Rough idle and misfires are some of the signs that you may need to pay attention to your spark plugs. Worn-out or damaged spark plugs can lead to frustrating driving experiences or even engine damage if ignored for long enough.
Read more to find the signs of worn or failing spark plugs, how long spark plugs last on average, and how to budget for their replacement.
What Does a Spark Plug Do?
Spark plugs are parts of an internal combustion engine that ignite fuel. The ignition process begins with electricity from the battery that goes to the ignition coil, and then to the spark plugs.
Electricity goes through the spark plugs and jumps a gap at the end of the spark plug, which is called an arc of electricity (if the current is high enough). The arc is the spark that ignites the flammable mixture.
Spark plugs can fail for a variety of different reasons. Anything that disrupts the electrical current to the spark plugs from the ignition coil causes the plugs to malfunction, including a buildup of oil, fuel, or carbon.
An incorrect gap will likely cause problems. Spark plugs wear out – an old spark plug may just have a larger gap.
Some people choose to “gap” their spark plugs or use a tool to bend the electrode to have the correct gap. This is something that a knowledgeable vehicle owner can do to assist in prolonging the life of the plugs.
Sometimes, the wrong spark plug has been put into a vehicle, and the heat range is off. This will also cause problems.
Of course, damaged plugs may not work. Plugs could be corroded or burned due to their long life or harsh conditions.
What Does a GOOD Spark Plug Look Like?
Here is a representative used spark plug that is also considered to be a “good condition”.
Light gray, yellow, or brown discoloration to the ceramic insulator portion around the center electrode and the clean side electrode.
Indicates normal combustion. Shows the ideal fuel-air mixture is burning properly in the cylinder.
There appears to be little oil consumption represented by light deposits on the insulator tip, but it is nothing to worry about, assuming the spark plug is firing well.
What Does a BAD Spark Plug Look Like?
Sooted/Carbon Fouled
The spark plug has a uniform layer of dark black carbon deposits.
This spark plug condition indicates the engine is operating too rich. Fuel is excessive.
This can be caused by: a clogged air filter, malfunctioning cold start fuel mixture controls, too many short trips, or the spark plug heat range is incorrect.
Oil Fouled
The spark plug is coated with a glossy black oil residue, which leaves wet deposits.
This type of oil fouling indicates that too much oil is entering the combustion chamber when the engine is running.
The underlying causes could be: overfull crankcase, not functioning crankcase ventilation, or worn out piston rings and/or valve stem seals allowing oil to leak past.
Lead Fouling
The spark plug has glazed deposits of yellowish-brown color that cover its inner surfaces. It is possible for there to also be a greenish hue that may be surficial evident. This glaze appears to be baked on and can be from a very thin coating to thicker layers that have partially flaked off.
This lead fouling is the result of contamination caused by contaminated leaded fuel, either from fuel with lead or leaded components of aftermarket fuel additives.
Ash Fouling
The spark plug appears coated in a thick off-white ceramic-textured material. This ash fouling is very brittle and is easily removed with a nail or a pocket knife blade.
This type of ash fouling will form with excessive use of additives, whether from fuel treatments, oil detergents/stabilizers, or aftermarket engine chemicals.
The products used leave behind a superfine white ash when it is burned through the combustion process.
Center or Side Eroded/Melted Away
The spark plug has completely lost material on the center and/or side electrode. The electrodes should show eroded, rounded, or melted away electrode material. The ceramic insulator tip may also be blistered, softened, or cracked.
This type of thermal damage is caused when the plug overheats due to extreme combustion temperature. Possible reasons for the condition can relate to low fuel octane, excessive spark advance timing, pre-ignition events, lean air-fuel ratios, or incorrect spark plug heat range.
Heavy Wear of the Center Electrode
Upon examination, the spark plug center electrode shows significant erosion that has worn away significant or possibly all of the electrode material.
This significant wear indicates that the plug has worn past its usable service life. Other possible causes could include an excessively wide spark gap or an incorrect heat range spark plug.
Heavy Wear on Side Electrode
Upon visual inspection, the spark plug side electrode shows significant erosion and wear of the metal. In extreme cases, most of the spark plug electrode material has vaporized.
This excessive sideways wear is typically caused by detonation damage inside the cylinder. Potential causes of the detonation can be caused by low fuel octane, pre-ignition events, and excessive oil/fuel additive usage.
Center Electrode Insulator Nose Fractured
Upon visual inspection, the spark plug ceramic insulator nose around the center electrode has a piece chipped or fractured off. The breakage creates a gap area or missing section from the insulator tip.
There are possible causes for this breakage and material loss structure, including dropping or damaging the plug, excessive conductive deposits building up and partially bridging to the center electrode, or simply the spark plug has reached its usable service life.
Symptoms Of Bad Spark Plugs
Because faulty plugs can cause misfires, there is an overlap with many of the general red flags. If you think you have a bad spark plug, it is best to have a mechanic check your vehicle sooner rather than later.
#1. Misfires.
