What are symptoms of a bad spark plug?
A bad spark plug can cause a range of issues with your vehicle's engine performance. Identifying the symptoms of a bad spark plug early on can help you address the problem before it leads to more serious and costly repairs. In this article, we'll explore the common signs that indicate a spark plug may need to be replaced.
Rough Idling or Misfiring Engine
One of the most noticeable symptoms of a bad spark plug is a rough or uneven engine idle. If the spark plug is not firing properly, it can cause the engine to misfire, resulting in a rough, shaky idle. This can be especially noticeable when the engine is first started or when idling at a stoplight.
Decreased Fuel Efficiency
A faulty spark plug can also lead to decreased fuel efficiency. When a spark plug is not functioning correctly, it can cause the engine to burn fuel less efficiently, resulting in lower gas mileage. This is because the engine is not receiving the proper spark to ignite the fuel-air mixture effectively.
Difficulty Starting the Engine
If you notice that your vehicle is taking longer to start or is struggling to start at all, a bad spark plug may be the culprit. A worn or damaged spark plug can have trouble generating the necessary spark to ignite the fuel, making it difficult for the engine to start. This can be especially problematic in cold weather when the engine needs a strong spark to start.
Lack of Power and Acceleration
A malfunctioning spark plug can also lead to a noticeable decrease in engine power and acceleration. When a spark plug is not firing correctly, the engine may not be able to generate the full power it is capable of, resulting in sluggish acceleration and reduced overall performance. This can be particularly evident when trying to merge onto a highway or pass another vehicle.
Increased Emissions and Exhaust Smoke
Finally, a bad spark plug can contribute to increased emissions and visible exhaust smoke. If the spark plug is not igniting the fuel-air mixture properly, it can lead to unburnt fuel being expelled through the exhaust system, resulting in higher emissions and potentially visible smoke. This can be a sign of a more serious engine issue that should be addressed promptly.
If you notice any of these symptoms, it's important to have your vehicle's spark plugs inspected and replaced as needed. Addressing a bad spark plug early can help prevent further damage to your engine and maintain optimal performance and fuel efficiency.
How does a car act when it needs new spark plugs?
When a vehicle needs new spark plugs, you'll likely experience a combination of performance issues, such as a rough idle, sluggish acceleration, and difficulty starting. Other common signs include a noticeable drop in fuel economy, engine misfires, and the "check engine" light illuminating on your dashboard.
Performance and efficiency issues
- Rough idle: The engine may shake or vibrate when idling.
- Poor acceleration: Your car will feel slow and unresponsive when you press the gas pedal.
- Engine misfires: You may feel a jolt, stutter, or loss of power because the spark plugs are failing to ignite the fuel-air mixture.
- Poor fuel economy: With a bad spark plug, your engine won't burn fuel efficiently, leading to more frequent stops at the gas pump.
- Difficulty starting: The engine may take longer to turn over, stall when starting, or not start at all.
This video demonstrates the symptoms of a bad spark plug: 58sEasyAutoFixYouTube · Mar 5, 2023
Warning indicators
- Check engine light: The "check engine" light on your dashboard can come on for many reasons, including faulty spark plugs.
- Engine noises: You might hear unusual sounds like knocking or pinging coming from the engine.
- Exhaust smell: A raw fuel smell from the exhaust can indicate that unburned fuel is exiting the engine.
What to do
- Check your maintenance history: Refer to your owner's manual to see when the spark plugs were last replaced. Spark plug lifespan varies by type, from about 25,000 miles for copper plugs to over 100,000 miles for platinum or iridium.
- Visit a mechanic: A qualified technician can inspect your spark plugs and other components to confirm the issue and replace them to restore your engine's performance.
How do you tell if a spark plug is bad?
You can tell a spark plug is bad by checking for engine symptoms like rough idling, misfires, and poor acceleration, or by visually inspecting the plug for damage, wear, or residue. Other signs include a decrease in fuel economy, difficulty starting the engine, and a lit "Check Engine" light.
This video explains how to identify a bad spark plug by observing its condition: 1mMotorWeekYouTube · Apr 4, 2019
Engine performance issues
- Rough idling: The engine shakes or vibrates more than usual when the car is stopped or in neutral.
