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While not much larger than AA batteries, spark plugs play a key role in the operation of any vehicle with an internal combustion engine. Simply put, they create the spark that ignites the air-fuel mix in the engine's cylinders, generating the power that turns the wheels.
As for how electricity reaches the spark plugs, this depends on your vehicle's ignition system. In traditional systems, spark plug wires perform this task. In Coil-On-Plug systems, wires aren’t needed as the plug sits directly atop the cylinder.
Spark plug wires are usually made of flexible, heat-resistant materials like silicone or synthetic rubber, so they last as long as possible.
Generally, spark plug wires need swapping every 30,000 to 70,000 miles, though they may work just fine at 100,000 miles or more. Ultimately, how long spark plug wires last depends on factors like the vehicle and engine type, your driving habits, weather conditions, and more.
This article covers the symptoms you might experience if you don't change your spark plug wires. For those interested in DIY maintenance, we'll also share step-by-step instructions for testing your vehicle’s spark plugs and spark plug wires.
What Happens If You Don’t Change Your Spark Plug Wires?
Spark plug wires fail for many reasons, including prolonged exposure to engine vibrations, heat degradation, friction wear, etc. If one or more of your spark plug wires gives out, there are a number of symptoms you might experience, including:
- Decrease in fuel economy
- Unstable idling or stalling
- Engine not running smoothly or at all
- Unresponsive acceleration
These symptoms often serve as early warning signs that it's time to replace your spark plug wires. Below, we'll examine each of these signs in more detail.
Decrease in Fuel Economy
A drop in fuel economy is one of the most common signs suggesting your spark plug wires might need replacing. When faulty, they can't effectively transmit the electrical current from the ignition coil to the spark plugs.
Without sufficient power, one or more spark plugs may misfire (where the cylinder doesn’t fire), resulting in the incomplete combustion of the engine's air-fuel mixture. As a result, your vehicle may consume more fuel than usual to make up for the lack of power, leading to decreased gas mileage.
Unstable Idling or Stalling
Another telltale sign of bad spark plug wires is unstable idling or stalling. When these wires are damaged or worn out, they may intermittently fail to deliver enough voltage to the spark plugs. This can cause the engine to run unevenly, often accompanied by rattling, pinging, or knocking sounds.
The engine may even stall, particularly when idling. This issue is most apparent (and potentially a safety risk) while in stop-and-go traffic when the engine shifts between active driving and idling.
Engine Not Running Smoothly or At All
Faulty spark plug wires can have a major impact on your engine's combustion process. When compromised, they may not deliver ample voltage to the spark plugs. This can manifest as your engine running roughly, with RPMs surging and falling and vibrations being felt through the steering wheel.
In extreme cases, your vehicle may struggle to climb hills or accelerate quickly. If multiple wires fail, the engine may not start at all. For example, on a cold winter morning, a vehicle with faulty spark plug wires might not be able to generate a spark capable of turning the engine over.
Unresponsive Acceleration
Lastly, faulty spark plug wires can result in unresponsive acceleration. Instead of a smooth, swift response when pressing the gas pedal, you may notice hesitation, stuttering, or a sluggish feeling where the vehicle struggles to gain momentum.
Like other symptoms on this list, this issue often stems from spark plug wires that aren't supplying enough power to the plugs. This problem is especially evident when the engine is under stress, like when overtaking another vehicle or hard-accelerating from a standstill.
Other symptoms you might experience if you don’t change your spark plug wires include dark exhaust smoke, a check engine light, backfires, and reduced engine power.
How to Test Spark Plugs and Wires (and Change Them If Necessary)
If you suspect one or more of your car’s spark plug wires has failed, you can either take it to a shop or test them yourself. While you're at it, you may as well test your spark plugs too.
In this section, we'll not only cover testing, but we'll also review how to change spark plugs and wires if any come back faulty.
When performing the below steps, note that there may be slight variations depending on the vehicle’s make and model. If you’re unsure at any point in the process, pause and consult your car’s owner's manual.
