Signs of Bad Spark Plug Wires [Causes & Symptoms]

Spark plug wires should always be examined first, whether you’re doing a thorough tune-up or just changing the spark plugs due to the aging wires because of exposure to high temps and electrical stressors.

Many factors can lead to bad spark plug wires. Loss in power and acceleration are common indicators of failed spark plug wires. A faulty wire might also be indicated by the engine light or evident damage to the cables.

Keep reading to know more about spark plug wires and general care to save you from the hassle.

What Are Spark Plug Wires?

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This YouTube video explains all that you need to know about spark plugs. He explained in the video how they work, why you should buy a performance wire, and why your spark plug is better with sufficient energy.

As explained in the video, the spark plug wire transmits the coil’s energy to the spark plugs, igniting the combustion chamber’s air and fuel.

Signs and Symptoms of Faulty Spark Plug Wires

Spark plug wires are critical components of your vehicle’s electrical system. You’ll notice a variety of engine-related issues once they start failing.

The following are signs and symptoms of bad spark plug wires:

Hard Starts

Various reasons might make it difficult to get your engine started. A deteriorated battery terminal, a bad starter, bad injectors, or defective ignition lines might all be blamed. In addition to the case mentioned above, several more aspects are to consider.

Defective spark plug wires might be the source of the problem warning. So, if you’re having trouble starting your engine, don’t give up yet. Make sure your ignition cables are in good shape.

Engine Misfiring

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This video explains how to diagnose your faulty wires and how determine if your engine is misfiring.

Engine misfire is something that no driver or motorist wants to encounter. However, it is inevitable. Regardless of the year, make, or car model, every road-going vehicle may encounter an engine misfiring at some point. It is brought on by incomplete or irregular combustion.

Malfunctioning spark plug wires will not convey the needed spark from the distributor, resulting in an engine misfire, as spark plugs are meant to emit sparks at specified intervals.

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Rough Idling Engine

In most cases, a vehicle’s idle should be smooth and consistent. Electrical current is transmitted from your vehicle’s power circuit to the engine’s spark plugs, in which the current ignites the air-fuel combination in the combustion chamber.

Ignition will not start correctly if the spark plug wires become defective, resulting in harsh, irregular engine idle. Rough idling, for example, can be caused by a variety of different problems, including a faulty ignition coil. So, before you replace your ignition wires, make sure they’re the source of the problem.

Knocking Noises

An unusual sound is mostly a sign of a problem. A knocking sound in the engine is a common sign of faulty spark plug wires, and if not rectified quickly, it can result in serious mechanical damage.

The noises you detect are coming from the pistons, indicating incomplete or delayed combustion. So, before you leap to a conclusion, check your ignition cords to be sure they’re not the source of the problem.

Engine Power Reduced

The engine spark plugs require unaltered electrical conduction to ensure a set spark throughout the combustion process. If the spark plug or its wires deteriorate, it will harm the combustion process, resulting in a loss of engine power.

Engine Stuttering

A problem with the fuel injection system is the most prevalent, causing your car’s engine to stutter. A faulty filtration, fuel pump, or fuel injector might be the source of this problem. Spark difficulties from defective spark plugs or ignition coils can also cause it.

Engine Surging

When the exterior coating of the wires splits, the electricity traveling through them encounters severe resistance. As a result, when the electrical current is sent to the spark plugs, it may be entirely halted or experience short bursts. This occurrence produces engine surging, which is bad for the vehicle’s general health.

Engine Light Turned On

Modern automobiles have sensors built in that give out indications about possible problems, such as the state of your engine and other things that impact your vehicle’s performance. However, if your car’s Check Engine Light turns on, it might be a sign that you have a damaged ignition cord.

Slow Acceleration

Bad spark plug cables can also cause sluggish acceleration and worse gas mileage. When your car’s ignition wires break, it loses its acceleration ability, resulting in increased fuel consumption.

Although additional factors contribute to low fuel efficiency, the situation is concerning and irritating. Replacing your spark plug wires is the only way to fix it.

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What Causes Spark Plug Wire Failure?

There is a lot to consider when it comes to your spark plug wires failing. These three reasons contribute to the signs and symptoms of bad spark plug wires.

Visible Wire Damage

Over time, spark plug wires might dry up, so examine for fractures in the insulation.

These wires are connected to the spark plugs placed on the engine’s cylinder head, so they should be easy to spot. If the wires come into contact with some of the engine’s hot components, they may begin to melt. One of the indicators of faulty spark plug wires is visible cracking or melting.

Here are the factors that cause the wire damages and calls for replacement:


Engine vibration can cause the electrical connections at the spark plug to weaken over time. The energy required to ignite the spark plug rises; as a result, potentially damaging the ignition coil and spark plug wires.


The engine’s heat might cause wire sheathing and boots to burn. A damaged boot might hamper the appropriate seating and operation of spark plug wires. Damaged wire insulation might cause electricity to go to the ground instead of leaping the space at the bottom of the spark plug.


The insulation is sliced and broken when spark plug cables grind against engine components, especially sharp edges. Electricity may jump to the ground rather than reach the spark plug when this occurs.

Carbon Buildup

Carbon clogging occurs when the air and fuel combination in the automobile is too rich, implying that the fuel proportion is too high. It causes incomplete combustion, resulting in a carbon buildup on the spark plug’s firing end.

Inadequate Spark Plug Gap

The width of the gap between electrodes is one of the most important aspects of whether a spark plug works properly. The spark will be too weak if the spacing is too tiny, causing the engine to perform badly or inefficiently.

