7 Symptoms Of Bad Spark Plugs and How To Read It!

The most common symptoms of bad spark plugs include start-up trouble, rough idling, sluggish acceleration, declining fuel economy, engine misfiring, and engine knocking. Failing to replace your spark plugs on schedule leads to poor performance in the short term, and potentially to serious damage in the long term.

For optimum performance, your engine’s spark plugs should be clean with no damage to the electrodes. If your spark plugs become dirty or fouled, it can affect how your car engine runs. A fouled or bad spark plug is a plug that has become covered with a substance like oil, fuel or carbon or one that is blistered from running too hot. Driving with fouled or bad spark plugs can cause a host of problems for your engine.

Keep reading to see if a simple spark plug swap can keep your vehicle and engine at their peak performance.

Symptoms Of Bad Spark Plugs

The most common signs of bad spark plugs include:

  • Trouble Starting the Engine
  • Rough Idling
  • Poor Or Sluggish Acceleration
  • Declining Fuel Economy
  • Engine Misfiring
  • Engine Knocking

1. Trouble Starting the Engine.

Having trouble starting your vehicle? While your first thought may be a dead battery or an empty gas tank, bad spark plugs could be the cause. If your engine takes a long time to start or won’t start at all, your spark plugs may be to blame.

If spark plugs have become clogged or worn down, the car’s engine has to work overtime in order to make up for the overworked plugs. Sometimes the car will not start at all because there is simply not enough spark created to turn the engine over. When the spark plugs are in this sad state, even harsh weather conditions can keep the engine from turning over.

2. Rough Idling.

An engine’s failure to turn over is not the only sign that the spark plugs are in dire need of replacement. If a car owner hears that the engine is emitting annoying knocking and rattling sounds, they should not put off inspecting it. These sounds mean that the car’s engine is roughly idling.

In addition to strange noises, an engine that isn’t idling properly may send noticeable vibrations throughout the vehicle. These vibrations can be disconcerting, to be sure, but they can also cause damage to sensitive components if left untreated.

3. Poor or Sluggish Acceleration.

If your vehicle has lost its get-up-and-go, bad spark plugs could be the culprit. When a spark plug becomes fouled or dirty, it doesn’t spark effectively which can cause your car to perform sluggishly.

It takes longer to speed up if your spark plug isn’t working properly. Even if you firmly press on the gas pedal, your vehicle may hesitate if you have a fouled spark plug. Since the old spark plug isn’t burning the fuel properly, your engine won’t be able to keep up and accelerate smoothly.

4. Declining Fuel Economy.

Making more trips to the gas station? A sudden drop in fuel economy can be traced to dirty or fouled spark plugs. If the spark plugs aren’t functioning properly, it can have a negative impact on your gas mileage.

Your vehicle won’t use fuel as efficiently with a bad spark plug. When your spark plug doesn’t burn fuel, it doesn’t provide as much power to your engine, which will make your vehicle use more gas. If you notice that your fuel efficiency drops by 3 miles per gallon (0.78 L/km) or more, then there’s a good chance that your spark plugs are misfiring.

5. Engine Misfiring.

Maybe you won’t notice declining fuel economy or lackluster acceleration right away. However, you might still notice the cause of both these issues: engine misfiring. Misfiring is the common term for what happens when some or all of the fuel in the chamber fails to ignite. When combustion fails, you’ll notice that your engine cuts out momentarily or pulls you forward in a jerky, uneven way.

A misfiring engine must work harder and consume more fuel in order to deliver the power you need, resulting in declines to your vehicle’s fuel economy and performance. It can also lead to significant, lasting damage if the problem isn’t fixed.

6. Engine Knocking.

If you don’t address your bad spark plug symptoms in time, you may start hearing engine knocking, which is a very serious issue that will 100% lead to severe engine failure and mechanical damage of important components like piston heads, compression rings, valves, and the engine’s cylinders themselves.

