Signs and Symptoms of Bad Spark Plug Wires

A bad ignition coil wire will not only affect your ability to operate your car, it will also lead to more serious damaged in the long run. And in some cases, replacing the other damaged components will cost a lot more than replacing the spark plug wires.

Keep reading to learn how to identify and replace damage spark plug wires and spark plugs.

What does a spark plug wire do?

The spark plug wires are the conductor for the electricity that causes the explosion that allows your engine to run. Without spark plug wires your carburetor would simply pour gas into the cylinders which would pour out into the exhaust.

The spark plug wires conduct a high current of electricity from the distributor, which has been boosted by the coil, to the spark plug in the engine, which causes a spark in the combustion chamber igniting the fuel.

Spark plug wires are essential to a smooth-running engine. When a wire is aged or damaged, the high current of electricity flowing through it can, and will, escape the protective rubber coating and connect with any metal object near it. This is called arcing.

Arcing reduces the current to the spark plug which will reduce or even eliminate the spark from that plug. Reducing the power of the explosion on that plug will affect the idle and performance of your vehicle, usually causing higher fuel consumption.

If an arc eliminates the spark completely, it causes a noticeable “lope” in the engine idle from the plug not firing. When plugs fail to fire, they can damage your catalytic converter by passing unburned fuel through your exhaust. And replacing a catalytic converter is more costly.

How do I know if my car needs new spark plug wires?

Below are some faulty spark plug wires symptoms;

Visible Damage on the Spark Plug Wires

Spark plug wires may start to dry out over time, so look for cracks in the insulation. The wires are attached to the spark plugs, which are located on the engine’s cylinder head, so they shouldn’t be too difficult to find.

The wires can also start to melt if they make contact with certain hot components of the engine. Visible cracking or melting is one of the signs of bad spark plug wires. Make sure you inspect the entire length of each wire to ensure you do not miss any bad or loose spark plug wire symptoms.

Be sure to first check spark plug wires for visual damage. Look for any signs of melting or cracking. The following situations call for immediate replacement of spark plug wires:

Vibration damage

Constant wear and tear caused by engine vibration can loosen the electrical connection at the spark plug. This causes an increase in the voltage required to fire the spark plug, which could damage the ignition coil as well as the spark plug wires.

Heat damage

Engine heat can burn wire insulation and boots. A damaged boot can impede proper spark plug wire seating and performance. Wire insulation that’s damaged by head can allow voltage to jump to the ground rather than jumping the gap at the bottom of the spark plug.

Abrasion damage

When spark plug wires rub against engine parts, particularly sharp edges, it causes cuts and breaks in the insulation. Once this happens, voltage can jump to ground instead of reaching the spark plug.

Engine Stalling

This is one of the most noticeable symptoms of bad spark plugs and wires, and overall, one of the most common car complaints is engine hesitation. Your car should smoothly accelerate once you apply pressure to the gas pedal. But, the engine may hesitate or hiccup during acceleration if it’s not getting a steady flow of electricity due to faulty spark plug wires.

Engine Misfiring

A car engine converts gasoline into energy that is used to power the vehicle in a process known as internal combustion. Bad spark plug wires can interfere with the electrical current flowing to the engine, making it difficult for the engine to complete the combustion cycle.

An incomplete combustion cycle can cause an engine to misfire, which means your car may jerk, stumble, or briefly lose power.

Replace the plugs more frequently if there’s oil on the plug’s tip when removed from the engine. The presence of oil is due to a cracked valve cover gasket, degradation of the spark plug O-ring, a faulty head gasket, or defective or worn valve guides.

A repair is necessary because oil can cause the engine to misfire or prevent it from starting. An engine that continues to misfire could damage the catalytic converter, which might cost $1,000 or more to replace.

Engine surging

When cracks and electrical leaks are present in the ignition lead’s insulation, the electrical current will experience brief spurts or completely stopped from transmitting to the spark plugs. This causes engine surging issues that you don’t want to share.

