The most common symptoms of bad spark plugs include difficulty starting your vehicle, rough idling, sluggish acceleration, reduced fuel economy, misfiring, and engine knocking. Ignoring scheduled spark plug changes can lead to underperformance in the short term, and possibly serious damage down the road.
For the optimal performance of your engine, your spark plugs should be clean and not damaged with the electrodes. Once the spark plug becomes dirty or fouled, it could lead to issues with the way your car engine operates.
A fouled or bad spark plug can be fouled with oil, fuel or carbon or blistered from operating too hot. Driving with fouled or bad spark plugs can create a whole host of issues for your engine.
So, keep reading to find out if a simple swap of spark plus can keep your vehicle and engine performing well.
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.
If your car won’t start, the first thing that comes to mind is that you might have a dead battery or nothing in the gas tank. However, bad spark plugs may be the issue. If your engine takes forever to start or won’t start at all, the spark plugs may be the cause.
When spark plugs are clogged or worn out, your car engine has to work overtime to make up for the poor functioning spark plugs. Sometimes, it won’t start at all because simply there is not enough spark created to turn the engine over.
With spark plugs this bad, harsh weather conditions might not be able to keep your engine from turning over.
#2. Rough Idling.
An engine that won’t turn over is not the only indicator a vehicle’s spark plugs are worn out. If an owner hears their engine creating bothersome knocking and rattling noises, they should not wait to look into it – these noises indicate the engine is idling rough.
In addition to weird noises, an engine that is idling rough may transfer noticeable vibrations throughout a vehicle. The vibrations may be unpleasant and alarming, however, if left unaddressed, they can cause damage to sensitive components.
#3. Poor or Sluggish Acceleration.
If your vehicle isn’t getting up and going it’s possible that bad spark plugs could be to blame. When a spark plug gets fouled or dirty it doesn’t spark like it should and makes your vehicle feel slow.
If your spark plug fails, it takes your vehicle longer to speed up. Even if you stomp on the gas pedal your vehicle still feels sluggish with a fouled spark plug.
Since the old spark plug isn’t properly burning the fuel and allowing for the engine to run properly (to keep up and accelerate) therefore it still feels sluggish, since the spark plug isn’t creating the best fighting chance for the engine.
#4. Declining Fuel Economy.
Making more trips to the gas station? A sudden change in fuel economy can be attributed to dirty or fouled spark plugs. If the spark plugs aren’t working right, then your gas mileage can take a hit.
With a bad spark plug, your vehicle won’t utilize fuel as effectively. If your spark plug fails to combust fuel, it doesn’t supply as much power to your engine, which can force your vehicle to use more gas.
If you notice your fuel efficiency has decreased by three miles per gallon (0.78 L/km) or more, then there’s a chance your spark plugs may be misfiring.
#5. Engine Misfiring.
You might not realize right away that your vehicle’s fuel economy has dropped or that your acceleration isn’t what it used to be, but you will likely be aware of the reason behind both problems: engine misfiring.
Misfiring is what happens when some or all the fuel in the combustion chamber fails to ignite, and when that happens, you will literally feel and notice an engine drop-out or jerking movement.
When an engine misfires it has to work harder and consume more fuel to generate the power you demand, which is why you will have noticed drops in fuel economy and performance. Engine misfiring can also lead to lasting and considerable damage if it isn’t resolved.
#6. Engine Knocking.
If you wait to fix your bad spark plug symptoms, you will eventually notice the engine knocking.
This is a major problem that will definitely lead to extensive engine failure and or mechanical damage to important components such as the piston heads, compression rings, valves and the cylinders of your engine.
#7. Lit “Check Engine” light.
If your spark plugs cause misfires, the light will come on. There are various reasons your “Check Engine” light may come on. Just because your “Check Engine” light comes on doesn’t mean that your spark plug is at fault.
To see if you could be having an issue with your spark plug, you need to check the engine code by plugging an OBD code reader, which is a device that reads codes from the engine control unit, into the link connector port just to the left of your steering column.
