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How Long Do Glow Plugs Last?

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Diesel engines have some slightly different components than their gasoline counterparts. Glow plugs are one such component. Glow plugs serve a critical purpose in a diesel engine's start sequence by providing the heat necessary for ignition. But just how long can you expect diesel glow plugs to last? We did the research to bring you the answer.

Unlike other engine components, glow plugs are only on for a brief period when a diesel engine starts. For this reason, the longevity of a set of glow plugs depends on the frequency of engine starts and the ambient temperature at which the engine is started (over time, on average). But in general, you can expect glow plugs to last for approximately 100,000 miles.

If you still have some questions about the life expectancy of glow plugs, don't worry. In this post, we'll discuss the topic in greater detail. We'll also talk about how to tell when glow plugs need changing, what causes glow plugs to go bad, whether you should replace all of the glow plugs at once, and how to test glow plugs to determine whether they are operational or faulty. Without further ado, let's get into it.

Four glow plugs for a diesel engine, How Long Do Glow Plugs Last?

The Life Expectancy Of Glow Plugs

Unlike engine oil, fuel injectors, and other similar components, the life expectancy of glow plugs isn't directly correlated to the number of miles on the odometer.

As mentioned, the glow plugs on a diesel engine simply facilitate the ignition process by heating the air-fuel mixture in the combustion chamber to ensure that there is adequate heat for ignition.

Since the ignition process is when glow plugs are used, they really only experience wear during this time. Thus, the frequency at which a diesel engine is started is largely what determines how long glow plugs will last. In other words, each engine start effectively shortens the lifespan of the glow plugs. 

Spare part for the car engine

In addition, the ambient temperature at which the engine is started also impacts the longevity of glow plugs. 

Diesel engines are notoriously difficult to start in cold conditions (that is, if the engine block heater wasn't plugged in, but that's a separate topic) since heat is one of the key ingredients of a successful engine start. So, in extremely cold conditions, the glow plugs might be cycled more often, resulting in additional wear.

So, the glow plugs in an engine that is frequently started in a cold climate (such as for frequent trips around town, etc.) will wear out sooner than glow plugs in an engine that is mostly started in a warmer climate and runs for longer periods (such as long highway trips).

But the general consensus is that you can expect glow plugs to last approximately 100,000 miles. 

How Do You Know When Glow Plugs Need Changing?

Luckily, there are a couple of good indicators that glow plugs need to be changed. 

Difficulty Starting (Or Failure To Start Altogether)

The best indicator that glow plugs are faulty is difficulty starting. Provided the battery is operational, difficulty starting indicates that the glow plugs are unable to provide the necessary heat for the ignition process.

How much difficulty the engine has during the ignition process corresponds with the number of faulty glow plugs. A somewhat hesitant start means that there might only be one or two faulty glow plugs. If it takes several attempts to start the engine, it's likely that several glow plugs are faulty. 

This is especially apparent in cold climates. When it's cold out and a diesel engine struggles to start, it indicates that the glow plugs are bad. 

Read more: Diesel Engine Won’t Start In Cold—What Could Be Wrong?

Smoke (White Or Black) Coming From The Exhaust Pipe When Starting

Incomplete combustion creates poisonous carbon monoxide from exhaust pipe of white car

Another indicator that glow plugs are faulty is white or black smoke coming out of the exhaust pipe upon ignition. Simply put, smoke coming from the exhaust pipe means that there is some kind of issue with the ignition/combustion process, and the glow plugs could be to blame. 

It's likely that smoke coming from the exhaust pipe will be accompanied by a rough start.

What Causes Glow Plugs To Go Bad?

Two types of glow plug

There are a couple of causes of glow plug failure

The first is simple wear and tear. Like any automotive component, use and time will take a toll, ultimately resulting in failure. 

But another common cause of glow plug failure is excess voltage. Simply put, if glow plugs are subjected to excessively high voltage, the tips of the plugs can be damaged, resulting in failure. 

In addition, there could also be issues with the wiring that lead to premature failure. For instance, glow plugs should only be on during engine start to facilitate ignition, but faulty electrical components/wiring could cause glow plugs to remain on when they aren't supposed to be.

How Do I Test My Glow Plugs?

If you suspect that one (or several) of your glow plugs might be faulty, there's a way to remove the guesswork and actually test each plug to confirm whether or not they function properly. 

Follow these steps to test your glow plugs using a multimeter:

  1. Remove the glow plugs from the engine.
  2. Visually inspect each glow plug for any signs of obvious damage.
  3. Set the multimeter to its lowest setting (around 200 ohms). The setting you choose here means the multimeter won't display a value if the electrical resistance being measured reads higher than the selected setting.
  4. Check the resistance of the leads of the multimeter by touching them together. Note the value (in ohms) displayed on the multimeter for later. This value will be accounted for later to ensure an accurate test.
  5. Place the black lead on the wide portion of the body of the glow plug just below the terminal, and place the red lead on the terminal. Ensure both leads are solidly contacting the respective portions of the glow plug.
  6. Observe the multimeter reading. If the reading is around 1.5 ohms, subtract the value noted earlier from the current multimeter reading. If the value is 1.5 or lower, this indicates that the glow plug is operational. A higher reading (which means that there is a greater electrical resistance) indicates that the glow plug is faulty. 
  7. Repeat the process for all of the glow plugs.

Click here to see this multimeter on Amazon.

For a visual demonstration of the glow plug testing process, check out this video:

Should You Replace All Glow Plugs At Once?

Should you discover at least one faulty glow plug in your diesel engine, it's recommended to replace all of them at once. Even if you identified one or two faulty glow plugs, you should go ahead and change all of them.

Even though some plugs might still function properly, they are all the same age (unless you or the previous owner only did a partial change previously). Thus, failure might be right around the corner. To prevent having to do multiple glow plug change jobs, it's best to have them all changed.

What Happens If You Start A Diesel Without Waiting?

Glow plugs light on car dashboard

Part of a diesel engine's startup procedure is waiting for the appropriate light on the instrument cluster to turn off, indicating that the glow plugs have gone through their heating cycle and that the engine is ready to start. 

Should you attempt to crank the engine before the glow plugs have adequately heated the combustion chamber, the engine could fail to start, especially in cold weather. 

In warmer climates, prematurely cranking the engine might not be problematic; the engine might be warm enough to ignite without the help of the glow plugs. 

However, it's recommended to always wait for the appropriate indication prior to starting the engine just to be safe.

In Closing

We hope this guide has taught you all you need to know about proper glow plug maintenance. Remember, glow plugs are an essential part of diesel engine ignition, so it's critical that they function properly. 

Before you go, be sure to check out these other related guides:

Do Cummins Engines Have Glow Plugs?

Do Diesel Engines Have Spark Plugs?