Engine rough idling

Rough idling is a common engine problem that many factors can cause. However, rough engine idling can be challenging to diagnose since many factors can be the culprit. How your engine runs on idle determines your engine’s total overall health.

Inspecting and fixing the problem sooner than later will save you from more significant issues. Bear in mind that rough idling is one of the major symptoms of bad ignition coil rather than spark plug wires. You need to ensure ignition leads are the cause before removing them.

Reduced engine power

The engine spark plugs need undeteriorated electrical conduction to ensure a predetermined spark during the combustion process. Any deterioration on the spark plugs or spark plug wires will negatively affect the combustion process, leading to loss of engine power. If defective spark plug wires interfere with an electrical passage, your vehicle will suffer a significant loss of engine power.

Fuel Economy Degradation

Dirty or fouled spark plugs reduce fuel economy because the plugs are not efficiently burning gasoline in the combustion cycle. Miles-per-gallon can drop by as much as 20% to 30%. A mechanic will replace the plugs and adjust each plug’s gap to factory specifications using a special tool.

Slower Acceleration

If it takes longer to accelerate, there’s less power to pass a car, for example, the issue could be worn spark plugs, they need replacing. However, a bad fuel filter, dirty or clogged fuel injectors, as well as issues with the oxygen sensor and ignition system can slow acceleration.

Difficulty Starting

Worn-out spark plugs can be the cause. Have a seasoned mechanic determine if the plugs need replacing. Simply, the engine will not start if the spark plugs can’t produce enough spark to start the combustion process. Some of the other causes for difficulty starting include issues with the ignition system, a battery needs replacing, or worn-out spark plug wires. Replace the car battery if it fails to produce enough voltage to start the engine.

Warning Lights

Finally, don’t ignore the “Check Engine,” “Malfunction,” or engine silhouette symbols. These warning lights may illuminate if the spark plugs fail, or if the spark plug wires need replacement. However, it depends on the automaker’s purpose for the light. It might be limited to warning the gas cap is loose, the vehicle is emitting emissions above U.S. regulations, the oxygen sensor or the mass airflow sensor may need replacing, and/or the catalytic converter is not operating properly.

Note: If one of the lights mentioned above flashes, immediately shut off the engine and call a tow truck. The flashing light may signal a serious problem like a misfire. Ignoring this warning could result in an expensive repair bill. Check the owner’s manual to know all the warning symbols for your vehicle.

How to test spark plug wires

It’s important to keep an eye on your spark plug wires to avoid subsequent engine issues. Here are several methods for testing spark plug wires;

How to Spark Test

Remove a spark plug wire from any plug. Attach a spark tester to the wire and to an engine ground. Crank the engine and check for a good spark at the spark tester gap.

A good spark will be blue-white and will be plainly visible in daylight. If a good spark is present, the problem is probably not in the ignition system. Check the fuel system and/or stark timing.

Weak sparks are orange or red and may be hard to see in daylight. If you did not see a spark, remove the coil wire from the distributor cap. Attach the spark tester to the distributor end of the coil wire. Crack the engine and check for a good spark at the spark tester. If a spark is present, the problem is probably in the distributor cap, rotor or spark plug wires.

How to use a screwdriver to test the insulation of the wires

To test a spark plug wire with a screwdriver, get a piece of wire that is long enough to reach from a ground source to the spark plugs on your car. Tie one end to the shaft of a well-insulated screwdriver. Tie the other end of the wire to the ground source Then drag the screwdriver tip down the length of each spark plug wire, around the coil and the boots.

If you see an arc from a wire to the screwdriver, you have defective wire. It is important to use a well-insulated screwdriver for this test. If you don’t, you run the risk of shocking yourself.

Testing for shorts with a test light

Tools Needed: 12 Volt light test

Attach the test light to a good engine ground. With the engine running, move the test light along the length of each spark plug wire. If a spark jumps from a plug wire to the test light at any point, this is an indication that the insulation has broken down, and the spark plug wire should be replaced.

