Bad Distributor Cap: Symptoms and How To Fix

When you hear “distributor cap”, the “cap” part might make you think that this unassuming component is not of major importance to the inner workings of your car. However, the truth is quite the opposite. The distributor cap serves as an integral part of the ignition system by regulating the engine’s firing order. Being just a small component as it is, a bad distributor cap can wreak havoc to the ignition and thus affect your engine efficiency.

Distributor cap problems, including clogged, cracked, corroded or faulty cap, can cause performance issues ranging from rough idling to stalling, or worse, your car might just refuse to start at all. In addition, being located in a harsh environment, the distributor cap and rotor are prone to wear and tear over time, thus it’s a crucial DIY car maintenance skills to detect the symptoms of a bad distributor cap early on before it fails completely. Read on to learn the important role of a distributor cap, the symptoms of a bad distributor cap, how to troubleshoot and fix in each case, and the cost for replacing a bad distributor cap. 

What Does The Distributor Cap Do?

Today, with the advancement of technology, full computerized electronic ignition systems have become the norm. However, prior to the mid-2000s, nearly every vehicle on the road didn’t come with such a computerized system and would have a distributor for facilitating ignition in the engine. 

In all cars, SUVs and trucks in this era, the distributor is a vital part of the engine management system. To put it short, the distributor’s role was routing electrical current from the ignition coil directly to the spark plugs in each cylinder in the correct firing order and at the right time. 

The distributor consists of a rotor rotating inside a distributor cap. As you turn the ignition key, the engine runs, sending high voltage  through the ignition coils to the distributor rotor, specifically sending the electricity into an electrode of the distributor rotor. The rotor is connected to the ignition coil thanks to a spring-loaded brush in the distributor cap. 

This electrical current turns the rotor, allowing it to transfer electrical power to separate electrodes located inside the distributor cap. From there, the distributor cap passes the electricity to the spark plugs through a series of spark plug wires in a specific way, and ultimately to different cylinders in the correct firing order at precise moments to ignite the air and fuel mixture inside each cylinder. Both the “correct firing order” and the “at precise moments” are crucial to achieving optimum engine efficiency. 

To facilitate the passing of voltage directly to the spark plugs, the second end of the rotor passes by the metal contacts in the distributor cap when there is high electric current.

This entire process happens each time a cylinder needs a spark to ignite the air and fuel mixture to generate a combustion.

In summary, the role of the distributor cap, together with the rotor, is to facilitate the distribution of incredibly high voltage to the appropriate spark plugs at the right time, while keeping the distributor’s internal parts separate from the engine, thus keeping them clean. It holds the contacts and posts between the distributor rotor and the spark plugs wires in each cylinder. 

If there’s any malfunctioning in the system that leads to a bad distributor cap, the spark plugs won’t receive the spark from the distributor when they need it to ignite the fuel mixture. Thus, your engine won’t be able to generate power efficiently, and the result is starting problems and a host of other issues.

How a Distributor Cap Can Go Bad

Natural wear and tear

Every time you turn on your vehicle, high voltage flows through the distributor rotor and cap. Because of this, they do wear out over time and need to be maintained and replaced on a regular basis to ensure optimal engine performance. 

Experts recommend that during routine service, when the distributor rotor and cap are replaced, the entire ignition should be inspected to ensure everything else is in good working condition, including the ignition timing.

There are several wear and tear that can cause the distributor cap to fail, which would then require replacement, including:

• Cracks in the housing: Due to exposure to freezing temperature in the winter and then sudden heat when the engine’s running, causing the housing to crack. Can cause a car’s engine to miss, which may lead to rough idling as well as hesitation during acceleration. 
• Burned distributor cap terminals: Due to over exposure to extremely high voltage constantly. Can cause the engine to skip a cylinder from the firing order.
• Corroded terminals: Excessive carbon build-ups and any oxidation due to moisture condensation on the distributor cap may cause contact to stop or frail.
• Caked up or greasy terminals: Over time the terminals can become dirty with dirt and grime buildup, and might cause the engine to skip a cylinder from the firing order and create strange engine noises. 