How to Change Spark Plug Wires

So, you’ve found out that your spark plug wires are faulty. Then it’s time to replace them. If you don’t know how to, continue reading below for a step-by-step tutorial.

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1. Remove the Old Spark Plug Wires

To change your old spark plug wires, start with the shortest cable and work your way up to the longest. This is to keep you from accidentally swapping the wires. Rotate or twist it to loosen the distributor or spark plug boot. Ensure you don’t tug or twist it to avoid damaging the cable connection, make sure you don’t tug or twist it.

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2. Putting in the New Spark Plug Wires

Layout all the new spark plug cables and match them with the old ones. More critical than matching the wires is making sure the ignition leads to fire in the correct order.

Improper installation of these wires can result in engine malfunctioning and surging. You will park your car in the garage in the worst-case situation until the firing order is tracked down and reinstalled correctly.

Push one side of the replacement ignition cable boot on the ignition coil or distributor first, then the other end on the spark plug to correctly install ignition cables. To avoid cables detaching from the cap, ensure they are securely fastened.

Hold the spark plug wires firmly against the boot and twist and push the cable in carefully. On the plug insulator, make sure the cable latches and snap-in. The wires will snap easily if you place them correctly, then you will hear a ‘click’ sound.

Check if you did a decent job with the installation. If the wires fall out easily when you pull them out slightly, it means the installation was done incorrectly and has to be corrected.

3. Routing the New Wires

To avoid damage, ensure the lead cable does not touch any hot components, such as the exhaust manifold. When arranging spark plug wires, make sure they’re not too near or too parallel to each other, and make sure they’re routed so they’ll be fired properly.

How to Test Spark Plug Wires

Testing your spark plug wires can be finicky. But, don’t worry. Below are ways to test your spark plug wires, continue reading.

1. Examine the Ignition Leads and Chips

Visually examine the ignition leads one by one while the engine is switched off. Wipe the dirt from the wire with a clean towel and inspect the spark plug wires for physical damage such as fractures or burns. Taking the wires off and inspecting them on the ground is the best method to accomplish this.

Examine the insulation between the ignition coils, distributor, wires, and boots for deterioration (the end of the spark plugs wires). Inspect the spring chips in the distributor that fit into the spark plug wires. Faulty springs may end up causing the wires to detach while remaining attached to the distributor cap. If everything appears to be in order, reconnect the cords and continue.

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2. Start the Engine and Look for Electrical Leakage

Start the automobile and inspect the wiring carefully. Electrical leaks are indicated by snapping noises around the cables and the distributor or ignition coil. To avoid electrical jolts, do not touch the cables while the engine is running.

3. Spray Bottle Test

While doing this step, you should be in a dark area. Why? Because electrical leakage might be difficult to detect in a bright, open garage. All you’ll need is a spray bottle and some water for this step.

Run your engine, spritz the distributor with bottled water, and look for electrical leakage and flaring. Spray the ignition lines from the distributor down to the spark plug locations with water and examine for leaks.

4. Do an Ignition Lead Resistance Test

Using an Ohm or analog meter, check the resistance of each spark plug wire to verify whether it is within the manufacturer’s standards. Remember to test and reconnect the wires one by one (if they are okay).

Set the Ohm multimeter and place the prongs on the metal connections of the ignition lead to test the spark plug wires. Before going on to the next wire, reconnect the one that is tested. If you do have up to two bad spark plug wires, you’ll need to replace all of them with a new set of leads.

5. Check the Wire Connections and Route

Check your owner’s manual to determine if your main wire is properly routed and connected. Wires won’t be efficient and drain more energy if they are routed incorrectly on hot engine components like the exhaust manifold. When this happens, the engine’s performance suffers.

Contacting heated parts over an extended period may cause fractures in the spark plug cable insulations and the passing of the electrical bridge. Check to see if the spark plug wires are properly connected. Engine misfiring, hesitation, surging, rough idling, and an engine warning light are all symptoms of incorrect connections.

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Frequently Asked Questions (FAQs):

How much resistance should a spark plug wire have?

It is based on the quality of the spark plug wires. The good ones have a resistance of 3000 to 5000 Ohms per meter. You must ensure that it is constant from one wire to the next. However, you may be getting odd readings due to a faulty meter connection.

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What is a spark plug wire made of?

A copper-core center electrode is found in the majority of spark plugs. Copper is a superior conductor of electricity to any other material used in spark plugs, and it also transfers heat more quickly.

They also feature exterior insulation consisting of an extremely flexible and heat-resistant substance like silicone or EPDM rubber that is often thicker than the conductor.

Can spark plug wires touch each other?

Never allow spark plugs to touch each other or the engine’s metallic foundation. Use non-conductive plug wire looms to avoid this from happening. You can also use spark plug wire holders to ensure the wires are separated by half an inch or more.

How frequently do you need to change spark plug wires?

​​Suppliers play distinct roles and have varied component replacement periods. On the other hand, your spark plug wires should live much beyond 60k miles. However, a few elements, such as deteriorated insulation and the quality of the wires, have a considerable effect on the replacement interval.

What are the signs of bad spark plugs?

The answers are frequently found in your spark plugs. By using this chart to analyze your engine’s performance, you’ll be able to make any required modifications to attain top performance.

Check Those Spark Plug Wires

If your engine displays two or more of the signs and symptoms of bad spark plug wires, you’ll need to identify the cause and repair it before it gets worse. Changing a spark plug wire is rather simple. If you’re not confident in your ability to diagnose and repair the cables yourself, have them inspected and replaced by a competent technician.

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