How Many Spark Plugs Does a Diesel Have |

How Many Spark Plugs Does a Diesel Have

There is a question that has always been on the minds of automotive enthusiasts and that is “How many spark plugs does a diesel engine have?”.

Diesel engines do not have spark plugs. Those strange elements that from one day to the next made our grandparents’ gasoline cars stop working. However, what they do have are other elements called glow plugs or glow plugs, whose failures can cause our diesel car to end up running very poorly when cold or to stop starting completely.

Nowadays, practically all diesel engines have a glow plug warning light. And those that don’t will at least illuminate the misfire light to indicate that something is wrong. When the ignition is switched on, all the lights come on for three or four seconds and then go out, indicating that everything is OK to start the engine. If during normal operation the light comes on, does not go out after starting, or remains to flash, the car may have a faulty glow plug(s) (diesel engines usually have one glow plug per cylinder).

Note that some models only warn of the malfunction for 20 to 30 seconds and then shut off so as not to interfere with normal driving. Other models may also display a warning on the on-board computer display, with text similar to: “check glow plugs”.

Do diesel engines have spark plugs?

A diesel engine has no spark plugs. Instead, diesel engines use compression ignition and glow plugs to heat the combustion chamber, which helps ignition when the engine is cold.

The purpose of these small components is to facilitate the cold starting of diesel engines. In addition, they may be required for a short period of time after engine start-up to ensure consistent engine performance and minimize pollutant emissions.

In reality, they are nothing more than metal components that become red-hot when the engine is started. As a result, they contribute to heating the air and combustion chambers of the engine. They are not as important as indirect injection engines as they were in earlier pre-chamber engines, but they are still necessary for extreme climatic conditions.

Why Does a Diesel Engine Have No Spark Plugs?

Diesel engines do not need spark plugs, as the explosion of air and fuel inside the cylinder is not necessary. The carburetor of four-stroke gasoline engines with a traditional fuel supply sucks in a combination of air and fuel, which is injected into the cylinder. It compresses it and, when the timing is perfect, a spark from the spark plugs ignites the mixture, allowing the engine to start.

In the case of diesel engines, only air is sucked in, which creates such high pressure in the combustion chamber that the temperature rises. At that point, diesel fuel is injected and starts to burn. Spark plugs are also unnecessary, as no spark is needed.

How Do Diesel Engines Work Without Spark Plugs

Basically, it is an alternative internal combustion engine with self-ignition thanks to the high temperatures derived from the compression of the air in the cylinder.

The main difference with a gasoline engine lies precisely in this point: diesel engines do not need a spark to ignite but have glow plugs that raise the temperature of the combustion chamber to improve cold starting and use this heat once the optimum temperature has been reached.

The four strokes of a diesel engine proceed as follows:


In this first stroke, air filling takes place as the intake valve remains open while the piston is descending towards the bottom dead center. The total amount of air is always admitted under any load condition, and the fresher it is, the lower the density and the more air can enter, thus increasing combustion (intercoolers or air radiators are used for this purpose).


The intake valve closes when the piston reaches the bottom dead center and begins the stroke to the top thus compressing the air inside the cylinder in a ratio of approximately 18:1 and significantly raising the temperature.


Shortly before reaching the top dead center, the injector sprays the fuel into the chamber, and it ignites immediately upon contact with the hot air. (Without the need of the spark plug spark, only with the heat transmitted by its incandescence).

Exhaust: The pressure generated by the temperature will push the piston downwards with force, and part of this energy will be used to return it to the top dead center, thus expelling the burnt gases and allowing the inertia to start the cycle again.

Diesel or gas oil has always been considered a more economical fuel and with a more effective performance per liter than gasoline. However, when talking about prices, we must not forget that diesel vehicles are more expensive to buy on the market and more costly to maintain.

How do diesel glow plugs work

A glow plug is a heating device that heats the fuel and air entering a diesel engine to promote efficient combustion of the fuel. The combustion of the gasoline starts your engine, allowing you to drive. Your engine has a glow plug in each cylinder, either in the pre-chamber (pre-chamber engines) or in the combustion chamber (direct injection engines).