General troubleshooting steps

Items needed

Jumper wire (30-60cm long)
Spark plug wire pliers (if available)
Clean cloth
Tape measure
Digital or analogue multimeter or ohm meter
Owners’ manual for your car
Rubber mat

Testing steps

Test each wire separately. In other words, disconnect one wire, perform the test, reconnect it and continuing to the next wire.

Note: When reconnecting the wire, ensure it is not too close to other electronics or to anything hot, like the exhaust. By doing this wire by wire, each wire will be reconnected to its corresponding cylinder, ensuring the firing order of the ignition system is correct so the engine will continue to run.

1. Inspect the wires and clips

In a well-lit area, remove and visually inspect the spark plug wires one at a time. Clean each wire with a cloth and then look for physical damage like cuts and scorch marks on the wires or boots, carefully examine the insulation around the wire and look for corrosion between the boot (the end of the spark plug wire), the spark plug and coil. If you can’t see any physical damage, check the spring clips that hold the wires in the distributor cap. Damaged clips may allow wires to slip and not remain in the correct position. If everything looks okay, put the wire back and move onto the next one.

2. Run the engine

Turn the engine on and look for electrical arcs around the wires or listen for a snapping noise that might show a high-voltage electrical leak. Be careful not to touch the wires while the engine is running due to the risk of electric shocks from the high voltage.

3. Perform a spark plug wire resistance test

Use a digital or analogue multimeter (or ohmmeter) to check if the resistance of each spark plug wire is within the manufacturers’ guidelines. Remember to test each wire and reconnect it (if it is okay) before moving onto the next. To do this, set the multimeter to the appropriate scale and measure the wire by placing the probes on each end of the wire lead, making sure they touch the metal contacts.

If the reading is within the manufacturers’ guidelines, the wire can be returned to the car before you start to test the next wire. Any defective wires should be replaced. If two or more wires are found to be defective, it’s recommended to replace all wires with a set of new wires.

Using a Digital multimeter

Use the digital multimeter, set to Ohms Ω. Simply attach or hold a meter probe on each end of the cable and read the total resistance on the meter. The Society of Automotive Engineers suggests the maximum resistance should be 12,000 ohms-per-foot. However, some OEMs have recommended different maximum resistances. Consult a tune-up manual for exact specifications.

Using a 12 Volt light test

Note: Try to avoid direct contact with metal parts of the vehicle when checking for shorts.

4. Check your spark plug wires are routed correctly

Consult your owners’ manual to see if your spark plug wires are routed properly. This is important as cross coupling can create an energy drain, which will lower performance. It is important to route the spark plug wires so that they do not come in direct contact with engine hot part such as exhaust manifold. Prolonged contact with engine hot parts could result in development of cracks in the insulation layer of spark plug wires.

How To Remove and Install Spark Plugs

Changing your vehicle’s spark plugs in most cases is a straightforward job that you can tackle on your own. Before changing the plugs, ensure you have the right spark plugs for your vehicle.

Removing the Old Spark Plugs

Locate the spark plug (some applications the spark plug might be hidden under the upper intake manifold, Ignition coil or ignition coil cover); locate the wire connected to the spark plug or ignition coil.
Pull on the boot – the connector at the end of the wire or unplug wiring connector on the coil. Be sure not to pull on the wire itself, you risk damaging the wire or wiring connector.
After the wire or coil is removed, use your socket wrench with the spark plug socket to remove the spark plug. Place the socket around the spark plug and turn the socket counterclockwise.
The spark plug should unscrew easily. If you are having problems removing a spark plug that is too tight, stop before you break it off. You can put a bit of penetrating oil like WD-40 to help loosen it.
Take a look at the old plug; it should be a little dirty on the end. If it is oily or white, this could point to other issues that you need to investigate with the help of a professional mechanic.
Set the old spark plug aside.
Examine the threads in the engine, if they are dirty; clean them with a thread cleaning tool.

