Spark plugs and glow plugs are small components that make a big difference in how your vehicle runs. While they’re easy to forget about, keeping them in good condition is essential for reliable engine performance, fuel economy, and smooth starts. If you're not sure when to replace them or whether your car even has them, here’s what you need to know.
What Do Spark Plugs and Glow Plugs Do
Spark plugs are found in gasoline engines. They create the electrical spark that ignites the air-fuel mixture inside the engine's cylinders. Without them, the engine wouldn’t start or run.
Glow plugs, on the other hand, are used in diesel engines. Diesel fuel ignites under pressure, not with a spark. However, in cold conditions, the air inside the engine may not be hot enough for combustion. Glow plugs warm the combustion chamber to help get the engine started.
Both types of plugs are essential to the starting process and overall engine efficiency.
How Long Do Spark Plugs Last
Most modern spark plugs are designed to last between 60,000 and 100,000 miles, depending on the type and your driving habits. Iridium and platinum plugs tend to last longer than copper ones. However, spark plug life can be shortened by frequent short trips, extreme temperatures, or engine performance issues.
If you're noticing symptoms like rough idling, misfiring, or reduced fuel efficiency, your spark plugs could be worn out even if they haven’t reached their mileage limit. Replacing them can often restore lost performance and improve fuel economy.
Signs Your Spark Plugs May Need Replacing
Worn or failing spark plugs can create several noticeable problems. These include:
- Trouble starting the engine
- Rough or shaky idling
- Engine misfires, especially under load
- Sluggish acceleration
- Increased fuel consumption
- A check engine light
If your car is experiencing any of these symptoms, it’s worth having the spark plugs inspected.
How Long Do Glow Plugs Last
Glow plugs don’t typically last as long as spark plugs. Most last between 60,000 and 80,000 miles, but their lifespan can vary depending on the brand and how often they’re used. Diesel vehicles in colder climates tend to use glow plugs more frequently, which can lead to faster wear.
Since glow plugs are essential for starting a diesel engine in cold weather, failing ones often reveal themselves during the first cold mornings of the season.
Signs Your Glow Plugs May Need Replacing
If one or more glow plugs fail, your diesel engine may still start, but not easily or smoothly. Common signs of bad glow plugs include:
- Hard starting, especially in cold weather
- Excessive white smoke during startup
- Rough idle after starting
- Decreased engine performance
- A glow plug warning light on the dashboard
Ignoring these signs can lead to engine strain and excessive emissions, so timely replacement is important.
Can You Replace Just One Plug at a Time
It’s usually recommended to replace all the spark or glow plugs at once, even if only one seems faulty. This ensures balanced performance across all cylinders and avoids uneven wear. If one plug has failed, others are likely not far behind.
However, in some cases, a technician may recommend replacing only the affected plug, especially if the others are relatively new. It depends on the situation and the specific vehicle.
Why Timely Replacement Matters
Delaying plug replacement can cause a ripple effect of problems. Misfiring spark plugs can lead to unburned fuel, which damages the catalytic converter. In diesel engines, faulty glow plugs make starting difficult and put extra stress on the battery and starter motor.
On the other hand, new plugs can improve engine response, lower emissions, and help prevent long-term damage. Whether you’re prepping for winter or just catching up on maintenance, plugs are worth prioritizing.
Let Moonlight Repair in Wilton, Iowa, Keep Your Ignition System in Top Shape
If your engine isn’t running as smoothly as it used to or you’re unsure when your plugs were last changed, Moonlight Repair in Wilton, Iowa, can help. Our technicians will inspect your spark or glow plugs, check for wear or carbon buildup, and recommend the right time for replacement.
Schedule your service today and keep your vehicle starting strong no matter the season.