A misfire is when there is an issue igniting the air-fuel mixture inside a cylinder. It can happen when the mixture ignites too early, too late, or not at all.
Unburnt fuel can lead to excessive exhaust emissions, which are harmful to the environment and make the tailpipe smell like fuel. It leads to engine damage if the issue is not addressed.
#2. Rough and/or Loud Idle (with Vibration).
An engine with bad plugs can have a rough or loud idle and may or may not move with it. The unburned fuel from misfires caused by plugs confuses the O2 sensors.
So, it removes fuel from the cylinder and turns the cylinder into a leaner mixture. This is why the rough idle occurs and causes more misfires.
During a rough idle, it is also possible to see the tachometer slowly jumping the RPMs. You may see the RPM jumping on the gauge on the dashboard, or hear it through the engine.
Higher pitch sounds are higher revs, and lower pitches are lower revs. The jumping can be related to the engine control unit (ECU) attempting to compensate for issues.
#3. Rough and/or Slow Acceleration.
Abnormal response to the gas pedal is highlighted when hard acceleration is needed. Hesitation when accelerating is another issue and is generally caused by misfires leading to a lean air-fuel mixture.
If you notice your car bog or stumble when you suddenly stab the gas, this may indicate you may need new spark plugs.
#4. Poor Fuel Economy.
A rich air/fuel combination and/or a misfiring cylinder is inefficient, which means wasted fuel. The vehicle needs to use more fuel to achieve the same level of performance that the driver is requesting.
Since some of the fuel is either escaping the vehicle unburned or combusting in the wrong portion of the system, where it does not contribute to any power.
It is always a good practice to keep track of the fuel consumed by your vehicle. There are many possible reasons for poor fuel economy. However, if the vehicle suddenly has a dip in fuel economy, then dirty and/or broken spark plugs may be the single reason creating the problem.
#5. Difficulty starting a vehicle.
Bad spark plugs can create a vehicle that stalls or fights to start. To overcome the inertia of a car that is sitting still takes a relatively large input of energy, which is why this is when they seem to struggle the most.
If the vehicle fails to start completely, the problem is likely the battery rather than a set of spark plugs, since spark plugs do not usually fail collectively.
#6. Check Engine Light.
A failed spark plug will misfire, which will cause the check engine light to be illuminated on the dashboard. The car is supposed to generate a code when the ECM fails to fire correctly, which you can check using an OBD2 reader.
Used from 1996 onward, there are a number of spark-related trouble codes that might appear (e.g. e.g. DTC P0351 and DTC P0352).
How Long Do Spark Plugs Last?
Spark plugs come with a recommended lifespan, and most vehicles fall in the 30,000 to 80,000 miles range, and even advertised “extended life” plugs will wear down.
Some plugs may be “extended life” or “long life” plugs and will usually last around 100,000 miles, and probably are severely worn down by then, meaning they may work, but not very well.
The best prevention is to check the spark plugs on the vehicle every 30,000 miles. If the plugs are accessible for your vehicle, you can remove each one, using a spark plug socket, and visually inspect them.
Good plugs should look like an older version of new plugs, but just a bit dirtier. They may have a red coating on the ceramic insulation, which most likely is from additives that are in some types of gasoline. This won’t affect the proper function of the plugs.
Fuel-fouled spark plugs will have a sheen around the side electrode and the tip electrode due to too-rich mixture (too much fuel for the amount of air in the cylinder) for the plug’s heat range being too low for the engine, or possibly faulty ignition. Carbon fouled spark plugs will have blackened side and tip electrodes.
Pre-ignition damage shows the side electrode has been burned away. Worn plugs will have either too big of a gap or some general signs of wear since proper plugs have a consistent gap.
Mechanical damage to the spark plugs can occur if the plug extends into the cylinder too far, which can do a number on the plug and make it look very hammered.
Average Spark Plug Replacement Cost
When replacing spark plugs, be sure to use the same type of spark plug that the vehicle originally had. There are many types – copper, single platinum, double platinum, silver, and iridium.
Each has its own strengths and properties best suited to certain vehicles, and of course, the price varies with some types being more expensive. Check your vehicle’s manual or ask an authorized dealer what type your vehicle should have.
Some people will only change the spark plug that is presenting the problem (chances are this will be identifiable via the code that the ECU kicks out, or by visual assessment), and others may elect to change all the plugs at once.
Keep in mind that spark plugs are sold individually, so know how many you need before you go shopping. Most spark plugs are not more than $10 each.
If you are getting the plugs professionally installed, they will charge the labor, which may be from $50-$200, and you can do it yourself relatively easily for free. For the most part, on most vehicles, the plugs are easy to reach.
If you do this job yourself, you may want to take into consideration some sort of repair manual to determine the proper torque spec for the spark plugs.
Since most spark plugs are threaded into an aluminum head, because aluminum is a fairly soft material, it is relatively easy to strip the head by cross-threading and over-tightening the plugs.
This means either you need to re-tap a bigger hole, or you need a new head, which would not be a cheap mistake.