Signs and Symptoms of Bad spark Plug Wires: Tests and Fixes

The high tension lead, typically known as spark plug wires or ignition cables, are ignition system components. While most newer cars are designed with ignition coils, some passenger cars and trucks still use spark plug cables. The ignition system works by creating Sparks at predetermined intervals in order to ignite the air/fuel mixture in the combustion chamber. The spark plug wire’s primary work is to transmit spark from the distributor or ignition coil to the spark plugs.

The spark plug wires are manufactured with low resistance, durable material that can stand harsh conditions under the hood, as well as high voltage of the ignition system. As they work as transmitters that transfer the spark required for the engine to operate, they will negatively affect the engine’s performance when they fail to transfer the necessary spark needed for the engine to run optimally. Knowing the symptoms of bad spark plug wires, how to test them, and how to replace them will save you a lot.

Signs and symptoms of bad spark plug wires

The spark plug wires play a vital role in any vehicle’s electrical system. When they start failing, you will notice several symptoms of faulty ignition leads that pop up to tell you something is wrong. Failure to track down the cause of these signs and fix them on time might lead to more significant problems. Let’s quickly look at the common symptoms of lousy ignition cables.

symptoms of faulty ignition leads

  • Hard starting
  • Engine misfire
  • Engine hesitation
  • Engine surging
  • Engine rough idling
  • Reduced engine power
  • Check engine light

Hard starting: Many factors can cause hard starting. It could be due to a corroded battery terminal, lousy starter, lousy injectors, or faulty ignition leads. Aside from the causes mentioned above, there are some other factors as well. However, defective spark plug wires can be the culprit without prior notice. So, if you have hard starting issues, you don’t have to jump to a conclusion. Ensure you inspect your ignition cables.

Engine misfire: Engine misfire is what no driver or car owner would ever want to experience. Unfortunately, it is inevitable. At one point or the other, every road-going vehicles experience engine misfiring, irrespective of the year, make, and model of your vehicle. It is caused as a result of incomplete or erratic combustion. As spark plugs are designed to create Sparks at predetermined intervals, Faulty spark plug wires will not transmit the required spark from the distributor, or ignition could resulting in an engine misfire.

Engine hesitation: When the ignition leads develop cracks or degrade, it will transfer irregular electrical spark to the spark plugs. This is true due to electrical misconduct transmitting from the ignition coil or distributor to the spark plugs. Electrical misconduct causes engine hesitation, mostly when giving gas to the engine. Engine hesitation is also one of the common symptoms of bad spark plugs.

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.

Check engine light: Cars nowadays are built with tons of sensors that send signals to the car computer. These sensors are meant to report the engine conditions to the ECU, which controls your car’s overall performance. For instance, the 02 sensor says what happens in the exhaust, and the MAF sensor sends a report about the air/fuel mixture ratio. If anything goes wrong, the sensor in charge of that domain will send a notification to the car computer, and as a result, the check engine light will pop up on the dashboard to notify the driver of a problem that needs attention. And engine misfire or bad spark plug wires is no exception.

How to test spark plug wires

Bad spark plug wires share similar signs and symptoms with many faulty engine components like spark plugs, ignition coils, fuel filters, to mention just a few. When you notice the symptoms above, the good idea is to test your spark plug wires to ensure they are the cause. Before you proceed with the test, you’ll need some unique tools and materials.

how does bad spark plug wires affect your vehicle

  • Ohm or analogue meter
  • Owner’s booklet
  • Clean rag
  • Jumper wire 30-60cm long
  • Tape measure
  • Spray bottle and water
  • Screwdriver
  • Spark plug wires pliers (if available).

It’s essential to test ignition leads one at a time. That means disconnecting the spark plug wires one after the other and connect them back after each test. Doing this one at a time, you will not mix up the cables. Mixed-up wires will cause severe engine misfire. Ensure you write down and follow the engine firing order so the vehicle will continue to run nice and smoothly.

Step 1. Visually inspect the ignition leads and the chips: while the engine is turned off, visually inspect the ignition leads one at a time. Take your clean rag and wipe off the dirt on the wire and check if there are any physical damages like cracks and scorch on the plug wires. The best way to do this is to take the wires off and inspect them on the ground.

Check if there is corrosion on the insulation between ignition coils, distributor, wires, and the boots (the end of the spark plugs wires). Check the spring chips that fit in the spark plug wires in the distributor. Faulty springs might cause the wires to disconnect and still stay firm on the distributor cap. If everything looks good, fix the cables back, and proceed to the next level.

Step 2. Turn on the engine and check for electrical leaks: Turn on the engine and carefully examine the wires. Snapping noise around the wires and the distributor or ignition coil indicates electrical leaks. Do not touch the wires while the engine is idling to avoid electrical jolts.

Step 3. Spray bottle test: I’m going to show you how to check spark plug wire with water right now. Preferably you need to stay where you can switch on and off the light. This is because it might be challenging to detect electrical leaks on a bright and clear garage. What you need is a spray bottle and water in it. Go ahead and start your engine, spray the bottled water on the distributor and check for electrical leaks and arcing. Next, spray the water in the ignition leads from the distributor down to the spark plug areas and inspect for leaks as well.