7. Lit “Check Engine” light.

The light will come on if your spark plugs cause misfires. Your “Check Engine” light can come on for a variety of issues, so it may not necessarily mean the spark plug is the issue. Check the engine code by plugging an OBD code reader into the link connector port near the left side of your steering column to determine if the spark plug is your issue.

Common engine codes for misfires and spark plugs are P0-300 through P0-308. Some “Check Engine” lights won’t come on until your engine has a certain number of misfires.

What Do Bad Spark Plugs Look Like? (How To Read a Spark Plug)

Examining your engine’s spark plugs can give you an idea of how the engine is running. You can do this as part of the routine maintenance of your car, ATV, lawnmower, or boat, or if you suspect something is wrong with the engine.

The first step is removing the spark plug you want to look at. Follow these instructions on how to remove a spark plug. Take the spark plug in your hand and begin looking at it for signs of damage. Look at the end of the spark plug that was in the cylinder and examine it. You will likely see one of the following conditions.

How To Read a spark Plug

Normal spark plug.

Before we take a look at bad spark plugs, take a look at what makes a normal spark plug work correctly. On a new spark plug, the surfaces will be clean, with a regular silver metal finish to all metal surfaces, matching the threads on the outside. The copper core and electrode are surrounded by a while insulator.

In good condition spark plugs that are used, the surface of the metal tip of the spark plug may be slightly gray or ashy in appearance, but this is normal and is just a byproduct of being a component in the combustion chamber.

Carbon fouled.

Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled plug. This can be caused by a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture or idling your vehicle for too long.

Your mechanic can offer you advice on what type of spark plug to buy to replace the damaged plug, but you may want to consider switching to a “hotter” spark plug (the higher the spark plug number, the hotter the plug).

Oil Fouled.

If there is oil leaking in your engine down past worn piston rings or through the valves on your cylinder head, the spark plug may be oil-fouled. Look for wet motor oil on the plug itself.

Oil-fouled plugs will be wet, whereas carbon-fouled plugs will be dry. You will need to address the oil leak to prevent fouling more spark plugs. An oil leak into the engine can cause significant issues. Bring your vehicle to a mechanic for diagnosis and repair.

Dry And Wet Fouling

Although there are many different cases, if the insulation resistance between the center electrode and the shell is over 10 ohms, the engine can be started normally. If the insulation resistance drops to 0 ohms, the firing end is fouled by either wet or dry carbon.

Look For Blisters or Burning.

If you have been having overheating issues with your engine, you may burn out a spark plug. Look for blistering on the insulator tip of the spark plug or signs of heat damage such as melted plastic or burned metal.

A heat-damaged spark plug will have to be replaced. Overheating issues may be caused by worn-out or insufficient levels of coolant. Add coolant if your vehicle is low, otherwise, you may need to drain and flush your radiator.

Erosion, Corrosion, and Oxidation

The material of the electrodes has oxidized, and when the oxidation is heavy, it will be green on the surface. The surface of the electrodes is also fretted and rough.

Worn electrodes.

A severely worn spark plug is simply a plug that has been in use for too long. Older vehicles that have never had their spark plugs replaced may experience spark plug failure without fouling them in the ways listed above.

Severe wear can cause a spark plug to come apart as you remove it. You may also see signs of eroded connections or cracking plastic. Severely worn plugs must be replaced.

Broken electrodes

If the electrodes are broken off or flattened, it is likely that the wrong spark plugs are installed.  A spark plug that is too long can cause extensive damage to your engine while a short spark plug can cause poor gas mileage and spark plug fouling. Check your owner’s manual to ensure that you have the correct spark plug for your vehicle.

Related Posts: How Much Does it Cost to Replace Spark Plugs?

Next steps

If you’ve noticed any of the following signs and symptoms of bad spark plugs, a trip to your trusted mechanic is in order. Most problems that are a result of a bad spark plug are relatively easy to fix. However, continuing to drive with fouled or dirty spark plugs can lead to more serious problems that are more costly to resolve.