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How to Replace a Bad Distributor Cap

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A bad distributor cap can cause a whole lot of problems for your car. On the plus side, it’s a fairly simple fix—even if you’re not mechanically inclined.

If you need to replace your distributor cap but aren’t sure where to start, then you’ve come to the right place. Keep reading to learn the signs and symptoms of a bad distributor cap and how to replace it.

What Is a Distributor Cap?

mechanic working on engine repairs, close up

In case you’re not car savvy, we’ll start by explaining what a distributor cap is—and why it’s important.

The distributor cap is the cover of your distributor. It’s made of hard plastic and copper, and it’s shaped like a crown with black cables coming out of the top—kind of like Predator. Its job is to protect the distributor’s internal parts and bridge the contact between the rotors and spark plug wires. It also distributes electricity from the ignition coils to the spark plugs via its rotors.

The distributor itself is the rotating shaft that sparks your car’s mechanically timed ignition. Without the distributor cap, it has no contact elements. Without contact elements, your car won’t start.

Signs of a Bad Distributor Cap and Rotor

Your distributor cap and rotor work together to support the high voltage of electricity that runs through your engine. When there’s a distributor cap and rotor issue, it typically affects the voltage and how that voltage makes its way to the spark plugs. Spark plugs play a crucial role in keeping your engine running.

So, when they’re not getting the right amount of spark—i.e., voltage via fuel mixture ignition from the distributor cap and rotor—you’re going to have some problems. Here’s how those problems present themselves:

Your Engine Misfires

There are a few things that can cause your engine to misfire. The misfiring is always related to the fuel-to-air ratio, whether you’re getting a weak spark or poor compression. Your distributor cap is typically the first thing you want to check when your engine starts misfiring.

Your Car Won’t Start

If your distributor cap isn’t on correctly, is cracked, or malfunctioning, your car may not start. This is because it can’t distribute the engine’s spark through the spark plugs to get the cylinders moving.

Of course, there are other reasons your car may not start. So, it’s important to pay attention to what happens when you try to start your car. For example, if there’s a clicking noise when you turn the key, the issue is likely your battery or alternator.

The Check Engine Light Illuminates

A lot of things can trigger your check engine light. A loose or malfunctioning distributor cap is one of them. When your check engine light comes on, it’s time to go to a trustworthy mechanic to scan read the engine’s fault codes.

You’ll Hear Some Unusual Noises

When your distributor cap and rotor begin to malfunction, you’ll start hearing some strange noises. These noises are from your cylinders failing to fire up and get moving. You’ll likely hear a tapping, sputtering, squealing, or even a clicking sound.

Distributor Cap Replacement 101:

Luckily, your distributor cap is easy to replace. All you need is a new one and maybe a screwdriver. The cost to replace a distributor cap varies depending on your vehicle and whether you need new spark plugs or a cap and rotor kit. On average a distributor cap runs between $25 and $125.

Here’s how distributor cap replacement works:

Locate and Unplug the Distributor Cap

First thing’s first: pop the hood and locate the distributor cap. It’s the gray or black plastic component neat the center of the engine. It’ll have black cables coming out of the top which are your spark plug wires.

If you have any doubts, check with your owner’s manual or just look for the component that looks like Predator’s head.

Unplug and Unhook the Cap

Before unhooking your distributor cap, you have to unplug your spark plug wires. Your sparkplug wires are in place by firing order, so it’s important to label them so you know where to position them on the new distributor cap. Be sure to check them and replace them if necessary.

Now you can unhook the cap. Depending on the make and model of your car, the distributor will either be held in place by clips or screws. If there aren’t any hooks or screws, all you have to do is push down on the cap and rotate it counterclockwise—like a child-safe pill bottle.

Remove the Rotor

The rotor looks like a fan blade and is located directly beneath the distributor cap. Check to see if it’s held in place by screws. If not, you can go ahead and slide it right out.

Once you put the new rotor in place, give it a spin with your finger to ensure that it’s rotating smoothly.

Note: this step is only necessary if you need to replace the rotor. If your rotor is fine, you can skip this step!)

Reattach Your Spark Plugs

Before replacing the new distributor cap, you’ll want to reattach your spark plugs. Hopefully, you remembered to label them so you can fit them to the corresponding spoke. Otherwise, you can end up with some serious electrical damage.

Replace the Cap

Now you can place the new distributor cap to its rightful compartment, screwing or clipping it back in. Make sure it’s in the exact position of your old cap and that the spark plugs are set neatly. If they’re coiled or bunched up, it could cause an electrical shortage.

Test it Out

Once your new distributor cap is in place, you need to test it out and make sure you put it in right. Start your car and see if there’s any misfiring or strange noises. If your car starts up and runs smoothly, you did the job right.

Maintain Your Car

Maintenance is an important part of owning a car, and a bad distributor cap isn’t something you want to leave unattended. If you can’t do the work yourself, be sure to find a mechanic that you can trust.

For more car care tips and how-to’s, check out the rest of our articles.

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