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When Is It Time To Replace My Spark Plugs?

Depending on spark plug types and automakers using them, replacement intervals are specified at anywhere from 30k to 120k. We discuss how to know when your plugs really need replacement.

Spark plugs are a key ingredient of gasoline engines when it comes to serving up a dish of solid power. More specifically, they conduct electricity derived from the engine's ignition system - using it to create a small bolt of lightning to ignite the air/fuel mixture sprayed into each cylinder between the compression stroke and power stroke.

With the exception of diesel engines which utilize compression to ignite air/fuel mixtures, spark plugs are also used on engines that run on alternative fuels such as ethanol or other blends.

We describe a spark plug's firing as a miniature bolt of lightning because there's really no better way to liken it. And because lightning bolts, regardless of size, are destructive in nature, the ones occurring inside your engine take a toll on spark plugs over time.

In this article, we seek to tackle the commonly asked question, "How often do I really need to replace my spark plugs?" A look at ten of the best-selling vehicles in recent years shows a range of recommendations for spark plug change intervals based on the type of plugs engineers chose to use. This includes intervals of 30k, 60k, 100k, and even 120k miles.

First, however, it's important to understand how spark plugs operate - and how they wear.

Basic Spark Plug Operation

All spark plugs are assembled with an exterior shell that covers an inner central core made of metal. This core is known as the center electrode (or just the "electrode"). Another metal piece known as the "ground electrode" protrudes from the bottom of the threaded area, curving around and ending directly under the center electrode. The distance between these two points is known as the gap, and electricity travels between them whenever the spark plug fires.

Layout Of Typical Spark Plug
The layout of a typical spark plug.

The metal used to coat the tip of the ground electrode is how different types of spark plugs are known and defined - with popular examples being "copper", "nickel", "platinum", "double platinum", "iridium", and "silver". For more details on these variations, see our article Understanding The Different Types of Spark Plugs.

The center electrode gets its power via an ignition coil. Many modern vehicles feature one coil per cylinder; the older, traditional model relied on a single coil connected to an ignition distributor, which then fed the spark plugs via a set of spark plug wires.

How Spark Plugs Wear

No matter which metal is used to coat the spark plug tip, it will be consumed gradually over time as the engine is driven. As the coating wears down, the gap between the center electrode and the ground electrode widens - reducing the potency of the spark.

Spark Plug Tip With Metal Coating
Metal coatings on the spark plug tip (located on the ground electrode) are naturally consumed over time by use. As the coating wears down, the gap between the center electrode and the ground electrode widens - reducing the potency of the spark.

It's important to understand that projected lifespans for spark plugs are based on the premise that the engine using them is otherwise in top tune and enjoying ideal running conditions during the entire period. The reality is that improper combustion can end up occurring because of excessive ethanol mixed in by gas stations, incorrect air/fuel mixtures, cold weather, too-low octane fuel, or other issues. These take a toll on spark plug life. Short trips where engine temperatures stay low can foul spark plugs due to carbon buildup on the tip. If engine wear starts to allow oil into the combustion chamber, spark plugs can become fouled by this unintended oil.

If it's been awhile since you've changed your spark plugs, they're probably more worn than you suspect. Even if you haven't reached the full mileage interval between recommended plug changes, pay attention to how your vehicle runs. Look for the common warning signs that spark plugs are at the end of their life span. Worn spark plugs will tend to create rough engine idle, misfires, hard engine starts, reduced power, and higher fuel consumption. In most modern engines, a "check engine" warning light will even come on if misfires are present after three or four consecutive starts.

Spark Plug Pulled Out For Examination

If you've noticed any of these running conditions, we recommend pulling one of your plugs out for examination (see our related article on performing a tune-up which covers the removal process).

If your engine is equipped with spark plug wires rather than individual ignition coils, observe the condition they're in as you remove them to access spark plugs. Wires that are past their prime may feel crispy in your hands or come apart completely when you pull on them. Another way to test your wires is to examine your engine compartment at night, with the engine at idle. Any visible glowing or sparking from the wires means that current is escaping from the insulation.

Automakers may or may not specify a recommended replacement interval for spark plug wires, but a good set will typically last 60,000 miles. However, that may not apply in all circumstances.

One of the most common reasons for expensive catalytic converter failure is unburned fuel going out the exhaust. While there are many reasons for this to occur, it's often worn-out spark plugs at the root of the problem. Considering the cost of a typical catalytic converter is well over $1,000, keeping your spark plugs fresh can save you big money over the long run.

Other Factors That Can Affect Spark Plug Life

The tip of the spark plug should be relatively clean. Oil residue on the tip means you've got an internal oil leak. If you find oil on the plug and the engine is otherwise running okay, investigate the source of the oil. If you must delay any corrective repairs, be prepared to change oil-fouled plugs a bit more often.

As we mentioned earlier, driving your vehicle on mostly short trips where engine temperatures stay low can foul spark plugs due to carbon buildup on the tip. If this is the majority of your driving, be prepared to change spark plugs before the recommended intervals. Carbon buildup can also occur because of a clogged air filter, or fuel injectors that are dirty due to low-quality gasoline. A helpful best practice is to occasionally take the car on a higher-speed highway run.

Spark Plugs With Different Types Of Buildup On Tips

Another factor worth considering is that replacing your spark plugs more frequently is cheap insurance against a real problem that can develop when they remain seated in place in the cylinder head for a long, long time. By this, we mean the plugs can chemically react with the metal in the cylinder head, becoming seized in place. If this happens, the plugs are difficult (if not impossible) to remove without breaking them apart - requiring extra labor and/or cost to get them out. Note that aluminum cylinder heads are more prone to this issue, but any engine can be affected.

Opportune Times To Replace Spark Plugs

While the traditional tune-up has gone the way of the rotary dial phone thanks to advanced electronics, it still is a good idea to check and replace the few remaining engine service items at regular intervals. On our 21st century cars, this is limited to air filter, fuel filter, and spark plugs. One approach is to create a schedule, based on either time or mileage, and replace all these service items at the same time.

When it comes to spark plugs, the elephant in the room is spark plug access. On some vehicles, notably older ones, it couldn't be simpler: open the hood, and there are the plugs, which only require plug wire removal before you can put a socket on them.

On many modern vehicles, opening the hood reveals…. a large plastic cover, with no hint as to where the plugs may lie. If you're lucky, the plastic cover pops right off, and the plugs can be accessed once the coil pack is removed. In other cases, just getting to the plugs involves considerable time in removing additional shields, covers, brackets, and so on.

If your engine still has its original plugs, and there is a related service to be performed, such as coil packs, fuel injectors, or valve cover gaskets, this is an opportune time to tackle the plugs, as much of the disassembly is done already.

If you have good access to the plugs, one of the best ways to check them is to physically inspect them. This is our recommendation, separate from any stated mileage interval from the vehicle manufacturer. If your vehicle owner manual recommends replacement at 30,000 or 60,000 miles, inspect the condition of the plugs when you reach about two-thirds of that mileage.

If longer service intervals of 100,000 or more miles are recommended, we'd say check at the halfway point to see how things look. Assuming the vehicle continues to run well, check again at the three-quarters point. Any time fouled spark plugs can be seen or otherwise detected, go ahead and replace them immediately. Given the rather affordable price of new plugs, it could be a case of cheap insurance to toss in a new set.

Spark Plugs Variety Gallery

And if it's time to order new plugs, don't settle for anything less than the perfect ones for your vehicle and driving style. Give us a call - we'll be happy to make personalized recommendations!

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