How to install new spark plugs

If necessary, set the gap on the new spark plug before installing it. In the past, it was necessary to use a gapping tool to make sure the gap in the spark plugs was the correct size. Today, Champion spark plugs come pre gapped, eliminating the need for this step most times. You should double check that the gap is correctly set; check your owner’s manual to find the distance for the spark plug gap and use a gap gauge or feeler gauge to check the distance.

Seat the new spark plug by hand; turning it clockwise at least two full turns. Once you have the spark plug snug, use the socket wrench with extension and spark plug socket to tighten it turning the plug clockwise. The plug’s washer should be pressed against the mounting surface.
Take care not to over tighten the plug; you can strip the thread on the head of the engine leading to costly repairs. You can also use a torque wrench to torque it to specification (OE service or repair manual).
Replace the spark plug wire boot or ignition coil and wiring connector. A plastic boot should click into place while a rubber boot may require a coat of dielectric grease on the inside of the boot for easy removal next time.
Repeat the removal and installation process for each spark plug. That’s all there is to it! You have breathed new life into your car with a set of new spark plugs. If at any point you run into issues or have a question, consult your mechanic.

How to Replace Spark Plug Wires

Start by locating the spark plug wires;

Prop open the hood of your vehicle. The hood latch is typically located to the lower left-hand side of the driver’s dashboard. Some cars feature hydraulic hoods that stay propped open automatically. Either way, it’s important to make sure that your hood will not fall onto you while you’re rooting around in the engine.
Locate the spark plug wires. The wires are typically located next to the valve covers in the cylinder head. On one end, each wire will be attached to a spark plug and on the other end, to a distributor or ignition coils.
Understand why spark plug wires wear out. Due to the high voltage constantly sent through the spark plug wires, they have the tendency to increase in resistance over time. Eventually, this creates too much resistance for any electricity to flow. With the increase in resistance in the wires, there is a decrease in the amount of electricity arriving through the spark plugs — which results in the incomplete combustion of gases inside the cylinder.
If there is damage to the protective shield encasing the spark plug wires, then you need to replace the spark plug wires.
Determine whether or not you need to replace the wires. Age alone does not necessarily indicate that you need new spark plug wires. Look for damage to the wires, and listen for engine malfunction. If you see sparks jumping from the wire to the engine, it is a sign that you need to replace the wires.
Be aware of some of the obvious engine symptoms: rough, bumpy idling, and a deep “coughing” sound. The engine symptoms can also be caused by faulty spark plugs and other, deeper issues, so you first must prove that your wires have gone bad and need to be replaced.
You may need to replace the wires if you see sparks jump to the ground at night with the hood up and engine running. Depending on the conductivity of your wires, there might be sparks leaping from the whole front of the car, or just from one spot.
Look for obvious flaws in the wires. You may encounter frays, cracks, and even burnt spots. Any or all of these damages can indicate that you need to replace the wires.
Determine how many wires you need. Now that you have determined the number and type of spark plug wires, you can buy them at any local auto parts store. The clerk should be happy to help confirm that you are getting the correct type and amount of wires.
Make sure to buy the right wire length. You must buy an entire set, even if you only need to replace one wire. Thus, if you have a six-cylinder engine, you must purchase all six wires, all of which will vary in length. You must be aware of the length of the wire you are replacing compared to the old wires on your engine. Try to stay as close as possible to the old wire length.
Different makers have different lengths, and replacement wires are often sold longer than the originals. This allows them to sell more assemblies of wires to fit more applications, so you may have slight differences. Check the length before you start, and it’ll be okay.
Quality is important. Stay away from most of the “make your own length” kits unless they are high quality and you are very comfortable installing your own boots.
Oftentimes, the manufactures do not allow repairs on their wires. Don’t start cutting the wires to a certain length unless you know for certain that new ends can be safely put on the wires you cut.

Note: Some spark plug wires can be purchased individually from some auto parts stores fully assembled.