Symptoms of Bad Glow Plugs in a Diesel

Spark plugs are critical components for the correct operation of a vehicle; when one of these components fails, several symptoms appear alerting the driver of a problem that must be attended to:

  • Failure to start the car
  • Poor acceleration
  • Engine starting problems
  • Black smoke in the exhaust
  • Engine light warning

Failure to start the car

Hard starting is one of the most common indications of malfunctioning glow plugs. These faulty components are unable to provide enough heat to properly warm the cylinders and start the engine.

When the glow plugs do not quickly generate the necessary heat, it will most likely take many attempts to start the engine. On the other hand, if the spark plugs are completely inactive and temperatures are too low, the engine will not start properly.

Poor acceleration

When the glow plugs are broken, engine performance suffers, especially when the ambient temperature is very low. If you keep pressing the pedal and the engine will not start, the glow plugs will most likely need to be replaced.

Poor acceleration occurs when numerous components of the car are worn or broken. Under these conditions, it is suggested to perform a thorough check before concluding that the main reason is a faulty glow plug.

Engine starting problems

When an engine will not start, it is usually because there is not enough gasoline in the cylinder. Any type of ignition problem is cause for concern, but when it is a faulty glow plug, it is a problem that must be resolved immediately.

The glow plug is intended to properly heat the cylinder to ignite; however, many misfires can occur due to various issues; therefore, do not discard the glow plug in the first place to check its condition; it may be defective.

Black exhaust smoke

When you start your diesel car and see black or dark gray smoke coming out of the exhaust, this indicates that it is defective. The tones emitted by this component indicate the presence of a problem in the combustion system.

The diesel engine can emit black smoke for a variety of reasons; however, it is wise to inspect the glow plugs; if they are defective, one of the indications is black smoke from the exhaust.

Engine light warning

An indication that the glow plugs are defective is the activation of the check engine light as a warning signal. To confirm this, it is essential to scan the system with a specialized tool to locate the problem.

The code P0380 can often be seen in glow plug circuit A fault indications. Other codes available are P0670, P0671, P0672, P0673, P0674, P0675, P0676, P0677, P0678, P0679, P0680, P0681, P0682, P0683, and P0684, P0381, P0382, P0384383.

If your car has a diesel engine, the warning light on the dashboard will come on, indicating that there is a problem with the glow plug system. The signal is a spiral-shaped line or coil that resembles an amber-colored wire.

How Often I Need To Change The Glow Plugs?

Glow plugs should last up to 100,000 miles; however, they will progressively degrade due to wear and tear during this time.

The good news is that glow plugs are not only very durable, they are also among the most cost-effective vehicle components to repair.

How Many Glow Plugs Are in a Diesel Engine?

Each cylinder of a diesel engine can have up to 10 glow plugs. The glow plugs are connected directly to the engine control unit, which analyzes engine data via a microcontroller. This controls when the current to the glow plugs is turned on and off, as well as the amount of current required to heat them. This is determined by the starting temperature of the motor.

Once the glow plug module has calculated the timing and electrical current required by the glow plugs, the current flow to the heating unit of each glow plug, causing it to produce visible light. The intake air is compressed and the injectors supply fuel to the heaters. This fuel combines with compressed air and evaporates to initiate the combustion process.

Final Thoughts

How many spark plugs are in a diesel engine? None. Spark plugs are not used in diesel engines. Instead, the diesel engine uses a spark plug known as a glow plug, which differs in form and function from the spark plug.

In diesel engines, only air is introduced, which creates such high pressure in the combustion chamber that the temperature rises. The diesel fuel is then injected and burned. Spark plugs are also unnecessary, as no spark is needed.


Hi, I'm the initiator and writer of this blog. Cars were and will be my first love, and my favorite hobby, that's why I decided to start this blog and write about my discoveries and techniques to improve my cars or repair them.