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Do Semi Trucks Have Glow Plugs? (17 Cool Facts)

Semi trucks most predominantly use diesel to power their engines. Diesel fuel is typically warmed by glow plugs near the cylinders of the engine. 

Glow plugs operate differently from spark plugs which spark to ignite the fuel. However, glow plugs gradually warm the diesel, decreasing the emissions produced.

Here is the answer to Whether Semi Trucks have Glow Plugs:

In general, semi trucks with diesel engines have glow plugs. Glow plugs are used to warm diesel fuel to aid in combustion. Most semi trucks have 6 cylinder diesel engines and will therefore have 6 glow plugs with one located at the top of each cylinder. Some diesel engines have grid heaters instead.

glow plugs

What are Glow Plugs and do Semi Trucks have them?

Glow plugs are usually located at the top of the cylinders of your engine.

This is because in a diesel engine the glow plugs heat the fuel, which then heats the engine and powers it.

The heat is generated by the electric from the battery which heats the plug, which in turn heats the cylinder.

Most semi trucks have glow plug powered engines. Most Diesel engines use them and most semi trucks have Diesel engines, which means you can usually guess a semi truck has glow plugs. 

Glow plugs differ from spark plugs in gasoline-powered engines and are preferred for the large semi trucks. Additionally, some semi trucks utilize grill heaters which work similar to glow plugs. 

How do Glow Plugs in a Semi Truck work?

Glow plugs are located near the fuel injection and are heated by electricity.

The heat is transferred to the engine where it slowly heats the fuel and powers the engine. The best way to explain the glow plug is to compare it to the gasoline tank, spark plugs.

Spark plugs are the ones people are most familiar with. Spark plugs ignite and spark to create combustion in the gasoline engine. In a nutshell, this is how a spark plug works. 

In comparison, glow plugs work by heating up and then transferring the heat to the engine and warming the diesel fuel.

This is accomplished through compression on the plug by the fuel injection spray, which heats the coil at the end of the glow plug. 

There are no sparks, but rather warming through electricity. 

Are Glow Plugs just for Starting?

Glow plugs are primarily used for starting the engine. They play a crucial role in the engine starting due to the heat they produce and the movement of the warmth. 

However, they have been updated to include more than this. Glow plugs have also been added to aid in reducing emissions during the start-up period. 

Glow plugs have been updated so the start-up cycle allows the fuel to warm up before starting during cold weather. Especially during cold weather, the fuel can create a lot of noxious fumes when the engine is started. 

What happens if you don’t let Glow Plugs warm up?

The glow plugs need time to heat the fuel in your tank, or else your engine can have a hard start.

This is especially true in the wintertime, or with cold weather, as the cylinders themselves need time to heat before heating the fuel. 

As far as permanent damage, there is not much of a threat to long-term damages. The emissions will be bad, and rather noxious.

The idling might be rough at first, but otherwise, there really isn’t any serious damage that would result from this. 

Do Gasoline Engines have Glow Plugs?

Gasoline engines do not utilize glow plugs, because the fuel is different from diesel and the ignition process is different. Diesel is designed to burn much slower than gasoline in order to allow the semi trucks to travel far with large loads without running out of fuel. 

Gasoline vehicles are designed for quick spark ignition and the process of slowly heating it up is not good for this type of vehicle.

Gasoline cars and trucks have a different engine design and using a glow plug could lead to pre-ignition combustion which can damage your engine. 

Diesel fuel is very different from gasoline, even though they do have similar origins.

Glow plugs would also be inefficient in a gasoline engine since the point is to get the molecules inside the fuel moving quickly to generate more energy to haul a lot of weight.

This isn’t really necessary for smaller gasoline vehicles. 

Do all Diesel Engines have Glow Plugs?

Actually, while glow plugs are very popular, there are alternatives.

There is the grid heater that operates with a similar idea as the glow plug (heating the fuel instead of using sparks), but it has some notable differences.

For some truck drivers, grid heaters are preferred, especially in cold weather and regions.

Where the glow plugs operate with the cylinders which sometimes struggle to move the hot air through the engine, the grid heater is more encompassing. 

The grid heater has more surface coverage of the engine than a cylinder and spark plug, so starting it on a cold day is much easier.

Despite this, they are not as popular as glow plugs, with only Cummins engines really using them anymore. 

How many Glow Plugs does a Diesel have?

Glow plugs are located at the top of each cylinder in a vehicle. Most class 8 semi trucks, which are some of the largest, have about 6 cylinders each. The number of cylinders is usually less in smaller models.

Some class 7 models have about 4 cylinders since the level of horsepower is less in these trucks which are designed for haul less weight. This means the number of glow plugs is usually between 4-6 for semi-trucks. 

The number of glow plugs is consistent with the number of cylinders in the truck, which varies per model. 

Does the Detroit 60 series have Glow Plugs?

The Detroit 60 series includes a rather large number of truck models including Peterbilt, Kenworth, and Freightliner.

