How Many Spark Plugs Does A Diesel Have?

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Did you know that diesel engines don’t use spark plugs at all?

This is true because this term is used in gasoline vehicles only. Instead, diesel engines utilize glow plugs, which are completely different.

Unlike spark plugs, a glow plug is a heating element that generates a spark, as its name suggests. On the other hand, the glow plug heats the diesel, igniting it.

In the case of a diesel engine, there is only one glow plug for each cylinder. So, for example, if your vehicle is a six-cylinder one, then there will be a total of six glow plugs.

Although the question of how many spark plugs does a diesel have been answered at the beginning of the article, to save you some time, it is recommended to continue reading because there are many other things to know about glow plugs listed below.

So, without any further ado, let’s get straight into it!

Are Spark Plugs Required In Diesel Engines?

If diesel trucks don' have spark plugs then how do they run? Let's find out

This is a common misconception that diesel engines use the same parts as gasoline engines.

Well, the reality is completely different. Parts are not interchangeable between the two engines.

Regardless, just to be clear, spark plugs are not needed in diesel engines.

They use glow plugs, which are used to heat the diesel instead of igniting it.

How It Works And How Different Is It?

If you are interested in buying diesel engines or were just enlightened that diesel engines don’t use spark plugs, you need to know how glow plugs work.

I will be comparing them with spark plugs to better define the difference.

The first main difference is that glow plugs gradually heat the diesel while spark plugs create a spark, as the name suggests.

Glow plugs need to work only once when the engine starts, while spark plugs have to work continuously.

If they stop, your vehicle will stop at the very same second. A fun fact is that glow plugs are not compulsory like spark plugs.

Now, this does not apply to all of you readers, but if you live in a hot enough environment, then chances are you could turn your vehicle on with the intake of the warm air.

The reason behind this is that diesel has a low ignition temperature. This does not mean that you can remove them immediately.

It is recommended to have them installed for reduced exhaust emissions.

What Do Glow Plugs Look Like?

As glow plugs are heating elements, there is not much to their shape and size. A glow plug would look like a long metal contraption with a flat and wide tip.

This tip is made up of conductive metals, which are also corrosion-resistant. The tip is responsible for heating the diesel.

The other end of the plug is simply a connecting bolt that allows it to be installed inside the cylinder.

What Happens When I Turn The Ignition?

This is where all the fun comes in. When you turn the key, the engine starts taking in air. If the air is warm enough, the glow plugs don’t need to be used.

The air taken in is compressed, which causes the pistons to rise. The high pressure allows the diesel to ignite and start the vehicle.

If the temperature is too low, the glow plug does the ignition part. The tip heats the diesel, helping it ignite.

The difference between the two is that glow plugs allow for reduced exhaust emissions while relying on the air will not allow for full combustion of the diesel, hence producing more toxic gases into the environment.

After The Engine Starts

The work of the glow plug is not done yet. It needs to work constantly to ensure that the engine is working properly.

Well, the first phase is the pre-heating phase. It is the part discussed earlier, the one in which the diesel is ignited.

Coming in next is temperature control. This is one of the most important uses of glow plugs.

The glow plug has to make sure that the temperature of the diesel is high enough that it combusts fully.

Suppose it is not, like when relying on compressed air, where incomplete combustion causes poor performance and more exhaust output.

A failing glow plug will have a similar performance too.

The third major work of the glow plug is to continuously heat the diesel without stopping. This ensures that favorable conditions are always met for the diesel to be ignited properly.

Different Types Of Glow Plugs

This part of the article is something that many of you may already be waiting for. There are different glow plugs on the market, with all of them having different performance levels.

There are three types of plugs. They are classified as:

  • Hot
  • Cold
  • Medium

There are other things to consider too, which will be discussed below. The first thing is to consider what type of fuel is used in your vehicle.

  • You have to focus on the percentage of nitro present in it. As nitro is responsible for being ignited, the higher the nitro levels, the colder your plugs should be.
  • If your engine is turbocharged, you may need to research a bit more. As turbo engines work differently from conventional diesel engines, you may need specialist plugs instead of standard glow plugs. It does not end there.
  • The size of the engine matters too. Larger engines are better at storing heat when compared to smaller ones. So, small engines require plugs that work at a higher temperature, while larger ones can use cold ones.
  • The use of your vehicle can also determine which glow plug would be suitable for you. If you step on the pedal occasionally, then it would be better to get a hot glow plug. This will reduce hesitation while accelerating, allowing you to enjoy your rides even more.

The basics of choosing spark plugs have been clarified, and now we shall be moving on to the different types of spark plugs that one can buy.

1. Pressure Sensor Glow Plugs

These are one of the most popular types. The science behind pressure sensor plugs is simple.

They work as intended and contact the ECU of the engine and keep it updated about the pressure.

This helps because the engine can control the diesel input, which can greatly increase fuel economy and help reduce exhaust emissions.

2. Ceramic Plugs

These plugs have been designed for cold weather usage. The thing about ceramic plugs is that they are designed to heat up faster than normal plugs.

So, if you are in a hurry and don’t want to wait a lot, this is the perfect option. Ceramic plugs are also great for fuel-efficiency.

They have one of the best fuel efficiency out of all the plugs listed here.

3. Metal Plugs

The thing about steel is that it is excellent at conducting heat. Well, that was put to use with these glow plugs.

They are perfect for cold weather because they can get much hotter than the previous two glow plugs.

Although they may take their time in doing so, it is better than your vehicle not starting at all.

All of these three choices are perfect for different uses. If you live in an area with moderate temperature, then the first option is perfect for you.

If you live in a cold area, where the temperature is around 0-degrees Fahrenheit, then Ceramic plugs will do the job for you.

