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How Many Spark Plugs Does a Diesel Have?

How Many Spark Plugs Does a Diesel Have?

Spark plugs are one of the most important parts of a gasoline-powered engine. They are the spark that creates the energy in your engine. If you own a diesel engine, you may wonder how many spark plugs your engine has.

This article will cover how your diesel engine works and exactly how many spark plugs your diesel engine has. I’ll go over the difference between spark plugs and glow plugs. In the end, you’ll have a firm understanding of how many plugs you need for your diesel. 

Here’s a quick answer to the question!

Diesel engines use glow plugs instead of spark plugs to create combustion, and there is a single glow plug per cylinder in the engine. For example, a 6-cylinder diesel engine would have 6 glow plugs.

With that in mind, lets dive in a little further to better understand how glow plugs are different from spark plugs, and how they work!

What are Spark Plugs?

A spark plug is a component of any gas-powered engine. Most engines will have one spark plug for each cylinder. So if you own a six-cylinder car, its engine will have six spark plugs.

The engine is designed to fire each spark plug in a precise order, and this provides an electric spark to your engine. The spark ignites a fuel and air mixture, causing a small explosion inside the valves of the engine. 

The explosion forces the piston in your engine down, creating energy and moving your car forward. 

Diesel engines create energy differently than gasoline engines. They don’t require a spark. Instead, they rely on heat from the plug.

Do Diesel Engines Need Spark Plugs?

No, diesel and gasoline engines operate differently. Diesel engines do not use traditional spark plugs. Instead, a diesel uses a heating device called a glow plug. 

While similar in shape and size, a glow plug serves a much different purpose than a spark plug. Instead of a spark, diesel fuel requires heat and pressure to ignite. The heat is provided by a glow plug.

What are Glow Plugs?

A glow plug is a heating device. Like a spark plug, it ignites the fuel/air mixture in the engine’s pistons. 

There is one glow plug installed on the top of each cylinder in your engine.

Starting a Diesel Engine

To start a diesel engine, air flows from the intake and is compressed. The piston rises, causing pressure. The pressure causes the diesel fuel to mix with the air and evaporate. 

Eventually, under pressure and heat, the mixture ignites. 

Under warmer weather conditions, a diesel engine can combust without the aid of a plug. Colder times make it difficult for the diesel/air mixture to combust. Heat needs to help the ignition. The heat is generated by the glow plugs. 

A glow plug will get your car started quickly and maintain engine temperature to ensure the highly efficient operation of your vehicle. 

How Glow Plugs Operate

Glow plugs operate in three phases. The glow plug heats the engine quickly, maintains temperature, and keeps the engine heated after it has been started.

  • Pre-Heating makes starting the engine easier and helps start the vehicle quickly.
  • Temperature maintenance ensures ideal combustion conditions.
  • Post-heating maintains heat for the engine during the combustion process. 

The three-phase system ensures your engine always has enough heat to ignite the diesel mixture in your engine, and generate energy for your vehicle.

Types of Glow Plugs

Choosing the right glow plugs for your vehicle is based on a few factors. Plugs are categorized as hot, medium, and cold. You will need to assess nitro percentage, engine type, engine size, and acceleration when purchasing new plugs.

  • High nitro-percentage means a colder glow plug. Low percentage use a hotter glow plug.
  • Turbo engines may require specific glow plugs based on the engine and manufacturer.
  • Small engines will usually need hot glows, while large engines can use colder plugs.
  • Hot glow plugs are best for acceleration or to improve sluggish performance 
  • Cold plugs can produce more power and torque.

Pressure Sensor Glow Plugs (PSG)

These plugs offer an integrated pressure sensor that can improve the efficiency of your engine. The sensor reports pressure in the cylinder to the engine control unit. 

PSG plugs can make starting your car in the cold easier and increase fuel economy. They are also great for lowering the emissions of your vehicle.

Ceramic Glow Plugs (CPG)

Ceramic glow plugs heat up quickly. They also can reach much higher temperatures than their counterparts. 

Since they heat up fast, you’ll be able to quickly start your vehicle in the coldest weather conditions. Diesel fuel is burned at optimal conditions while using ceramic plugs. CPG plugs offer the best efficiency for most vehicles.

Steel Glow Plugs

A steel plug is also efficient and reliable. While they may take longer to start, a steel glow plug will have no problem starting your vehicle under extremely cold conditions. 

How Many Glow Plugs Does Your Diesel Engine Have?

If you made it this far, let’s go over how many glow plugs are in a diesel engine. A diesel engine requires one glow plug for each cylinder in the engine. If your vehicle has a six-cylinder engine, you would need six glow plugs for the engine. 

A diesel engine needs zero spark plugs and one glow plug per cylinder.

How Often Do You Change Diesel Glow Plugs?

Replacing glow plugs is essential for the peak performance of any engine. Luckily replacing plugs is easy and affordable. A regular tune-up around 12,000 miles is the ideal time to inspect your used glow plugs. 

Many vehicles may have different maintenance schedules. You can consult the owner’s manual of your vehicle to find the appropriate time to perform scheduled maintenance.

On average, most glow plugs last at least 100,000 miles. When replacing a damaged plug, you should replace every plug in your engine. This will ensure each cylinder is providing optimal performance.

Replacing glow plugs is easy and affordable. Installing new glow plugs can be done by most amateur mechanics right in their driveway or garage. Of course, it’s affordable to have a professional mechanic replace them as well. 

Troubleshooting Glow Plugs

Are you having problems with your car? From not starting to poor efficiency, your plugs might be the culprit. Here are some symptoms you should look out for that might be caused by malfunctioning glow plugs.

  • Your vehicle won’t start, especially in cold weather.
  • Poor fuel efficiency is caused by bad glow plugs.
  • Damaged plugs can cause vehicle misfiring.
  • Smoke from your engine could be signs of faulty glow plugs.
  • Malfunctioning glow plugs can cause poor fuel efficiency. 
  • Diesel knocking noises are often caused by plug malfunctions.
  • Vehicles that can’t accelerate or don’t have enough power might have glow plug problems.
  • Activated vehicle warning lights can indicate issues with your glow plugs.

Any loss in power, performance, and efficiency may be attributed to malfunctioning glow plugs. The good news is that glow plugs are easy to replace and a cost-effective repair. 

Final Thoughts on Glow Plugs and Diesel Engines

I hope I provided some insight into how a diesel engine runs. Many car owners might assume their diesel engine requires spark plugs. They do not, diesel engines use glow plugs instead. 

While a glow plug is similar, there are many differences between the two components, and serve different functions.

In this article, I covered that diesel requires zero spark plugs and instead uses glow plugs

No matter what size engine you own, you will need one glow plug per cylinder. For example, a four-cylinder diesel will require four glow plugs. If you’re not sure how many cylinders your vehicle has, you should consult the owner’s manual. 

Of course, if understanding your diesel engine is confusing, it’s always best to visit your local shop or dealer for any repairs or maintenance. 

Zach Reed

Hi, I'm the founder of! Having owned a wide variety of vehicles in my life, I was astounded at how hard it can be to find answers to common automotive questions. Rather than sit idly, I decided to create this website to help others!