Signs of Faulty Spark Plug Wires (Signs & 3 Tests)

In this article, ill teach you how to find the signs of faulty spark plug wires and how to test them. 

A spark plug is responsible for supplying the spark needed to ignite the engine to life. It is usually made from durable material that lasts millions of uses. But, like any engine component, it can get worn out due to aging, corrosion, or exposure to extreme temperatures. 

Avert further damage to your engine by learning the signs and symptoms of bad plug wires. 

Spotting the Signs of Faulty Spark Plug Wires

The key to preventing further damage is quickly spotting the signs of a bad spark plug.

Damaged spark plug wires cause noticeable effects on the engine of the vehicle. Here are the common signs of a faulty spark plug wire to look for:

1. Engine Surging

Engine surging is when a vehicle suddenly slows or speeds up when the accelerator is kept steady. 

A bad spark plug causes electrical leaks and cracks in the insulation of the ignition lead. This results in sudden spurts or halted electrical current transmission within the engine. 

2. Rough Engine Idling

Rough engine idling is commonly detected when a vehicle is started. 

It is characterized by shaking, vibrating, or bouncing sensations throughout the vehicle. It can also cause a skipping or slipping sound from the engine. 

Take note that several issues can cause rough engine idling. It is not a sure indicator of faulty spark plugs.

3. Engine Misfire

Engine misfire is the most alarming sign of bad spark plugs. 

Engine misfire is caused by interference in the combustion. A bad spark plug does not properly transmit the spark needed by the ignition or distributor. 

4. Engine Hesitation

A bad spark plug cannot consistently transfer electrical currents. 

Many vehicle owners complain when their engine lacks power or stumbles during acceleration. This is because of the inconsistent supply of electrical current coming from the spark plugs. 

Testing the Condition of Spark Plug Wires

Different engine issues can cause the same set of signs and symptoms. 

Testing the condition of the spark plug wires is the best way to confirm the cause of the engine issues. Multiple tests, from simple visual examination to intensive checks, can be done to check for bad plug wires. 

Examine the Spark Plug Wire Condition

The first test a vehicle owner should do is a visual examination of the condition of the spark plug wires.

There are two things to look for when examining the spark plug wires, cracked or melted insulation. The insulation of spark plug wires is prone to drying out over time. It can also be damaged through contact with heated engine components. 

Check the entire length for any signs of damaged spark plug wires. 

Examine the Wire Connection

Misconnected wires can cause engine issues like engine surging and engine misfiring. 

Vehicles come with a manual depicting the engine wires’ route and connection. Compare the proper wire connection indicated on the manual with the current connection on the engine. The connection should look similar, if not exactly, to the one in the manual. 

Wire rerouting is needed if the current wire connection is not similar to the one indicated in the manual. 

Examine the Ignition Leads and Spring Chips

Turn off the engine and examine each ignition lead. 

Take the wires off the engine and examine them on the ground. Clean off any dirt with a clean rag to see any damage. Check for any corrosion on the insulation between the ignition coils, distributor, boots, and wires. After that, check whether the spring chips are fitted on the spark plug wires in the distributor. 

Proceed to the next tests if there are no visible damages to the spark plug wires. 

Check for Electrical Leaks

Place back any removed wires and components and turn on the engine. 

Snapping noise while the engine is running is a common indication of electrical leaks. Listen for any snapping noise around the wires, distributor, and ignition coils. 

Be careful not to touch the wires while the engine runs to avoid electrical shocks. 

Resistance Check

A multimeter is needed to perform the resistance check. 

spark plug test with multimeter
Video | ChrisFix

Remove the spark plug wires and attach the multimeter probes to each end. Check whether the resistance measured is within the range indicated on the vehicle manual. Reconnect the wires back to the engine if the resistance is within the normal range. 

Replacement of the wires and leads is needed if the resistance measured is not up to par. (1)

Spark Check 

A spark tester is needed to perform the spark check.

spark plug tester
Video | MrBundre

Remove the spark plug wire from the plug. Attach the wire end to the spark tester and attach the other end to an engine ground. Turn on the engine ground. Look for the presence of a spark on the spark tester gap. 

A weak spark is hard to see in daylight and is orange or red. On the other hand, a good spark is indicated by the presence of a blue-white spark that’s visible in daylight. The ignition system is functional is a good spark is observed. (2)

Remove the coil wire from the distributor cap if no spark is observed. Attach the distributor end of the coil wire to the spark tester. Turn on the engine and observe for a spark. If a spark was observed, bad spark plugs or issues with the distributor cap or rotor are expected.  

Wrapping Up

Vehicle owners generally know when something is wrong with their vehicles. 

It’s not uncommon for vehicle owners to be alarmed by car performance issues like reduced gas mileage and rough engine idling. The key to preventing damage to the engine is finding the cause of the problem. 

Observe any symptoms of bad plug wires to determine if there are issues with the electrical and ignition system of the vehicle. Multiple tests can be done on the spark plug wires to confirm whether it is the cause of the issues.

Vehicle owners can start on the necessary repairs once they confirmed the presence of faulty spark plug wires.

Take a look at some of our related articles below.

(1) resistance measured –
(2) ignition system –

Video References



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About Sam Orlovsky

b1d87d2ee85af3e51479df87928bdc88?s=90&d=mm&r=gCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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