Step 1. Gather Your Tools and Ensure Safety Precautions
Before you begin, you'll need to gather all the needed tools, including:
- Spark plug socket
- Ratchet
- Spark plug gapping tool
- Anti-seize lubricant
- Dielectric grease
- Ohmmeter (or a multimeter)
- Safety gloves and glasses
You’ll also need replacement spark plugs and/or spark plug wires if any test faulty.
Also, ensure you're working in a well-ventilated area and that your engine is shut off and is cool, which will help prevent accidental burns.
Step 2. Locate and Remove the Spark Plugs and Wires
Next, locate your spark plugs and wires. They're usually found on the top or side of the engine, with one wire leading to each spark plug.
Then, carefully disconnect the wires, being sure to pull on the boot (the thicker part at the end of the wire) rather than the wire itself to avoid causing any damage. Once the wires are removed, use your spark plug socket and ratchet to gently remove the spark plugs.
Remember to keep track of which wire connects to which spark plug, which will be important when installing new wires or reattaching the old ones.
Step 3. Inspect the Spark Plugs and Measure the Gap
Once you've removed the spark plugs, take some time to inspect them for signs of wear or damage that might indicate they need replacing, like a worn-out electrode or deposits on the insulator.
Next, measure the gap between the center and side electrodes using your spark plug gapping tool. Compare this measurement to the recommended gap in your vehicle's owner's manual.
The gap is crucial as it determines the spark size necessary to ignite the cylinder's fuel-air mixture. Most vehicles require a gap size of between .025” to .060”.
If your vehicle has a distributor-based ignition system, this is a good time to inspect the distributor cap and rotor. Look for corrosion, cracks, worn contacts, or other signs of damage, and if any are present, it's likely time to replace the distributor cap and/or rotor.
Step 4. Test the Spark Plug Wires with an Ohmmeter
Now, it's time to test the resistance of your spark plug wires, one at a time. Set your ohmmeter or multimeter to the ohms setting, usually denoted by the Greek letter Omega (Ω).
Connect one probe to one end of a spark plug wire and the other probe to the other end. Record the resistance readings for each wire. A good rule of thumb is that resistance should be less than 12,000 ohms per foot of wire.
If the resistance is too high, it could indicate the wire is damaged or worn out, hindering the flow of electricity. If resistance is too low (for example, below 1,000 ohms), it suggests a short in the wire. Both cases suggest the wire needs replacing.
Compare these readings with the manufacturer's specs, which can usually be found in your vehicle's service manual. Consistency in readings across all wires is a good sign, while significant deviations suggest a problem.
Step 5. Install New Spark Plugs and Wires if Necessary
If your spark plugs or wires show signs of wear or fail the resistance test, you'll need to replace them.
For spark plugs, check the gap of the new plug with your gapping tool and adjust it if necessary to match your vehicle's specifications. Before tightening the plug into place, apply a small amount of anti-seize lubricant to the threads of the spark plugs to make future removal easier.
When installing new spark plug wires, make sure to connect them in the same order as the old ones to maintain the correct firing order. Apply a small amount of dielectric grease to the inside of the wire boots to prevent sticking and corrosion.
Once everything is in place, hand-tighten the spark plugs with the socket and ratchet. Be careful not to overtighten, which can damage the plug or the cylinder head.
For the wires, push them onto the spark plugs until you feel a click, indicating they're securely connected.
Finally, double-check your work before starting your vehicle to ensure all spark plugs and wires are correctly installed and securely connected.
Need To Replace Your Spark Plug Wires? Trust JEGS for All Your Automotive Parts Needs
While they rarely require attention, spark plug wires do eventually need replacing, otherwise your engine performance will suffer. Ignoring the signs of worn-out spark plug wires can lead to decreased fuel economy, unstable idling, and even engine failure.
While there's no specific mileage interval for testing spark plug wires, a general rule of thumb is to check them during each major tune-up or when you replace the plugs, usually between 30,000 to 100,000 miles, depending on your vehicle's make and model and the type of spark plugs you're using.
For replacement spark plugs, spark plug wires, or any other car components, visit JEGS, your trusted source for quality automotive parts. Have more questions? Contact our team today.