Removing the Wires

Make sure that the car is turned off. Never try to replace plug wires on a running motor. Likewise, do not try to replace plug wires on a motor that’s too hot to touch.
Keep an inventory. Once located, take note of the length and location of each wire. You will need to put each new wire back where you pulled its corresponding worn wire – and it will be much easier if you write down what you’ve done. If you connect the wires in the wrong order, your engine will misfire and run poorly.
Try marking each wire with tape and a number (corresponding to a spark plug location) so that you don’t lose track.
Be methodical. Replace the wires one at a time, and in a specific order or direction. This may help you remember which wire goes where, and it will reduce the risk of putting the firing order out of sync with the engine. Take your time. Start with one wire, and completely finish replacing it before moving on to the next one.
The wire is connected at both ends. You must unplug each side before installing the new wire.
The spark plug must fire when the piston is nearly at its highest point in the cylinder. Thus, it is imperative that you do not get this sequence out of order. Try starting at one end of your engine, and working across.
Unplug the wires. Use a spark plug wire removal tool to unplug and remove the wires. Use caution when you pull the wire from the plug. The newer engines have rubber boots that fit tightly over the plug, keeping it dry and clean.
Remove the wire by pulling on the boot. If you pull on the wire instead of the boot, you may destroy the wire, and some of it will be left on the plug.
Some of the wires can be very tightly stuck to the spark plug. Grasp the rubber boot firmly. If it doesn’t pop off immediately, try twisting it back and forth while you pull it free.
Inspect the boot for signs of carbon tracking. This will appear as a black line running from top to bottom inside the boot. If you notice this line, the spark plug must be removed for inspection.

Installing New Wires

Work backward. Connect the new wires in the same order as you removed the old wires. Before installing the boot on to the spark plug, add a small amount of dielectric grease to the spark plug boot. The boot will be fully seated onto the plug when a small click is felt.
Plug wires run from the distributor or coil to the plug, and must be replaced exactly as factory run. Running from a coil to the wrong plug will prevent the engine from running, which could lead to damage.
Keep the wires away from exhaust components that might damage them, and keep each wire from crossing over another wire.
Spark plug wires usually rest in wire looms or standoffs. A wire resting on the engine or a wire crossing another can short or leak or break down due to heat. Thus, make sure to rout the replacements properly through the loom, well away from resting on any metal.
If replacing wires with a high-performance coil and kit, be aware existing looms may not fit. In this case, you can buy larger diameter standoffs to attach or widen the holes in the looms.
Close and latch the hood. Try to lift up your hood after you’ve closed it, and make sure that it isn’t loose. You should not be able to pop the hood without using the switch inside your vehicle cab.
Listen to your vehicle. After you have carefully installed the wires in their correct places, start the engine. It should run and idle smoothly. You may notice new power and efficiency, especially if your old wires were heavily worn down.
If your engine does not run, runs very roughly, or backfires after replacement, then check for improperly run wires, wires run to the wrong cylinder, wires that are grounded along their length, wires that are not properly seated in the boot, or boots that are not properly seated to make contact with the coil or plug.
Never touch a wire on a running engine, or you might receive a painful shock. There are tens of thousands of volts generated in the ignition system, and an improperly seated wire is much more likely to shock you. It has less of a ground at the plug end, making you a more viable path.
If you notice bad idle backfire or other performance problems, you have likely put a wire in the wrong place. Consider hiring a local mechanic to diagnose and fix the problem.
Take the car for a test drive. While on the test drive, try to put the engine under a load by driving up a hill or slowing down in a higher gear, then accelerate with downshifting to put the ignition system under a load. Ignition systems have a better chance of failing under a load.

How to prevent spark plug damage

Here are tips to maintain your car’s spark plugs;

1. Clean before removing the old spark plug.

Before removing the old spark plug, use a compressed air gun to blow away any dirt or debris that may have collected around the exterior. The dirt cannot be allowed to enter the cylinder bore because it will score the wall and wear the rings. If the spark plug does not have a tapered seat, then a washer-style gasket is required. The gasket is installed with the small side toward the cylinder head. The length of the threaded area is called the reach and positions the electrode properly in the cylinder bore.

