• Lachlan Palmer

Why Do Diesels Take Longer To Start?

Glow plugs are an essential part of a diesel motor and are the reason why diesels are able to start almost as easily as petrol’s. With diesels becoming more and more popular, the glow plugs have become even more important as commuter and day to day use has taken over as one of the main uses for diesel cars.


Glow plugs are a tiny, pen shaped piece of material that sits inside the motor and is heated using battery power until glowing red (hence glow plugs) to help with the starting of diesel motors from cold. This is necessary because of a number of unusual differences between diesels and petrol’s.

Unlike diesel, petrol is an incredibly flammable and volatile liquid, meaning that it evaporates quickly and is easily ignited or lit even at very cold temperatures. However, diesel is a thick fuel source that requires high heat and extreme pressures to combust in the way that is similar to a petrol motor.

This is one of the key differences between petrol and diesel cars, as a petrol-powered car uses a spark plug to ignite the fuel. Diesels, in fact, don’t use a spark plug at all and instead rely on the heat of the motor and the pressures inside to burn the fuel as soon as it enters.

When a diesel motor is cold though, there is often not enough heat to ignite the fuel, meaning long cranking times and even (sometimes) no starting at all. This is where glow plugs are important, as even when the motor is cold, the fuel is sprayed directly onto or near to the glow plugs, heating it enough to burn properly.



The glow plugs are important as they can prevent extra wear to the whole starting system, from the battery to the starter motor and can save you a lot of lost time and money. To know your diesel is ready to start, all you need to do is look for the curled wire symbol on the dash to go out once the ignition is on.

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