The most common engine codes for misfires and spark plugs are P0300 – P0308. In some cases, the Check Engine light won’t come on until you’ve had a certain number of misfires.
What Do Bad Spark Plugs Look Like? (How To Read a Spark Plug)
Finding out how your engine is running will come from looking at your engine’s spark plugs. This will be effective as a part of the routine maintenance of your car, ATV, lawnmower, or boat, or with suspicion of problems.
The first step is to remove the spark plug you want to look at. Follow those instructions about how to remove a spark plug. Once you have or hold the spark plug, begin looking for damage. Look at the end that was in the cylinder first. You will find either one of these conditions.
#1. Normal spark plug.
Before seeing bad spark plugs, let’s examine what a normal spark plug looks like in working correctly. A spark plug that’s new will have a clean surface. All the metal surfaces have a uniform silver metallic appearance which is the same for the threads on the outside. The conductive copper core and electrode will be enclosed by a white insulator.
When used but in good condition, most the metal tip of the day spark plug may resemble gray or ashen metal, this is normal and is only a byproduct of being one of the components located inside the combustion chamber.
#2. Carbon fouled.
Black, dry soot on the electrodes and insulator tip indicates a carbon-fouled spark plug. This can be caused by a dirty air filter, excessive driving at low speeds, too rich of a fuel/air mixture or too long of an idle.
Your mechanic can provide you advice on what type of spark plug to purchase to replace that fouled plug, and you might also consider possibly going with a “hotter” spark plug instead (the higher the number of that spark plug, the hotter the spark plug).
#3. Oil Fouled.
If your engine has oil leaking past worn piston rings or through the valves in your cylinder head, your spark plug may also be oil-fouled. Check and see if there is any wet motor oil on the spark plug itself.
Oil-fouled plugs will be wet, while carbon-fouled plugs will be dry. You need to address an oil leak into the engine or you will continue to foul plugs with oil. An oil leak into the engine is a big problem, so you probably want to have your vehicle looked at by a mechanic.
#4. Dry And Wet Fouling.
Every situation can be different, but if the insulation resistance between the center electrode and the shell exceeds 10 ohms the engine would be able to start without issue, and if there is zero insulation resistance the firing end is fouled whether it be wet or dry carbon.
#5. Look For Blisters or Burning.
If you are having overheating problems with your engine, you might have burned a spark plug. Look for blistering on the insulator tip of the spark plug or look for heat damage (melted plastic or burned metal).
Once the spark plug is heat damaged, it needs to be replaced. Additionally, overheating issues could be due to worn or insufficient levels of coolant. If your vehicle is low on coolant, top off your coolant levels, otherwise you will need to drain and flush your radiator.
#6. Erosion, Corrosion, and Oxidation.
The electrodes have become oxidized, often resulting in a green color when the level is high. The electrode surface is also fretted and rough.
#7. Worn electrodes.
A spark plug that is extremely worn is simply a plug that has been in service too long. Older vehicles that have never had their spark plugs serviced can fail in a way that is not fouled as indicated by the above chart.
Extreme wear can result in a spark plug breaking apart as it is being removed. You may also have visible erosion, and possibly even cracked plastic, where an insulated plastic mothers the terminal connection. Whenever an electrode is visibly worn it must be replaced.
#8. Broken electrodes.
Electrodes that are broken off or flattened generally indicate wrong spark plugs were used.
When a spark plug is too long, it can severely damage your engine. If it is too short, you will experience poor gas mileage, excessive electromagnetic interference with the vehicle’s electronics, and possible spark plug fouling.
When in doubt, look up the correct spark plug for your vehicle in your owner’s manual.
Next steps
If you’ve seen any of the following symptoms indicating bad spark plugs, it would be a good idea to make an appointment with your mechanic.
Most problems related to bad plugs are fairly simple to fix. Continuing to drive with dirty or fouled spark plugs can lead to more significant and costly repairs.