How to replace your spark plugs

The NGK Difference

All NGK ignition coils must pass extensive testing procedures and quality checks through-out the manufacturing process to ensure fit and performance.

  • Validation lab certified to ISO17025 to maintain calibration standards
  • Electromagnetic interference testing to confirm that the coil does not interfere with other vehicle parts
  • Mechanical vibration testing
  • Accelerated life testing at higher voltages and frequencies to identify and correct failure points

Why does an engine need one?

Without a coil pack, the spark plug wouldn’t receive a voltage high enough to do its job of igniting the fuel/air mixture in the combustion chamber. The pressures are so high in the cylinder that the voltage has to be extremely high for the spark to be effective. A coil that is going bad can deliver a voltage that only fires the plug under certain conditions, which is what causes an intermittent misfire.


Common causes of ignition coil problems

Ignition coils can fail for a variety of reasons. They are not considered wear items, but often, age/use is the cause — thousands of miles of vibrations and high temperatures can lead to failure of the internal parts. 

Moisture damage is also very common — usually from leaking engine oil, but also from other fluid leaks or condensation. 

Lastly, spark plugs that are excessively worn can put extra stress on the ignition coils and lead to premature failure. This is because as spark plugs wear out, the spark plug gap widens. This means the ignition coil will need to provide a higher voltage to overcome this gap. Having to repeatedly produce a higher voltage can put a strain on the ignition coil and cause it to fail prematurely.

A little dab’ll do ya

Before replacing the coil, add a dab of dielectric grease to the tip prior to installing it over the spark plug terminal. Plug it back in to the ignition wires. You will hear and feel a click when it’s seated properly on the plug. Screw down the coil to the manifold, repeat this step for all cylinders, then, replace the manifold cover when you’re done.

Why does a coil fail?

Ignition coil packs hate heat and vibration, so a hot engine bay is a challenging place for it to live. Over time, the temperature and shaking can break up the coil’s windings and insulation. If the internal insulation breaks down, it can cause a short in the winding, limiting the amount the voltage gets stepped up.

Another way coils can fail is by developing cracks in their insulated case. These cracks can then allow moisture in to short out the windings intermittently, causing rough running.

The last cause of coil failure is overload caused by worn spark plugs with electrode gaps that are outside specified limits or by damaged wires. In time, the needed voltage can rise to levels, causing the coil to overheat and short circuit.

How much does it cost to replace ignition coils?

A simple coil-on-plug can cost anywhere from a couple hundred dollars, where a cassette-style system can be upwards of $1000. The cost is dependent upon the style of coil as well as the difficulty of access.

Prep is key

Lay a towel over the fender before you get started to protect the paint as you will be leaning over the engine for some time. On this particular car, we needed to remove four allen bolts to reach the plugs. Remember to mark the location of each wire before you remove anything. Likewise, you can take a quick picture with your phone for safety.

On modern cars, coil packs are used in place of distributors and ignition wires because they provide more efficiency and reliability by using no mechanical or moving parts like distributors. The coil pack is a collection of ignition coils controlled by your car’s electronic ignition that transforms the power from your cars battery down to the spark plug, which of course ignites the fuel and drives the pistons. Putting a wrong wire or in this case, a wrong coil pack on a plug, is going to make the engine run terrible or even not at all, so keep track of which plug goes on which cylinder. Remove the bolt holding the ignition coil. Next, unplug the ignition wires connected to the coil. Now, remove the coil by lifting straight up and out of the manifold.

Can you fix this on your own?

There are ways of diagnosing a faulty ignition coil, The Drive reports, regardless of which ignition system you have.

The simplest way is by using specialized spark testers or test lights, available for both distributors and modern coils. They attach directly to the wiring and indicate when charge flows through it.

Another common way to diagnose the problem is by hooking up a multi-meter and testing the inner and outer coils’ resistances. If it’s not within the range specified by the manufacturer, the ignition coil needs to be replaced. However, it’s possible for bad coils to still pass this test. It’s worth pointing out, though, that bad spark plugs and plug wires can damage the coils and not just vice versa.

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Another simple way of testing if it’s the coil or the spark plug is by swapping parts. Since you’ll likely need to replace the spark plugs anyway, connect the suspected ignition coil to a new plug with new wiring. If it still doesn’t fire, it’s the coil. Or, simply unplug the wires from each cylinder one at a time. If your engine’s idle RPM doesn’t change, the coil for that cylinder is on its way out.

Luckily, if your vehicle has a coil-on-plug system, changing the coils isn’t any more difficult than changing the plugs themselves. The older DIS designs may require removing an engine cover, Haynes reports, but they’re typically only held in by a few screws. Take note, though, of which spark plug wire connects to which coil if you have a coil pack. Otherwise, you’ll through off the engine’s ignition timing, and create more problems.

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