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How to Tell If Coil Pack is Bad: Signs, Problems, and Solutions!

Bad Coil Pack Symptoms

A coil pack is a pack of ignition coils that is controlled by the car’s computer. Since the coil packs are electronically controlled by a computer, vehicles with coil packs don’t need condensers or distributors in its system. Its task is to start the combustion process by accumulating energy and when the car’s computer calls for it, it releases the voltage through the spark plug cables until it reaches the spark plugs. Coil packs are known to provide better engine performance compared to the distributors found in older vehicles. As impressive as it sounds, coil packs are also subjected to failures. Knowing how to tell if your coil pack is bad will be helpful to prevent other potential problems. Signs of a bad coil pack include engine misfiring, loud engine, lack of power, RPM drops, intermittent smoke from the exhaust, and illuminating warning lights. 

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How to Tell If Coil Pack is Bad: What happens to a car when the coil pack goes bad?


The coil pack is the one of the ignition system’s main components. Its task is to help the fuel ignite in the cylinders by providing a high voltage of electricity to the spark plugs. It is vital since when the spark is created at the spark plug, the fire must start from a high voltage supply since it is needed to burn as much possible fuel in the cylinder. 


The coil pack can build up energy as high as 50,000 to 75,000 volts whenever the combustion process begins. If the car’s computer or the engine control unit transmits the signal to the ignition control, the voltage from the coil pack will then be released through the spark plug cables then to the spark plug. But as the spark is travelling to the spark plug, it will jump to the ground from the spark plug gap which causes a fuel air mixture explosion inside the cylinder chamber. You can easily locate the coil pack by following the spark plug wires since it is normally installed in the top area, under the hood in most vehicles. 


Like most of the car’s components, the coil packs are also subjected to normal wear and tear and other issues. If it becomes faulty, there will be some signs or symptoms that can let you know how to tell if the coil pack is bad.   


Knowing how to tell if coil pack is bad can better help you understand what causes it and how to avoid other potential problems. Here are some of the signs of a faulty coil pack.

  • Engine misfires


When your engine misfires, it could be an indication that you have a bad or weak coil pack. It is also one of the earliest signs of a faulty one. Engine misfiring is also commonly known as hard starting. You will notice it every time you start your vehicle and you hear a sputtering or a coughing noise. 


A misfiring engine can lead your vehicle to shake or sound choppy. Jerking and spitting can also occur every time you drive at high speeds or vibration when it is idling at a stop light. A failing coil can cause a low-spark intensity that can result in reduced power or performance.

  • Rough idle


You will know if your vehicle is rough idling when you feel it bouncy or shaky while using it. You may also notice strange sounds and inconsistent RPM counts. It could also feel like your vehicle has lesser power when you are driving. If you are experiencing these symptoms, you need to have it inspected right away to fix the problem. 

  • Backfiring


When the unused fuel in the engine combustion cylinders exits through the exhaust pipe, it can cause your vehicle to backfire. This is one of the early symptoms of a bad ignition coil that causes the coil pack to become bad. When this happens, you will notice black smoke coming from the exhaust pipe. It is important that when you encounter this problem, you need to have it checked and fixed immediately to prevent it from becoming a problem that can be too expensive to fix.   

  • Visible damages


One of the ways to know how to tell if coil pack is bad is by doing a visual inspection. You can check your coil pack and inspect it for any damages or cracks and for loose connections. You need to open your hood and look for the coil pack. It is usually box-shaped that has corresponding coils for each cylinder. These coils need to be also checked regularly because they are also subjected to normal wear and tear since they are often exposed to heat and other driving conditions. 

  • Reduced fuel economy


If you notice that your car has been consuming more fuel than usual, it might be caused by a bad coil pack. A bad ignition coil won’t be able to provide enough energy to your car’s spark plugs. Your car will need to burn and use extra fuel to compensate for the problem. Using and burning extra fuel will be the only way for your car to continue running. 

  • Weak yellowish spark


A coil in good, working condition will produce a clear and bright bluish spark and it can be seen clearly in the daylight. You might also hear a loud cracking sound when it sparks. If it is producing a weak yellowish spark or something within that shade, then it is an indication that you have a bad or a weak coil.   


You can test the coil spark by using a fully charged battery since it is one of the ways to know how to tell if coil pack is bad. But since it involves holding the spark plug wire with tools, it is not recommended if you are not an expert. It is best that you have a professional check and test the spark for you. 