• Exposure to liquids: Another type of one-off event that might make your distributor cap fail is if you drive through a deep puddle. The water can get into the distributor cap and short out the electrical current, leading to failure. Depending on the seriousness of the damage, you might need to replace the cap, or simply dry it out to restore it to working order. 
• Broken spark plug wire tower.

Bad Distributor Cap Symptoms 

As above, a worn, corroded or wet distributor cap is a bad distributor cap, and these incidents are common. You need to become familiar with the following symptoms of a bad distributor to repair the damage in time, or else your engine will suffer:

Starting and accelerating problems

A functional distributor cap is essential to the generation of engine combustion. Therefore, a bad distributor cap, no matter what type of damage, would most likely cause difficult starting problems. 

This is especially so in cold weather, because extremely low temperatures may cause the cap to freeze. And when the engine runs, the cap which is covered in plastic, will be subjected to a sudden burst of heat, which would cause it to crack. This is very common with vehicles being parked outdoors or in an unheated garage with low temperatures. 

A cracked distributor cap can cause a car’s engine to miss, which may lead to rough idling as well as hesitation during acceleration. Or worse, if the cap were to fail completely, well, it will not be able to facilitate the distribution of electricity to the spark plugs, thus there will be no combustion to get the vehicle started.

Rough Idling

In addition to cracked distributor cap housing, another reason for rough idling is burnt distributor cap terminals. The terminals are the small electrodes on the bottom of the distributor cap. Over time, with constant exposure to very high voltage, these terminals will naturally become covered in carbon buildup or worse, become burnt. When this happens, the engine will skip a cylinder from the firing order, causing noticeably rough idling. 

Stalling and Backfire

Another common symptom of a bad distributor cap is stalling or backfiring from the exhaust while coasting with the car. 

This is a sign that you have a broken distributor cap. The distributor rotor must turn properly to transfer the electricity to the spark plugs. If it does not, optimum combustions won’t happen, the engine loses power and stalls out. The longer this problem is neglected, the worse it will get.

Another reason is a failed rotor that is not sending enough voltage to the spark plugs.

Shaking

If you have a bad distributor cap, you will most probably experience some degree of vibrating to a more pronounced shaking that can be felt throughout the vehicle. This is because the distributor rotor is not spinning properly and the cap is not distributing the voltage properly in the right power stroke of the cylinder, thus messing up the timing and firing process.

If you often notice this sort of shaking during idle, or when the car is stopping at a red light, then it’s highly likely that the culprit is a bad distributor cap.

Unusual engine noises

Another symptom of a bad distributor cap or failing rotor is loud, strange noises from the engine, specifically because the cylinders are trying to fire but fail, including a tapping, clicking, or sputtering sound.

Another telltale sign of a bad distributor cap is a high-pitched squealing noise upon starting the engine. The most likely reason for this squealing noise is the distributor cap being caked up with dirt and grease or other pollutants, and as the air circulates through the engine, the clogged distributor cap will create a squealing noise. 

If you find a dirty, clogged distributor cap upon inspection, just clean it with WD40, which will be detailed further below. If the squaling continues, you might need to have the whole distributor replaced. 

Difficulty Turning Over

Sometimes you will notice that along with engine stalling, the car does not turn over. This usually happens when the engine is cold and not when it is hot. 

When your engine starts, your battery sends electricity to your spark plugs, and the resulting spark will power the initial ignition. This then drives your crankshaft. The “turning over’, which refers to this movement of the crankshaft after the initial ignition, is what allows your engine to start moving. 

Your engine starting but failing to turn over points to issues with a bad distributor cap. In case the cap is intact, the problem would lie with the crankshaft or timing belt.

Check Engine Light comes on

In case there is an improper combustion process in the cylinders caused by a bad distributor cap and rotor, this will be detected by the engine control unit (ECU) and thus the Check Engine warning light will definitely illuminate on your dashboard.

In most cases, the Check Engine Light will come on when the distributor cap is cracked or corroded, or if it is distributing voltage incorrectly or inconsistently, thus messing up the firing order and timing.

Of course, your Check Engine light can mean a number of different things, but if you see this light on along with other symptoms of a bad distributor cap above, you will need to inspect the distributor.

