How Do Bad Spark Plug Wires Affect Your Vehicle? (7+ Issues)

Even though spark plugs are a small part of your vehicle, they can be the heart and soul of your engine. They are one of the main components of the ignition system.

When the spark plug wires are bad, the spark plug performance drops, and it could lead to a lot of dangerous problems. This came up a lot when I worked as a mechanic and today I’ll show some of the things I ran into.

In general, when spark plug wires go bad, your vehicle will show symptoms such as

  • Reduced gas mileage
  • Slow starts
  • Engine misfire
  • Rough engine idle
  • Engine hesitation
  • Check the Engine Light ON
  • Slow acceleration

 All of these issues will affect the vehicle’s engine performance.

Issues When Spark Plug Wires are Bad

spark plug in zoom
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Spark plugs play a major role in a vehicle’s ignition system. They provide the spark for the air/fuel ratio. This allows pistons to move downward and create power for the engine. But when the spark plug wires go bad, this process gets interrupted, and it will lead to many bad outcomes.

1. Reduced Gas Mileage (MPG)

gas mileage on a speeding car
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Reduced gas mileage is the most common issue with bad spark plug wires. If you aren’t familiar with MPG, here’s a quick explanation.

What is MPG?

Your vehicle’s gas mileage is measured in MPG. Sometimes, you might have heard the vehicle mechanic say 30 mpg or 20 mpg. For instance, if your vehicle has 30 mpg, it travels 30 miles per gallon of gas.

If your vehicle’s gas mileage is less than normal, it could indicate bad spark plug wires. As I mentioned above, when the wires are bad, the engine performance gets reduced. This performance deduction directly affects the use of gas. That means more gas will be used than the normal circumstances. Hence, you’ll experience reduced gas mileage.

Slow Start

slow start gauge meter
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You might have difficulty starting the vehicle when the spark plug wires are bad. This happens because of the poor performance of the pistons. As you already know, spark plugs provide the spark to the air/fuel mixture to operate the piston. When the plug wires are bad, the spark is less powerful. It will cause starting issues.

2. Engine Misfire

engine misfire
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You’ll have to deal with engine misfires when the vehicle’s spark plug wires are bad. Bad or damaged wires won’t produce the needed power to the cylinders, and it could lead to an engine misfire.

What is Engine Misfire?

An engine misfire can happen when one or more cylinders don’t produce the required power. It could lead to sudden power loss and acceleration issues. An engine misfire is a serious situation you must take care of immediately.

3. Rough Engine Idle

engine idle and hesitation
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Rough idle occurs because the engine cannot fire when the vehicle is parked and no gasoline is applied. Most vehicles tend to idle between 600 RPM to 1200 RPM. When the spark plug wires are bad, it prevents the engine from firing consistently due to a lack of power. Hence, the vehicle will idle below or above regular speed. You might feel a vibration or shake.

4. Engine Hesitation

Engine hesitation can occur regularly when the air/fuel ratio is too lean. That means the engine does not have enough power from the spark plug’s operation. And the faulty spark plugs or wires can mess up the ignition system’s air/fuel ratio.

5. Check the Engine Light ON

engine light icons
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Your vehicle’s dashboard shows different types of indicator lights. The check engine light is one of the main indicator lights you can spot on the dashboard. If the check engine light is ON, that might be because of the bad spark plug wires. When the spark plug wires are bad, they will disturb the spark plug operation, and eventually, the engine won’t get enough power from the spark plugs. All these occurring will lead to the check engine light flashing.

6. Slow Acceleration

acceleration gauge
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When the spark plug wires are bad, your vehicle’s acceleration will be affected. You might experience slow acceleration than usual. As you understand, less power will lead to less acceleration.

Testing Spark Plug Wires

testing spark plug wires
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You now know how bad spark plug wires can affect your vehicle. Hence, you can easily identify the symptoms of bad spark plug wires. If the spark plug wires are bad, you should replace them quickly before any more dire issues pop up. But before replacing it, you should confirm this issue. Here are a few simple steps to check spark plug wires.

Important: Bad spark plug wires can cause trouble for many vehicle components. For instance, they affect spark plugs, fuel filters, and ignition coils. All of these parts are major components of the engine. Hence, the engine will get damaged eventually.

Things You’ll Need

  • Digital Multimeter
  • Screwdriver
  • Vehicle’s instruction manual
  • Clean rag
  • Measuring tape
  • Water and spray bottle

Step 1 – Visual Inspection

First, turn off the vehicle’s engine and access the ignition leads. Clean any dirt on those ignition leads. Check the spark plug wires for any physical damage. This is the first thing you should do, and if the spark plug wires are damaged, replace them soon as possible.

Step 2 – Spray Bottle Test

In this step, you can check for any electrical leaks. However, detecting electrical leaks is difficult, I use a spray bottle with water for this step.

Spray the water on the distributor and the ignition leads. And spray water on the spark plug area too. Remember to keep the engine ON during this process. Check for any electrical leaks and arcing.

Step 3 – Ignition Lead Resistance Test

If step 1 and 2 doesn’t give you good results. You can always check the resistance of the ignition leads using a digital multimeter. First, set the multimeter to the ohm setting and check the resistance of the ignition leads. Then, compare the test value with the manufacturer’s standard values.

Video References

Auto V Fix

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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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