They will also retain fuel efficiency, which is a huge plus. If you are the odd one out and live in temperatures that are even colder, then metal glow plugs are your best bet.

How Long Do Glow Plugs Last?

Although engines in warm temperatures can somehow function without glow plugs, you still need to keep them installed.

They are important in ensuring consistent engine performance. As glow plugs work at high temperatures, they slowly wear out. They need to be replaced every few years.

The expected lifespan of a glow plug is around 100000 miles. This is not a fixed number and can vary according to usage.

If you drive at high speeds, then you can expect the glow plugs to wear out sooner. The least you can expect from them is around 80000 miles.

It is better to get the glow plugs inspected with every oil change. This way, you will be up to date with their condition.

Symptoms Of Failing Glow Plugs

You should always keep track of how much the glow plugs are used.

If there are sudden performance issues, such as higher exhaust output and decreased acceleration, then there is a very likely chance that the glow plugs may be failing.

Let me list down some of the most common glow plug failure symptoms:

  • The vehicle may not start on the first go. This symptom may not be noticeable for people living in warm environments, but if the temperature where the vehicle is pretty low then a failing glow plug may cause the engine to not start that easily.
  • As glow plugs are used more and more, their functionality is compromised. In cases like these, there may be smoke being emitted from the hood of the car. Now, smoke could be caused by a variety of reasons, but failing glow plugs has the highest probability of happening. This issue could also be caused by an overheating vehicle.
  • As has been discussed in this article several times, a faulty glow plug does not allow fuel to burn efficiently. This greatly increases fuel consumption.
  • You may notice that your vehicle may not be performing as well. The acceleration could be affected, and you may notice a lag or a delay when stepping on the pedal. Well, a worn-out glow plug is to blame for that.
  • If the engine has suddenly started misfiring, then there is an excellent chance that the glow plugs have reached their end of life.

Some Other Common Symptoms

Some other common symptoms of failing glow plugs include knocking sound coming from the engine and engine malfunction lights on the dashboard.

These indicate that there is an issue with the engine. There is one thing that must be clarified.

Although these symptoms can be caused by failing glow plugs, they can also be caused by a variety of different issues.

With a vehicle’s engine being made up of dozens of different parts, anything could be the reason.

When any of these occur, it is better to get your vehicle towed to the nearest mechanic and get the issue fixed.

Ignoring these symptoms and continuing to drive will lead to more and possibly irreversible issues with the engine.

So, instead of diagnosing yourself, it is better to get a proper inspection from a mechanic.

This way, you will be sure what the problem is, and you will not have to worry about buying and installing new parts that will not fix the problem.

Light may also be shed on some other problems if they exist, which can be fixed before they cause any issues.

Getting New Glow Plugs

Getting new parts for a vehicle is one of the best things that one can do.

From a simple tire change to engine ones, such as new glow plugs, you can control the performance and fuel efficiency of your vehicle.

There are some things that a person must keep in mind when replacing a damaged plug or a worn-out one. Even if one is damaged, it is recommended to replace all of the plugs in one go.

The reason for this is that a mix of new and old hardware can cause a mismatch, which can lead to problems such as poor performance and worsened fuel efficiency.

If one plug has worn out before the others, it is an indication that others may be close to the end of their lifespan too. So, a full replacement is advised.

When it comes down to talking about the cost and the ease of installation, well, spark plugs and glow plugs take the lead over all other replaceable parts in an engine.

They are extremely cheap and can be installed without calling a mechanic. This way, you will not only save on the installation costs but will also allow you to get familiar with the engine.

Total Number Of Glow Plugs In A Diesel Engine

This is the question that led you to this article. Although it was answered in the beginning, I thought it would be better to dedicate a small portion of the article to clarify that once again.

This depends on the number of cylinders in the vehicle.

If there are a total of six cylinders, then the total number of glow plugs in that engine would be six. In layman terms, one cylinder has one glow plug.

This is the case with all diesel engines. Just to clarify, there are no spark plugs in a diesel engine.

This has been said multiple times in this article, and I understand that it is not easy to wrap your mind around learning something new that is completely different than what one must have thought.

5 Steps To Changing The Glow Plugs

So, you have finally done all of the provided steps and now want to save on the installation costs by installing your brand-new glow plugs yourself.

The procedure is pretty simple, and I am going to list down the steps:

  1. Start off by warming the engine up. Keep the vehicle in this position for ten minutes and then switch it off.
  2. Without wasting any time, open up the hood and disconnect the battery. The next thing to do is to disconnect the glow plugs’ wires.
  3. Use a thick cloth to unscrew the plugs. If they are stuck, then use oil, or better yet, use WD-40. Let it sit for a few hours so it can do its magic. Then you can unscrew and continue.
  4. Use a hatchet wrench to screw the glow plugs back. The process must be done very slowly in order to avoid any damage to the glow plugs.
  5. The final step is to connect all of the wire caps of the glow plugs and connect the battery too.

One thing that must be kept in mind is the force used to screw the new glow plugs back in. If you are unsure, a torque wrench can be set to the desired value.

This way, you will not be risking anything. The force you choose is normally mentioned by the glow plug manufacturers in the user manual.

So, I suggest taking a look at it. This is all it takes to complete the process. If there is no need to spray WD-40, then the process will take a mere hour to complete fully.

After that, you should be ready to go!


If you were looking for the answer to the question, how many spark plugs does a diesel have then you definitely got more than what you were expecting.

A glow plug is an important part of a diesel-based engine, and one must have the proper knowledge about it to make sure that the performance and the fuel efficiency of the engine are retained at all times.

One thing that many of you may have already concluded is that choosing a new spark plug for a vehicle’s engine is not an easy task.

You need to take many factors into consideration before making the final decision.

I hope that this article may have enlightened you to some extent, and I hope that it may have cleared up some confusion.