regular basis, check the distributor cap for cracks. If you see one, you need to have the cap replaced. Even if there aren't cracks in the cap, there may be corrosive materials on the inside of the cap, which would also render it ineffective.
Stalling and Backfiring
If your car starts okay but acts up once you're on the road, stalling and/or backfiring, this could be because your distributor cap has a fault. Since the cap contains the rotor that spins to help the voltage travel from the coil to the spark plugs, if it is cracked, clogged with oil, or anything else that prevents it from spinning or slows the spinning down, it will only work part of the time.
If you notice that your revolutions per minute are all over the place, it could be because the connection between the distributor cap and the distributor wires has gone bad. Again, check the cap for cracks or evidence of burning, singeing, or corrosion.
If you turn your engine on and everything seems to be working, but you can hear a high-pitched squealing noise, turn your car off and remove the distributor cap. There may not be any visible signs of wear and tear immediately, but inspect the underside where the coil sits. You may be surprised at just how much muck has accumulated under the cap. Replace the cap with a new one and that should correct the problem. If you still hear that sound, though, you may need a whole new distributor.
Trouble Turning Over
Sometimes people notice that along with their car stalling, their car seems to turn over some times and not others. You will probably realize that it will turn over when the engine is cold, but not when it is hot. This can definitely mean that there is something wrong with the distributor cap, so have it checked out. If the cap seems okay, it might be another component in the distributor that is throwing off the ignition sequence.