Step 4. Carry out the ignition lead resistance test: check each spark plug wires’ resistance and see if they are within the manufacturer’s guideline using an ohm or analogue meter. Do not forget to test the wires one at a time and fix them back (if they are okay). To test the spark plug wires, set the ohm multimeter and place the probes on the ignition lead’s metal connectors. If the tested wire is good, reconnect it before moving to another. If you have up to two lousy spark plug wires, you have to get a new set of leads and replace all the wires.

Step 5. Cross-check the spark plugs wire connections and routing: Visit your owner’s booklet and see if your lead wire is routed and connected correctly. Incorrect routing can cause the wires to relax on hot engine components like the exhaust manifold and drain the energy. When this happens, it will result in a loss of engine performance. Prolonged contact with hot parts might result in cracks in the spark plug wire insulation layers and bridge electrical passage. You need to check if the spark plug wires are connected correctly. Wrong connections will result in engine misfiring, engine hesitation, engine surging, rough idling, and engine warning light.

How to change spark plug wires

Replacing your spark plug wires with high-end wires when they damage is essential for your optimum engine performance.

Take off the spark plug wires.

To replace your old spark plug wires, you need to replace them one by one starting with the shortest cable. This is to prevent you from switching the wires. Rotate or twist the boot on the distributor or spark plug to loosen it. Ensure you don’t tug or twist the cable itself to avoid breaking the cable connection.

Installing the new spark plug cables

1. This is a perfect time to clean spark plug holes and replace your spark plugs. But if you replaced the plugs recently, proceed with the spark plug replacement. Spread out the new spark plug wires and match the shortest one in the vehicle with the shortest one in the new pack you bought, then the next shortest till you get to the longest cable. Ensure you follow the above matching process and do not match identical lengths as this varies with different ignition lead manufacturers.

What’s more important than matching the cables is not missing up the ignition leads’ firing order. Improper installation of these cables can cause engine misfiring and surging, and in the worst scenario, it will cause your car to sleep in the garage till you track down the firing order and reinstall it correctly.

2. To properly install ignition cables, push one side of the new ignition cable boot on the ignition coil or distributor first and fix the other end on the spark plug. Make sure the cables sit well to avoid wires dislodging from the cap.

3. Hold the spark plug wire firm on the boot and gently twist and push in the cable. You have to ensure the cable clips and snap-in on the plug insulator. When you position the wires well, it’ll snap easily, and you will hear a ‘click’ sound.

Reconfirm the installation and see if you did a good job. Try to pull out the wires slightly, and if it comes off easily, it indicates improper installation that needs your attention.

Routing the ignition wires

It is essential to make sure the lead wire does not sit on any hot components like the exhaust manifold to avoid damage. When spacing spark plug wires, you have to ensure the wires are not close or parallel to one another, and it’s important to route the wires in the sequence of their firing order.

Routing the wires away from hot components prevents them from losing voltage between engine metal components and the conductor. High-quality wires are manufactured with some set of metallic springs that clicks or snaps when you install them properly. Ensure you go for high-end wires and avoid the cheap ones.

FAQs:

Can a bad spark plug wire cause the car not to start?

Can a bad spark plug wire cause the car not to start?  Absolutely yes. The spark plug wires’ primary work is to supply the necessary spark from the distributor or ignition coil to the spark plugs. If the spark plug wires fail, it can provide the insufficient spark to burn air/fuel mixture in the combustion chamber. So, bad spark plug wires can cause rough idling and, in the worst case, not to start.

How often do you do to change your spark plug wires?

Vehicle manufacturers have different roles and component replacement intervals. Although, your spark plug wires should last beyond 60k miles. However, few factors play a significant role in the replacement interval, such as compromised insulation and the wires’ quality.

What does a bad spark plug feel like?

Bad spark plug wires will ultimately affect your engine’s performance. When you have bad spark plug wires, you’ll experience engine surging, engine hesitation, engine surging, and rough idling.

Does changing spark plug wires improve performance?

The answer is yes and no. The no is because if you decide to change your spark plug wires in good condition to improve performance, you’ll be disappointed. And yes, because since a bad spark plug wire can transmit insufficient spark  to ignite the air/fuel mixture, replacing them with new high-end wires will improve your engine performance.

Is it easy to change spark plug wires?

Replacing your spark plug wires can be as easy as changing your car battery – but get it wrong and have funny engine behavior. Your vehicle might not leave the garage until you fix the wires correctly.

Can you change spark plugs without changing the wires?

Yes, the high tension wires are the different component that transfers electrical conduct from the distributor to the spark plugs. So, you can change spark plugs without changing the wires. The time of changing the spark plug could be the best time to change the wires.

Final word

Suppose your vehicle engine is exhibiting two or more of the symptoms of bad spark plug wires. In that case, you need to diagnose and replace any faulty ignition wires to prevent them from escalating to bigger problems. If you are reading till this point, it means we have cleared your mind from wondering how does spark plug wires affect your vehicle.

In a nutshell, replacing a spark plug wire can be pretty straightforward. But if you’re not confident enough to diagnose and replace the cables yourself, contact a professional mechanic for thorough inspection and replacement.

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Osuagwu Solomon

Osuagwu Solomon is a seasoned automotive technician for the past 9 years, and a technical writer. He loves writing about auto professional repair guides, DIY repair guides, and buyer’s guide. After spending six years in the automotive workshop, he decided to impact his knowledge to people aside his domain, and he has achieved this by centering his Automotive writing skills on REPAIRS.

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