With a vehicle after the 1996 model year, you would only need to read the code from your OBDII on-board self-diagnostic system to find out what’s wrong. However, the problem is that if your vehicle has a distributor, it was most likely made before the area of this electronic diagnostic system, so it’s not a simple matter of scanning for codes using your code reader.

Bad Distributor Cap: How To Fix

Cracked distributor cap

To prevent cracking, which would lead to rough idling and hesitation during acceleration, the only thing you can do is to inspect the distributor cap regularly for telltale signs of cracking. Whenever you can, try to park your vehicle in a protected environment in the winter, or at least try to protect it as best as you can from freezing conditions.

Dirty or corroded distributor cap

Distributor cap terminals can also become corroded due to water vapor in the crankcase reaching the distributor shaft (the shaft sitting on top of the distributor and driven by the camshaft gear), and then concentrated inside the distributor cap. When the engine cools down, the water vapor condenses and reacts with the metal contacts inside the cap to form corrosion. 

Another source of excess moisture is when the alternator overcharges the battery, producing battery acid that may build up inside the distributor cap. Another reason for corroded distributor cap terminals is that the wires may be porous and therefore attract moisture into the distributor cap. 

In addition, the distributor cap is prone to collecting dirt and grime or other pollutants that can make its way into the system. 

Fortunately, upon regular inspection, if you detect a corroded or dirty distributor cap, you just need to clean it with a multi-purpose cleaner/lubricant like the WD40,  which can remove corrosion, dirt and grime from metal car parts as well as protecting the metal from corrosion. Furthermore, if you notice a loose distributor cap, you must replace the distributor cap gasket at once to prevent moisture condensation. 

Below are to steps to clean distributor points to ensure perfect contact:

• Locate the distributor: Your distributor is a plastic grey component near the centre of the engine under the hood. The distributor cap looks like a crown, and the spokes on its top are connected to thick black cables. These cables are spark plug wires.
• Label and remove spark plug cables: Label the spark plugs cables so that you can reassemble them in the correct order once you’re done with cleaning or replacing the distributor cap. You can then pull the spark plug cables gently out from the distributor cap.
• Unscrew the distributor cap: Unscrew two screws on the side of the cap and try to pull it up gently to remove it.
• Clean the terminals with a contact cleaner: Using a soft bristled detailing brush that is abrasive enough on the metal contact terminals to remove any carbon build-ups and any oxidation, which may cause contact to stop or frail. Spray some contact cleaner and lubricant like the WD40 on the internal contacts.
• Dry out: Use a clean rug to rub the cap dry completely. If you have an air compressor, use it to speed up the drying process.
• Re-assemble: Now just reassembled everything, a reverse process of everything you did. Put the distributor cap into place. It is best to use a new distributor cap seal and for extra protection, apply a silicone seal around the base of the cap. Connect the spark plug wire in the correct order. You can then start the car to see if it is running smoothly without any strange noise again.

Distributor Cap Replacement

Cost

The distributor cap and rotor replacement cost can be anywhere from $70 to $130, including both parts and labor costs. In most cases, you can expect to pay between $30 and $60 for parts and another $40 to $70 on top for labor. 

Because distributors and distributor caps are prone to wear and tear, thus can malfunction or fail if not replaced regularly, many newer vehicles now make use of a distributorless system known as an direct ignition system.

READ MORE

• Distributorless Ignition System (DIS) 
• Top 5 Reasons Behind No Spark From Distributor

How often should I replace the distributor cap?

Most vehicles are recommended for routine servicing every 25,000 miles, during which the distributor cap and rotor are often inspected for signs of wear and tear and will be replaced if needed. 

Regardless of whether or not they are damaged, the shop will often replace the distributor cap and rotor at the same time during checkups that happen every 50,000 miles.  If you don’t drive your vehicle that much, it’s also a good idea to replace these parts together at least every three years or so.