  • Illuminating check engine light


An illuminating check engine light can mean a lot of things and one of which is a bad coil pack. An easy way to know how to tell if coil pack is bad is to scan it using a car scanning tool. You usually will get a trouble code P0351 for a faulty coil. If you read this code, your coil pack may need a repair or a replacement. 


How to Tell If Coil Pack is Bad: What causes a coil pack to go bad?


Knowing how to tell if coil pack is bad can be useful but knowing what causes it to go bad can help you prevent it from happening. Like the other parts or components of a car, coil packs are also susceptible to wear and tear. They can age too and become overworked when some issues occur.  


The ignition coil can fail for a number of reasons such as heat, moisture, vibration, or it can be caused by some issues in the other parts of the ignition system. These coils are usually found on the cylinder head and it can be left exposed to heat and vibrations. Excessive exposure to these conditions can result in their insulating materials to break down and cause an internal coil failure. 


Faulty spark plugs and faulty spark plug wires can also cause the coils to become damaged. The coils will have to work harder and will require more voltage if the spark plug wires have too much resistance or the spark plugs are worn out. When this happens, it can significantly reduce the lifespan of the coil. 


In addition, the spark plug might be supplied with less than its needed electricity when a coil becomes bad. This could lead the electricity produced inside the secondary windings to search for a new path of least resistance to ground which is usually through the boot or body of the coil. 


When dirt, moisture, or oil has been connected electrostatically to the boot or insulator, carbon tracking occurs and makes a way to ground. If a carbon tracking is discovered upon inspection, the coil and its corresponding plug will have to be removed and replaced. Ignoring and not replacing a faulty coil and its plugs can also damage the car’s computer and other components such as the catalytic converter. Repairing and replacing the affected parts can help recover the vehicle’s lost power and reduced fuel economy.  


How to Tell If Coil Pack is Bad: How do you test a coil pack?


Knowing how to test a coil pack can be as important as knowing how to tell if coil pack is bad. Modern ignition coils and coil packs are generally sturdy but like any other car parts, they can fail and will begin to show symptoms like the ones mentioned above. However, these symptoms can also be caused by other issues and other faulty car components. How to tell if a coil pack is bad? You need to test the ignition coil to determine if it is the one that is faulty or not. 


Don’t be confused why you need to test a coil to determine a faulty coil pack. Experts usually use the term coil pack when referring to any type of modern ignition coil. A coil pack is basically a group of ignition coils put together in one molded block. Each of the cylinders will get its own ignition tower in this design but its coil will be shared with a companion cylinder. But this one should not be confused with the COP or coil-on-plug ignition system since with this system, each cylinder will have its own ignition coil that can be found on top of the spark plug. 


There are several ways to test a coil pack and determine if it is indeed faulty.

  • Visual check


If you want to check if your ignition coil has no visible damage, you have to perform a visual check. Ignition coils or coil packs’ location can vary from vehicle to vehicle. You will want to check your owner’s manual to make sure that you can find it. If ever you do find it, be careful or wear protective gears to make sure that you don’t harm or electrocute yourself. 


If you found the coil, check and inspect it for any signs of damage such as cracks, burn marks, and carbon tracking . Check the ignition wirings for any wires that are deteriorated or damaged. You will also have to check the coil harness and connectors for loose connections and bent terminal pins. You also need to check for any signs of moisture since it can also cause problems.

  • Spark tester


Using a spark tester can also help you tell if a coil pack is bad. A spark tester can be installed inline with the ignition coil to check if the coil can create a spark. Make sure that you get the right spark tester for your car since not all testers can work with all types of ignition systems. 

  • Ohmmeter


Measuring the primary and secondary resistance of the coil using an ohmmeter can help determine if it is in good working condition or not. To check the primary resistance, you need to set the ohmmeter to the 10 ohms range and touch a meter lead to the middle prong on the coil pack’s electrical connector. A good coil pack should read between 0.3 and 1.0 ohms. 


For the secondary resistance, you will have to connect the ohmmeter to the high voltage output terminal and the positive terminal but this time, the ohmmeter should be set to 20,000 ohms . If you read between 5,000 and 10,000 ohms, then it’s good.. 


To ensure that you get the right ohm values for your coil pack, you need to refer to your owner’s manual or search your engine online to know what the appropriate readings should be.   


Once you know how to tell if coil pack is bad and you find out that you really have a bad coil pack, it is important that you have it fixed immediately. A faulty one can damage other components of your vehicle. It can also reduce its performance, its fuel economy, and it also makes your vehicle hard and not safe to drive. 





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