How Do Glow Plugs Work on Diesel Engines?

If you own a vehicle with a diesel engine, you know there are unique components and maintenance items that gasoline-powered car owners do not face. One of these components is the glow plug. Sometimes mistaken for or compared to a spark plug, the glow plug is entirely different in its function. If you are new to diesel, you are probably unfamiliar with how glow plugs work. In this article, we are going to cover glow plug function, types, and maintenance.

Glow Plugs are NOT Spark Plugs

Before we go further, we need to say it. Gasoline-powered cars use spark plugs to “touch off” combustion by providing the air/fuel mixture with a spark during each power stroke. Diesels utilize extremely high cylinder pressure to create the conditions for combustion. To the untrained eye, a glow plug might look similar to a spark plug, but in terms of function, the most common thing between them is the word “plug”.

How Do Glow Plugs Work?

When a diesel is running, the compression stroke increases the temperature of the air in the combustion chamber. This compression provides the heat needed to induce combustion, driving the piston down and creating power. Diesel engine blocks are large (to support high cylinder pressure), and as a result, are slow to warm up. A cold engine block will sap away heat from the compressed air, and without a spark (like gasoline engines get) the initial combustion on startup is difficult or impossible. This is where glow plugs come into play.


Glow plugs are heating elements that warm the air-fuel mixture in the combustion chamber or pre-chamber (IDI engines). The extra heat provided by the glow plug aids in combustion when the engine and intake air is cold on initial startup. This process is known as pre-heating.

Pre-heating changed as diesel technology improved. Early thermostat glow plug systems (we’ll cover glow plug types later) required the driver to manually activate the plug for 20 seconds or more before starting the engine.

The advent of in-cylinder glow plugs allowed the plugs to be activated when the key was turned to the “on” position. A “wait to start” light on the dash would let the driver know when to turn the engine over.

Modern glow plug systems require far less time to achieve the temperature required for startup, typically around 6-8 seconds. To further improve pre-heating speed, newer vehicles activate glow plugs when the doors are unlocked or opened.

Types of Glow Plug

There are two variates of the glow plug. In-manifold systems utilize a single plug, in the intake manifold to heat the air before it enters the combustion chamber. The pre-heating time for an in-manifold plug is longer due to the amount of air that needs to be warmed for combustion.

The second type of glow plug is located in-cylinder. This arrangement uses one glow plug per combustion chamber (or in the case of an IDI engine, one per pre-chamber). The preheating phase is much faster with in-cylinder glow plugs because the area they need to warm is significantly smaller.  

How often do glow plugs need to be replaced?

Glow plugs typically last around 100,000 miles before needing to be replaced. You will not need to worry about them too often during the time you own your diesel vehicle.

Are my Glow Plugs Bad?

There are a few indicators of faulty glow plugs, most of which will be pretty obvious.

Hard Starting or No Starting

The core function of glow plugs is to aid in starting the engine. If your plugs are faulty, it may be difficult or impossible to start your diesel vehicle on a cold morning.

Misfires on Startup

Faulty glow plugs will impact combustion and can cause misfires. These will not last long as the engine gets warm.

White or Black Smoke on Startup

Unburnt fuel can exit the tailpipe in the form of white or black smoke. This can be a sign of bad glow plugs. Once the engine is running, the fuel will be burning like normal, and the smoke will stop.

Issues NOT caused by faulty glow plugs

Some issues are often attributed to defective glow plugs but aren’t caused by them. The most common issues we see attributed to failed glowplugs are reduced fuel economy and reduced power or acceleration.

These issues really can’t be attributed to glow plugs. The plugs are active for a few seconds before and after startup, so they are not even on when the vehicle is moving. If a glow plug is causing a startup misfire, then for those first few seconds of operation fuel economy might be lower, but again, once you are driving, any reduction in fuel economy would stem from a different problem

Issues with Glow Plugs? Lake City Auto Care Can Help

We are diesel experts at Lake City Auto Care. The teams at our three locations in Hayden, Rathdrum, and CDA, ID are equipped to work on your diesel, whether it’s an F-350 Powerstroke or a Jetta TDI. Give us a call or schedule an appointment at one of our locations today!

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