In the larger models, it can be difficult to warm them with glow plugs without having something more intensive, such as a grid heater. 

Most Detroit 60 engines are what are called EGR engines. These engines benefit greatly from the use of glow plugs, which help to prevent wear and ensure efficiency.

EGR engines have difficulties converting particles in the combustion process without the help from the heat generated by the glow plugs. 

Do Cat Engines have Glow Plugs?

The CAT engines do not commonly use glow plugs. Especially the models used in Freightliner semi trucks. 

CAT engines don’t even use the air heating systems, rather than the plug system. The air heater system is the grid heater system which means the air is heated without the glow plugs by heating the surrounding air.

CAT engines use a system of direct injection. This means there isn’t a period for warming the fuel, but rather a system of ignition. This is part of the reason why CAT has not been able to meet most new EPA standards regarding emissions.

Can a Diesel run without Glow Plugs?

A diesel engine can run without glow plugs because glow plugs do not start the engine.

Rather, compression starts the engine through compressed energy being applied to heat the fuel and allow it to power the semi truck.

Some large engines do not even use glow plugs and may opt for the grid heater of the way in which a CAT engine operates.

These trucks do not need glow plugs in order to start. Some may think that you need a glow plug in the same way you need a spark plug. 

This is not so. Glow plugs do not so much as star the semi truck, so much as they heat the fuel to maximize the use of it, and reduce the emissions produced.

They also help your ride to be smoother and not grind as you drive. 

Does a First Gen Cummins have Glow Plugs?

Cummins engines do not use glow plugs to power their engines. This means the Cummins engines heat the air in the intake manifold rather than the combustion chamber.

The Cummins engines use the heater grid rather than the glow plug. They rely less on combustion and rather function as an overall heater. 

These types are less popular under EPA regulations due to the production of fumes, but they are well liked in the trucking community. 

Do Cummins ISX have Glow Plugs?

The Cummins ISX does not have glow plugs, and rather relies on the grid heater to warm the engine.

The ISX was actually the successor to the N14 back in the early 2000s. The older model was not able to pass EPA emission standards so the ISX model was made. 

The ISX has had its own issues with regulations of this manner over the years, but this truck is still being used today and is well-liked.

Why do Cummins not have Glow Plugs?

The Cummins engines were originally meant to have a higher horsepower to give them more strength on the road.

The heater system is preferred for this, especially in the larger semi trucks like the ones used in Freightliner

More recently, Cummins has been somewhat forced to adapt their engines to meet the EPA regulations over emissions.

They adjusted the system to burn off more of the fumes to prevent greater pollution in order to meet these standards. 

The systems have the added Diesel Particulate Filter (DPF) which helps to convert fumes through oxidation. This can interfere with the engine’s horsepower, but the issues seem to have been resolved in the ISX15. 

Do new Semi Trucks have Glow Plugs?

Glow plugs are fairly popular, even amongst new semi trucks. Glow plugs are integral to the efficient start-up and maintenance of most semi trucks.

Some have opted to use the grill heaters, or bypass the heating process altogether like CAT engines. 

However, CAT engines are not overly popular anymore due to the fact they do not have glow plugs or other heating mechanisms. So, most semi trucks have adopted either the grid heater system or glow plugs. 

Not only do they help preserve fuel consumption, but they reduce emissions as well. 

How do you know if your Semi Truck has Glow Plugs?

Glow plugs can be found at the upper part of your truck’s cylinders. They are located at the top, and the heat is transferred from this point down to the coils at the bottom. 

You can determine if your truck has these by inspecting this area of your truck and seeing if they heat from the electricity from the battery.

You can also always check your user’s manual and see if it specifies how your engine is heated. 

How to know if your Semi Truck Glow Plugs are Bad?

One of the best ways to determine if your glow plugs are bad is if you notice they do not get hot and “glow” when they are running.

They usually glow a low orange color when they are working, so you want to see this glow and if you don’t, then there is something wrong.

Other signs your glow plugs aren’t working are rough starts to the engine and difficult idling. These may feel like shaking or rumbling because the fuel is not warmed prior to running the engine. 

Last, if you see smoke or smell bad fumes when starting your truck’s engine, it could mean your glow plugs are bad and need to be replaced.

It is also possible they aren’t receiving the right amount of power from the battery, so be sure to check this too. 

Can you change your Semi Truck Glow Plugs yourself?

You can certainly change your own glow plugs in your semi truck. It requires knowledge of the internal workings of your truck, and you want to be careful to disconnect the battery before you do anything. 

Remember, the glow plugs get their energy from the battery. If it’s plugged in while you work on them, you can get hurt. 

Next, find your glow plugs located at the top of your vehicle’s cylinders and carefully remove the glow plug after disconnecting the energy wires.

This should then be replaced by the new, working glow plug.

Make sure you check your own vehicle’s manual before attempting any maintenance on it!