There's some sort of a relay happening in your vehicle's ignition system, and it all starts with the distributor. Through the distributor cap and rotor, it sends high voltage running from the coil to the right cylinder. The rotor is linked to the coil. As it spins within the cap, it also passes a series of contacts that lead to the engine cylinders. High-voltage pulse from the coil is produced every time the rotor tip goes past each contact. The pulse is arced across the gap in-between the contact and rotor and then goes all the way to the wire and the spark plug to reach the correct cylinder. When the cap eventually breaks or wears out because of arcing, the ignition sequence will be messed up.
Signs of a faulty distributor cap
Vibrations: If the car vibrates excessively when it idles, this could mean that the rotor inside the cap isn't turning properly. Timing for ignition may be off.
Trouble starting the car: If the engine doesn't crank up easily, this ignition problem may be caused by a cracked cap. Aside from cracks, this may also be brought about by corroded parts inside.
Stalls and backfires: If the car suddenly stalls or backfires even when it was started seemingly well, the cap might be at the helm of the problem. Cracks and oil clogs may be keeping the rotor from turning or causing it to spin slowly.
Faulty RPMs: If the RPMs seem messed up or out of order, the connection between the distributor wires and the distributor cap might be broken or no longer in good condition. There could be cracks, corrosion, or burns in the cap.
Squeals: If you crank up the engine and everything seems fine except for a high-pitched squeal, the cap may be hiding a dirty secret: there may not be tears or signs of damage, but underneath the cap, you may find that there's a lot of grime. Cleaning or replacing the cap usually solves the problem; if not, distributor replacement is a must.
Irrational turn overs: If the car not only stalls but also turns over only at certain times (when the engine is cold and not when this is hot), you may be dealing with a faulty cap. This may also be caused by a busted ignition component somewhere.
Checking the cap for signs of damage
- Remove the distributor cap by disconnecting the clips or undoing the screws. (Note: There are different ways to remove the cap based on the variations or designs used in different vehicles.)
- Inspect the cap and look for carbon buildup, dirt, burns, cracks, or any visible damage.
- Clean the dirty cap with a small, round brush and put this back properly. If damaged or if dirt buildup is stuck, replace the cap.
- Reinstall the cap on the distributor. Make sure that the wires are connected to the right spots and the cap is secured properly. Also check for slack.
Tips and warnings
- Keep the ignition wires, which run all the way to the spark plugs, connected when you remove the cap so you can easily bring back the connection when you reinstall the cap.
- Do not pull on the ignition wires too hard.
- Also check for a broken terminal.
- Do not use solvent or compressed air when cleaning out the vent of the distributor cap.