Spark plugs are the little things that spark the fuel-air mix so the piston moves down. Without a spark plug, the combustion would not occur and there will be no movement. Simply said spark plugs are a very important component of your car’s engine. It is best recommended to replace the spark plugs every 30-50 thousand miles. You can refer to the owner’s manual about the correct interval change on your car but you get the idea.

How To Determine A Bad Spark Plug: Common Issues

Spark Plugs will cause the following symptoms on your car:

  1. Rough Idle
  2. Misfires
  3. Lag In Acceleration
  4. Staring Issues

Your car will have different symptoms when having bad spark plugs. Sometimes the OBD reader, commonly known as the computer scanner will not detect bad spark plugs because of different variables. Bad spark plugs will cause your car not to start as fast. You might think that is a battery issue but late starts sometimes are caused by bad sparks. 

Bad plug symptoms will also cause a misfire in your car. A misfire is the result of incomplete combustion (or zero combustion) inside one or more of an engine’s cylinders. You should be careful with misfires as the more you leave a bad plug work its way the more problems it will cause. Misfiring will lead to other problems such as worse acceleration or very rough idle.

If all that is not your case that a hesitating car is one of the signs of bad plugs. You are accelerating normally and the car hesitates to increase the RPM and sometimes lower the RPMs while you press the accelerator. All things combined will also cause bad fuel mileage. Because the engine will not run as efficiently the car will have more unburned fuel into the engine which can also cause other problems.

How To Check Bad Spark Plugs

When it comes to checking for bad spark plugs you need to consider 2 things before checking. The spark plugs are fed with electricity from the spark plug wires. If your car does not have spark plug wires then it uses coils set but they do the same job. Sometimes the bad plug wires cause the spark plugs not to spark at all so make sure to check them as well. 

How To Check Bad Spark Plugs Wires?

Coils or Spark Plug Wires transfer the 12V electricity from the battery to the spark plug but with a much higher voltage that goes up to 60,000V to make the spark plugs fire. Sometimes these spark plug wires can get clogged up with oil or carbon deposits.

Swap The Coils

You need an OBD reader for this.

The faulty plug wire symptoms are rough idle, bad starts and hesitation on acceleration. An easy process for you to determine is to swap the wires. The coils can be unscrewed after you remove the connector.  You swap the coil that comes up as defective on the code reader with another coil that does not show any codes. If the error code for the bad coil will go away and transfer to the other cylinder you have a bad coil. 

General Inspection

No special tools required.

To check for bad plugs and wires symptoms you can make a general inspection. If you see any signs of cracks, burns, melting, or leakage in older canister coils you are likely to have a bad spark and coil. 

Coil Hack 

No tools required. 

Have your car running and open the hood. Once open you will have access to the coils(some cars have plastic covers you need to take off), remove the coil connector from the first coil. If you see that the vehicle starts idling or running worse that means that you cut the electricity supply to a fully functional coil. Your car will also turn off but don’t worry, put the connector back and place and turn on the car.  On the other hand, if there is no change in the engine performance that means that the particular coil is having issues and needs to be replaced. Make sure you check all coils.

Other ways of checking your spark plug wires require tools that can be found in professional shops such as a multimeter. If your coils or spark plug wires seem to be in fully working condition you move on to spark plug checking diagnosis.

How To Check Bad Spark Plugs?

the new spark plug and the damaged spark plug

If you checked all the spark plug wires or coils and the same issue persists, spark plugs need to be checked. For this, there is no plug or unplug method but you have to remove the spark plug and carefully check for the following :

  1. Carbon Deposits 
  2. Melted Tip 
  3. Oil or Liquid Leaks
  4. Gap
  5. Gasket Seals
  6. Insulator 

Before you start removing the spark plug you first have to have the coil or spark plug wire removed. To remove the spark plug you need a magnetic socket wrench or any usual wrench.

Tip: Make sure to check if there are any liquid deposits like water fuel or oil over the spark because once removed you don’t want that to enter the cylinder head.  

Once you remove the spark make sure to check all the listed signs of bad spark plugs. Starting with the carbon deposits, if you see signs of carbon deposit over the spark plug electrode make sure to clean it as it causes the spark plug not to fully spark. 

Secondly, check for the electrode of the spark as it might melt and not have that round shape it used to when you bought it. This is caused because of mileage, not bad driving. Together with that make sure to check the gapping. If the ground electrode and the electrode are too far out the spark is likely not to occur. If they are too close they are likely to cause a little spark not powerful enough to fire the fuel-air mixture.

Thirdly check the body of the spark. If the insulator, usually ceramic or porcelain material has cracks, that is one of the signs of bad plugs. The spark plugs also have gaskets which can get loose over time.

Other Considerations

There are other cases like my 2005 Mercedes Benz E 240 that use 2 spark plugs for each cylinder. You need to check both spark plugs in this case for each cylinder. The case with my car was a very rough idle and the car was hesitating a lot when accelerating. I checked all the coils and they all seemed to work well. I had 2 spark plugs that had a huge gaping between the ground electrode and the main electrode. I cleaned the spark plugs from the carbon deposits but the car was still running rough so I decided to replace both spark plugs for that particular cylinder. 

Fix Bad Spark Plugs FAQ

Q. Do bad sparking plugs cause a misfire?

A: Yes bad sparking plugs will not ignite the fuel-air mixture and cause a rough idle and misfire.

Q. What are bad spark plugs and wires symptoms?

A: Rough idle, bad starts, or hesitation when accelerating are some of the most common symptoms from bad plugs and wires.

Q. Should I use anti-seize on spark plugs?

A: Premium spark plugs come with a layer that protects from corrosion and rust. Do not use anti-seize on these spark plugs. Anti Seize works as a lubricant altering torque values up to 20 percent, increasing the risk of spark plug thread breakage and/or metal shell stretch.

Final Thoughts 

We recommend you take care of your car and replace the spark plugs based on the interval recommended by the manufacturer.

The tips provided are DIY-friendly and easy to apply. Before you go and see your car to an expensive shop make sure you check all the steps mentioned.



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