When Should I Change My Spark Plugs?

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By |2018-06-19T15:49:25-04:00April 27, 2018 - 08:42AM|Maintenance|

Because of the way a typical car engine works, each spark plug fires once for every two engine rotations. That means the plugs have to spark 400 times per minute even though your car is barely idling at 800 rpm. But a popular SUV such as a 2018 Nissan Rogue has a maximum engine speed of 6,200 rpm. Pushing the Rogue’s engine that high would need more than 50 sparks per second. It’s a lot of stress on parts that play an important role in your car’s drivability.

The good news is that spark plugs can last for years, and when they need to be replaced, the process is relatively inexpensive.

What Do Spark Plugs Do?

To recognize the signs of a bad spark plug, it’s helpful to understand more about how good ones are supposed to work. Each spark plug screws partway into an engine cylinder. The section of the plug that sticks out is the terminal. It’s connected to the car’s ignition system, which provides electricity for the spark. Inside the cylinder, the plug has two electrodes exposed. The electricity from the ignition system first flows through the plug’s center electrode. A spark is then created as the electricity has to jump a small gap to reach the second electrode. This spark ignites the fuel-air mixture inside the engine cylinder. The result is a contained explosion that delivers power by pushing against the top of the piston.

How Do I Know If It’s Time To Change My Spark Plugs?

With the tip of the spark plug located inside the cylinder, it’s directly exposed to high pressures and high temperatures alike. That ongoing wear and tear can be the source of a number of issues. The most common include waste-deposit buildups on the individual electrodes and changes in the size of the gap between the two electrodes. This can affect both the strength and timing of the spark, leading to problems during combustion.

That will often lead to difficulties starting your car and rougher idling once it’s started. Listen for the engine seeming to miss a beat, or for knocking or metallic rattling noises. These can come from a piston fighting against the force of combustion when the spark occurs at the wrong time. For example, a spark plug is supposed to spark when the piston is at the top of its stroke, to make the most power on the downstroke. If the spark happens as the piston is still moving upward, there’s a powerful collision between the head of the piston and the expanding ball of burning fuel and air. It literally can shake the piston inside of the cylinder.

This will cause obvious performance and efficiency issues. With spark problems, all the fuel in the cylinder isn’t completely used up, so the engine makes less power. Then your car won’t accelerate as expected. It can begin to hesitate as well. Further, the unused fuel is just wasted. The outcome for drivers is a noticeable drop in fuel economy.

How Long Do Spark Plugs Last?

As a good rule of preventive maintenance, you should change your spark plugs at set intervals even if you never notice any problems. The best way to determine a schedule is by checking your owner’s manual or routine maintenance guide. As an example, consider a pair of late-model cars and trucks from mainstream automakers. The Bowtie brand tells owners to replace spark plugs in the current Chevrolet Silverado pickup after 97,500 miles. Spark plugs for recent Toyota Camry sedans are good for up to 120,000 miles.

Further, it’s important to know that different kinds of spark plugs will have different service lives. A key here is the kind of metal used at the tip of the plug’s center electrode. Older vehicles usually came with copper-tipped spark plugs, while today’s cars and trucks rely on platinum or iridium tips. More advanced spark plugs also have switched to using platinum or iridium tips for the ground electrode. Generally speaking, the plugs’ expected durability goes in the same order, too. Thus, the long-lasting 120,000-mile spark plugs for the Camry are iridium-tipped.

Another tip: Be sure to have the spark-plug wires checked and changed along with the spark plugs themselves. Those wires carry the electricity to the plugs, so if they don’t work right, the plugs won’t get what they need to spark correctly in the first place.

If you have questions about this story, please contact us at Editors@carfax.com

About the Author:

Charles Krome is a long-time automotive journalist who got his start working on dealer publications with General Motors. He has covered nearly all aspects of the industry since then and continues to explore auto-related topics for Carfax today. Away from work, Charles enjoys reading, photography and spending time with his family. Charles owes his interest in cars to his father, whose daily drivers when Charles was growing up included a Lotus Elan, a Jensen-Healey and a pair of Toyota Celica Supras.