What are the Symptoms of a bad Distributor Cap and Rotor?

The distributor is a crucial component of the ignition system in older vehicles. And the distributor cap is the cover to the distributor itself. The cover also plays an essential function in the ignition system. If you own one of these petrol-powered older vehicles, you would want to know bad distributor cap symptoms. This will help you detect issues at the earlier stage and rectify them before they escalate.

Here, I’ll outline the most common signs of a bad distributor cap, the replacement procedures, and the cost of replacing it. By the end, you’ll know how to test for a bad distributor cap and how to replace it at home. So, grab a seat and a cup of coffee.

Symptoms of a bad Distributor Cap

What are the symptoms of a bad distributor cap and rotor?

The most common signs of a bad distributor cap are the car not starting, difficulty starting the engine, rough idling, engine misfiring, and check engine light. Other possible signs include poor acceleration, engine backfiring, and engine knock as a result of detonation.

Now, let’s take an in-depth look at these signs. Depending on how bad the cap is, you can suffer one or more, or even all of these issues.

The car won’t start.

If the distributor cap is not sitting correctly or is faulty, it will not deliver the right amount of spark to burn the air-fuel mixture in the combustion chamber. And if the air-fuel mixture is not burnt, the engine will not start.

However, a car not starting could be one of the symptoms of bad distributor timing, symptoms of a bad distributor shaft, or any other ignition system issues. It can also be an issue in the fuel delivery system.

Difficulty in starting the car

If the distributor cap is cracked, has carbon deposits, or the center terminal is worn, it won’t deliver the right amount of electricity needed to start the vehicle. This will cause the engine to struggle to start.

Rough idling

Another common sign of a bad distributor cap is rough idling. If the distributor cap is carbon-fouled, cracked, or has worn-out pins, it’ll affect the smooth idling of the engine. If you are asking what does a bad distributor cap looks like, you now know the answer. A bad distributor cap looks cracked or carbon-fouled.

The spark plugs on older vehicles need voltage from the distributor, which travels from the distributor cap to the plug wires before reaching the spark plugs. If any pin on the distributor cap are carbon fouled, the plug on that respective cylinder will not have the right voltage to properly burn the fuel in the combustion chamber. This will invariably cause rough idling.

If the engine is running rough, you’ll feel shaking or vibration in the cabin. The vibration will be noticeable when you first start the vehicle or during gear changing.

Engine misfire

Engine misfires on old vehicles are mostly traced to a defective distributor cap. If your older car starts misfiring, the distributor cap is one of the first places you should check. It could have physical damages or cracks, and the terminals could be carbon fouled. A car with a misfiring engine will have a bucking sensation, jerk when accelerating, and stumble as it runs.

Rough acceleration

Spark issues always affect performance, and a lousy distributor is no exception. Whenever there’s a spark issue, the performance level will decrease based on the severity of the problem.

The most noticeable part is the response you get when you depress the gas pedal. Once any cylinder is not firing well, it’ll affect how the engine accelerates when you push the accelerator. So if you are wondering, can a bad distributor cause loss of power? You now know the answer.


In reality, can a bad distributor cap cause backfire? A defective distributor cap can lead to a backfiring engine. In fact, it’s important to note that several other factors can cause an engine to backfire. Therefore, you must diagnose your engine when it backfires before pointing accusing fingers at the distributor cap.

Engine knock

Engine knock is commonly referred to as pre-ignition or engine detonation. Engine knock is that loud pinging sound you hear when accelerating. When you hear this noise, you may think your engine is about to fall out of the hood. However, the sound is not a joke. So, you should stop once you hear this noise to prevent causing more damage.

Check engine light

Once the engine notices improper ignition in the combustion chamber, it’ll log an error code on its memory and trigger the check engine light. The purpose of the light is to tell the driver something is wrong with the engine.

The light doesn’t point to a specific thing. So, in this case, you have to diagnose the vehicle to find out what’s wrong. However, some error codes are generic, meaning several system issues can prompt the car computer to log one error code. Also, it’s important to note that not all older cars will display the check engine warning light.

How do you replace a bad distributor cap and rotor?

Once you confirm the distributor cap is broken or faulty, the best solution is to replace it with a quality one. Here’s the most simplified way of replacing a defective distributor cap and the rotor.

Step 1: Locate the distributor

The distributor is typically located at the tail end of the cylinder head. If you can’t find it on your vehicle, trace the spark plug wires to where they are connected. All the spark plug wires connect to the distributor. You can also use your specific car service manual for more directives.

Step 2: Remove the distributor cap

Most distributor caps have two or three screws holding them together with the distributor. Other caps may have bolts or clamps holding them in place. Get the right screwdriver or wrench and remove these bolts or screws. I don’t like taking off the plug wires before removing the cap. You can remove them if you want, but the most important thing is to label the wires. Misplacing them will cause misfiring, rough idling, and poor acceleration.

Step 3: Remove the rotor

With the cap out of the way, remove the rotor by pulling it out straight. The rotor is a plastic part seen right after taking off the cap. It goes into the distributor shaft. Before pulling the rotor, check if any bolts or screws are holding it in place. Most older vehicles don’t have any screw or bolt, while some newer ones have a bolt or screw that holds them. So, check well before pulling the rotor.

Step 4: Install the new rotor

Get the new rotor and slide it in, following the reverse removal process. Ensure the rotor faces the formal position and it’s aligned properly. Reinstall the screw or bolt holding it in place.

Step 5: Transfer the plug wires

Remember I told you early that I don’t remove the spark plugs. Get the old cap and transfer the wires one at a time to avoid mixing them. When removing the plug wires, hold and pull them from the base. Do not pull the wire itself.

Step 6: Reinstall the cap

Fix the new cap and ensure it faces the same position as the old cap. Most caps will only fit in the right place. But you have to cross-check and ensure you get things right. After that, start your vehicle and ensure it is working smoothly.

Okay, I get it. Not everyone understands text instructions without a visual aid. Watch this YouTube video for a visual clarification on how to replace a distributor cap and rotor.

How much does it cost to change the distributor cap and rotor?

All things considered, a distributor cap and rotor replacement will cost between 50 dollars and 500 dollars. This covers the parts and labor charges. The price differences are because of various car make and models, locations, and the mechanic doing the replacement.

Final Words

As an owner of an older car who wants to keep it running as smoothly as possible, you have to cultivate a regular maintenance habit. You’ll maintain your engine’s optimal power by following schedule maintenance, like changing the distributor cap every 50k miles. With this guide on bad distributor cap symptoms, you can tell when the component fails earlier than expected and replace it accordingly.

If you trust your guts, you can replace your distributor cap by following the guidelines above. It’ll take you 20 to 45 minutes to finish the replacement and hit the road. But if you don’t, contact your car mechanic.



While growing up, I knew I had a thing for car repairs though my parents never wanted me to learn mechanics. I always visit a mechanic garage in my small neighborhood after school. As I grew older, at age 16, I got addicted to anything automotive. My parents had to enroll me in that mechanic garage since giving up was never an option for me. As a dedicated mechanic who got into the industry from an early age, I'm graced with an addiction to diagnosing and rectifying automotive problems with ease.

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