2. Always inspect a new plug for damage.

If dropped, check for cracks in the porcelain. Make sure the side electrode lines up with the center electrode and is straight. If not, gently reposition it with Needlenose pliers. Next, employ a spark plug feeler gauge to obtain the proper gap between the electrodes.

When correct, the gauge will drag evenly on both the center and side electrodes. Make sure the gauge is straight and not cocked when checking. If the side electrode needs to be excessively angled up or down to create the proper gap, the spark plug is of the wrong design or the specification is incorrect.

3. Spray the cavity around the center electrode.

Using carburetor cleaner, spray the cavity around the center electrode while having the plug face downward so any material will fall out. This will remove any porcelain dust or debris from manufacturing that can cause a misfire, especially under load.

4. Coat the thread with antiseize compound.

This is especially important with an aluminum cylinder head or equipment that is not stored in a building. This will aid in removing the plug in the future without galling the threads in the cylinder head.

Keep the antiseize compound away from the electrodes. If it gets on the electrode, wipe it off with a clean rag that is moistened with carburetor cleaner.

5. Coat with dielectric compound.

Coat the secondary wire connection and the porcelain insulator where the boot sits with dielectric compound (available at most auto parts or farm supply stores).This compound deters corrosion and prevents the boots from sticking to the insulator, which can lead to wires breaking during future service.

Thread each plug in by hand and then tighten with the proper socket and wrench. The plug needs to be tight against the cylinder head to maintain heat transfer and to keep the bore sealed. Never use air tools to remove or to install a spark plug.

What causes spark plug damages?

Oil in Combustion Chamber

A leading cause of spark plug problems is a flow of engine oil into the combustion chamber. If oil leaks into the combustion chamber, it can cause the tip of the spark plug to get oily and dirty leading to premature failure. Especially a problem in older vehicles, take notice if your vehicle starts to burn oil, as it can be an indicator that your spark plugs may be damaged, and therefore have a shorter lifespan.

Overheating

Repeated overheating of the spark plug tip can cause the plug to prematurely fail. Overheating can be caused by many things like pre-ignition and a malfunctioning cooling system. Pre-ignition can lead to heat building up in the combustion chamber causing the spark plugs to fail. In addition, if the cooling system isn’t functioning correctly, it can cause the engine and spark plugs to overheat. This overheating can lead to the spark plug’s electrode wearing out faster.

Carbon Buildup

Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled spark plug. This carbon buildup decreases the lifespan of a spark plug and can lead to hard starts, decreased acceleration, engine misfires and the check engine light coming on. Causes of a carbon-fouled spark plug include a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture, dirty fuel injectors or idling your vehicle for too long.

Improper Spark Plug Gap

The gap between the spark plug’s center and side electrodes needs to be calibrated perfectly to ensure optimal engine performance. Having the right gap ensures that the arcing occurs at the proper voltage to ignite the fuel and generate the combustion that makes the engine run. If the gap isn’t set correctly, extra stress could be placed on the spark plug tip which could cause it to erode and wear out prematurely.

Can bad spark plug wires cause engine knock?

Yes. Bad spark plugs sap engine power and power and performance. The internal combustion engine is a great engineering feat that revolutionized how we live our lives. That engine needs three things to work: fuel, oxygen, and spark.

Engine knocking is caused by your spark plugs not detonating properly and igniting all the fuel. The fuel and vapor that did not ignite eventually will catch fire and detonate. When that happens, you hear a knocking sound from your engine. Bad spark plugs are common causes of engine knock, but simple to fix. Fortunately, they are simple and affordable to replace.

When should you change spark plug wires?

The ideal interval for changing your spark plugs and wires depends on the manufacturer’s recommendations. Refer to your owner’s manual for the replacement interval. Some automakers require spark plug change at 18,000 miles, some at 30,000 to 35,000 miles, and others at 100,000.

A quality set of spark plug wires can generally last around 60,000 to 70,000 miles. Again, it is a good idea to replace these parts prior to a failure, like a misfire. A vehicle that is running rough or misfiring can cause catalytic converter damage.