The steps to replace the distributor cap

The design of the distributor caps and rotors might differ depending on your vehicle’s make and model, engine size and other factors. Fortunately, the process for replacing them on most engines is quite straightforward and rather similar, so if you fancy yourself a DIY type of man, you can do this in your own garage. Furthermore, in most vehicles with a distributor cap, the cap should be very easy to access. 

There are quite a few steps though, so feel free to consult online video tutorials to ease your work.

Most service manuals suggest that replacing a bad distributor cap should take you about one hour only. And the most time consuming step will simply be removing ancillary components so that you can reach the distributor. Below is a step-by-step guide on how to replace a bad distributor cap:

Purchase a tool kit: The first step is to get a replacement kit for the distributor cap and rotor. Most manufacturers of OEM parts sell the cap and rotor together in a kit, since usually they are replaced at the same time. You might find certain kits that come with replacement hardware, gaskets as well as new spark plug wires, or kits made for specific vehicle models, 

What you will need:

• A replacement distributor cap and rotor kit
• A few clean rags
• A contact cleaner and lubricant, like WD-40
• Socket set and ratchet
• Screwdrivers flat and Phillips head

Remove battery cables: An important safety warning for anytime you are working with electrical components: make sure you consult your manufacturer’s service manual first, and you must always remove the battery cables from the terminals. Remove the positive and negative terminals before proceeding to remove any components on the vehicle. It is always recommended to fully before attempting to complete this job. 

Remove any ancillary component to gain access to the distributor: In most cases you’ll have to remove a few components to have free access to the distributor cap and rotor, but most likely only the engine cover or air filter housing. Consult your service manual to safely remove these ancillary components.

Locate the distributor: You won’t have to raise the vehicle on a hydraulic lift or on jack stands. Just pop the hood open and you’ll find the distributor typically located on the top of the engine or on the side. 

Mark the spark plug wires: Before and during the removal process, take some time to take note of or mark down the location of each component, including the distributor cap, spark plug wires and the rotor on the bottom of the distributor, so that it will be easier later on to put everything back in the correct place and order. Failure to correctly connect the spark plug wires and install the new distributor cap can result in ignition problems, such as engine misfire.

First, in this step, we’ll start with the spark plug wires. A good tip is to use a marker or tape, mark the location of each spark plug wire in a clockwise order, starting at the 12 o’clock position on the distributor cap. This will ensure that when you reinstall the spark plug wires in the original order.

Remove spark plug wires: After you’ve marked the spark plug wires, gently pull them out from the distributor cap.

Remove distributor cap: Now that the plug wires have been removed, you can remove the distributor cap. It is usually secured by two to three bolts or clips on the side. Use a socket, extension and ratchet to remove them one by one then take the distributor cap off.

Mark the location of the rotor: With the distributor cap removed, you’ll see the rotor in the center of the housing of the distributor, which has a pointed end and a blunt end. Take a note of the rotor’s placement and place a screwdriver along the edge of the rotor to mark where the pointed end of the new rotor should be placed.

Remove the rotor: This part requires attention. If your rotor is secured by a small screw, usually in the middle or along the edge, it is important to use a magnetized screwdriver to suck it out. This is to avoid this screw falling into the distributor shaft and finding its way into the engine, which might create a host of problems.

If there’s no screw and the rotor just fits freely on the distributor shaft, simply take the old rotor out of the distributor. Before you throw it away, you should compare it with the new rotor to see if there’s any mismatch.

Put in the new rotor: With the distributor now free and open, some folks might want to use an air compressor to loosen any debris or excess carbon buildup inside the distributor. That is fine, but the following are what you must do to install the new rotor:

• Put the new rotor in the exact same position and direction as the old rotor. Use the marks you made before to make sure the pointed end is facing the right direction.
• If the rotor is secured with a screw, always install the new screw from your replacement kit in the rotor hole instead of recycling the old screws. 

Install new distributor cap: Make sure the screws or the clips align to the holes on the distributor cap, and the cap fits flush on the distributor.

Reinstall spark plug wires and coil: Use the marks for the spark plug wires to reinstall each of them on the same tower as before. The coil wire goes to the center post on the distributor cap.

Put engine cover and air filter housing back in and reconnect battery cables. Now you’re done. Start your vehicle to see if everything is working properly.