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How Many Spark Plugs Does a Diesel Have?

Close up photo of four spark plug laying on the ground.

Diesel engines do not have spark plugs, because they do not need the explosion of air and fuel inside the cylinder. In conventionally fueled four-stroke gasoline engines, the carburetor sucks in a mixture of air and fuel, which is drawn into the cylinder. This compresses it and, at the right moment, a spark jumps from the spark plugs that makes the mixture explode, causing the engine to work. 

In the case of diesel, only air is sucked in, which acquires such a high pressure in the combustion chamber that its temperature rises. At that moment, the diesel is injected and burns. Since no spark is required, spark plugs are also not required.

The Difference

Diesel engines run on glow plugs instead of spark plugs. In most configurations there is one glow plug for each cylinder. So, your 8-cylinder diesel engine would have eight glow plugs in a normal setup.

Gasoline engines and diesel engines work differently. The diesel engine requires the use of a heating device, so glow plugs are used. The glow plug and glow plug are similar in design, but the glow plug offers more features.

What are spark plugs?

Close up photo of a hand holding one spark plug

The spark plug is present in all gasoline engines. Most gasoline engines contain one spark plug for each cylinder. So, if you have a 6-cylinder petrol engine, you’ll probably find six spark plugs, one for each cylinder. 

The car’s engine fires these plugs in a certain order to provide the spark needed to ignite the mixture in the combustion chamber. A small explosion occurs inside the engine, igniting the air-fuel mixture.

Related: Do EVs Have Spark Plugs?

Function of diesel heaters

The glow plugs are primarily used as a cold starting device in diesel engines. Unlike a spark plug, the heater does not ignite the fuel-air mixture, but rather heats the combustion chamber (or previously also the pre-chamber) of the diesel engine and thus also the fuel-air mixture additionally by means of a spiral of incandescence.

This heating provides the fuel-air mixture with the energy or heat that the mixture lacks to self-ignite during cold starts. When the diesel engine is at operating temperature, the thermal energy of the engine block and the heat generated during the compression of the mixture are sufficient for self-ignition.

Glow plug: what is it?

Unlike a gasoline engine, a diesel engine requires, in order to start, to be raised to a certain temperature. Concretely, the air that causes the ignition of the fuel must be hot enough to start the engine. In practice, this consists of the driver having to wait a few seconds after turning the vehicle key to the preheating position, while the indicator associated with this stage comes on.

The glow plug consists of a heating element screwed into the cylinder head which, when electrically energized, heats the combustion or pre-combustion chamber in or above the cylinder (depending on the type of injection). There is one per cylinder. When it no longer manages to fulfill its role, it then becomes necessary to change it.

Glow plug: when to change it?

Zoom in photo of four glow plugs diesel engine.

Several clues suggest that a change of glow plugs is necessary:

  • It is increasingly difficult to start the vehicle.
  • The vehicle has difficulty starting in cold weather.
  • An abnormal emission of smoke.

On average, a glow plug should be changed every 100,000 kilometers. It is also strongly recommended to change all the glow plugs simultaneously so as not to create an imbalance between the cylinders when the engine is started.

Glow plugs: how to change them?

Changing glow plugs is a process that can be done quite easily by anyone with some knowledge in the field of automotive mechanics. However, a few basic principles must be respected:

  • Acquire spark plugs from the original brand by checking the references to used spark plugs.
  • Use a torque wrench with the references recommended by the manufacturer.

Do not force to remove the defective spark plugs (even if it means using an anti-seizing agent if necessary), screw in the new spark plugs well. If you have any questions, car professionals will be a safer alternative.

Glow plug price

Two premium glow plugs laying in the ground.

Be sure to take the references of your used spark plugs before choosing your new spark plugs: keep the same references and the same brand to have no doubt about their proper functioning. The price of a glow plug is between 10 and 20 euros per unit, depending on the model and brand.

If a glow plug pulls out, there is nothing you can do, and you will need to take the vehicle to the shop to have the glow plug drilled out. This requires special tools, and something can easily break.

Therefore, drilling the glow plugs is a job for professionals. If only one glow plug is broken, you can continue driving for a while with the remaining glow plugs intact. However, do not wait too long before replacing them.


If the glow plugs are torn out and can no longer be removed from the cylinder head, they must be drilled out. Drilling the glow plugs can cost 1,000 dollars or more, since the cylinder head has to be uncovered and this takes time. If drilling is unsuccessful, the entire cylinder head must be replaced. A new cylinder head costs between 500 and 1500 dollars, depending on the car model.