- Engine misfires: You may feel a jerking or stumbling sensation, particularly when accelerating.
- Difficulty starting: The engine may crank for a longer time or require multiple attempts to start.
- Sluggish acceleration: The car may feel slow to respond and have a noticeable lack of power.
- Poor fuel economy: You might notice that you are filling up your gas tank more often than usual.
Visual signs of a bad spark plug
- Damage: Look for cracked porcelain or electrodes.
- Wear: The tip of the spark plug should have a clean, defined edge. A rounded-off or worn-down electrode indicates it needs replacement.
- Contamination: Spark plugs should be free of heavy deposits, oil, or fuel. Excessive black soot or oil on the plug suggests a problem.
Other indicators
- "Check Engine" light: The check engine light can illuminate to indicate an engine misfire, which is often caused by a faulty spark plug.
- Engine knocking: A pinging or knocking sound coming from the engine can signal a problem with the spark plug.
This video demonstrates the importance of inspecting spark plugs for signs of damage: 52sDriver's TherapyYouTube · May 22, 2025
How to check
- Locate and remove the spark plug: This typically involves removing the spark plug wire or coil, then using a special socket to unscrew the plug.
- Inspect the plug: Visually check for the signs of damage, wear, or contamination mentioned above.
- Check the gap: Use a spark plug gap tool to measure the distance between the center and ground electrodes. This should match the specification in your vehicle's owner's manual.
This video shows how to check spark plug wires and coils: 44sChrisFixYouTube · Apr 21, 2014
What will happen if my spark plugs go bad?
When spark plugs go bad, they can cause rough idling, poor acceleration, and engine misfires, leading to poor engine performance and reduced fuel efficiency. Other signs include hard starting, the check engine light illuminating, and potential engine knocking or catalytic converter damage over time. These symptoms occur because a faulty spark plug cannot create a strong enough spark to ignite the air-fuel mixture efficiently.
This video explains the symptoms of bad spark plugs: 28shaile car care YouTube · Apr 23, 2023
Performance issues
- Rough idling: The engine may shake or vibrate noticeably when the car is stopped, as the engine struggles to run smoothly.
- Poor acceleration: The car may feel sluggish and hesitate when you press the accelerator.
- Engine misfires: The engine may sputter, hesitate, or feel like it's skipping a beat, especially at low speeds.
- Hard starting: The engine may crank for a longer time before starting, or you may need to press the gas pedal to start it.
Efficiency and other warnings
- Reduced fuel economy: Incomplete combustion means unburnt fuel is wasted, decreasing your gas mileage.
- Increased emissions: Unburnt fuel also contributes to higher harmful emissions from the tailpipe.
- Check engine light: A misfire or other combustion issues will likely trigger the check engine light, which may be flashing if the problem is severe. A flashing light warrants stopping driving immediately to prevent damage.
- Engine knocking: In severe cases, continuing to drive with bad spark plugs can lead to serious internal engine damage, such as knocking sounds.
Can bad spark plugs cause shaking while driving?
Yes, bad spark plugs can absolutely cause a car to shake while driving, especially due to engine misfires. A misfire creates an imbalance in the engine's combustion process, which can result in vibrations felt at idle, during acceleration, or at certain speeds.
How bad spark plugs cause shaking
- Misfires: When a spark plug is worn, faulty, or improperly gapped, it fails to ignite the fuel and air mixture correctly.
- Imbalance: This failure causes a cylinder to misfire, which disrupts the smooth, even rhythm of the engine.
- Vibration: The uneven firing creates an imbalance that you will feel as a shake or vibration through the steering wheel, seats, or floorboards.
Other related symptoms
- Rough idling
- Loss of power, especially during acceleration
- Stuttering or jerking when you press the gas pedal
- Decreased fuel economy
- Check engine light may illuminate
What to do
- Inspect spark plugs: Check if the spark plugs are worn or if the wires/connections are loose.
- Check for misfires: A mechanic can use a diagnostic scan tool to read codes that may indicate a misfire and help identify which cylinder is having the issue.
- Consider related components: If replacing spark plugs doesn't solve the problem, other issues like a faulty ignition coil, vacuum leak, or issues with the fuel system could be the cause.
