How Many Glow Plugs Are in a Diesel Engine?

Last Updated on December 1, 2023 by Jaxon Mike

Diesel engines have been growing in popularity in recent years, with more passenger vehicles opting for diesel power over traditional gasoline engines. But what exactly makes a diesel engine work?

An important component is the glow plug, which helps diesel engines start in cold weather. So how many glow plugs does the average diesel engine have?

The number of glow plugs in a diesel engine can vary depending on the size and cylinder configuration. But first, let’s look at what glow plugs are and why diesel engines need them.

What Are Glow Plugs and How Do They Work?

Glow plugs are small heating elements located in the combustion chamber of diesel engines. Their purpose is to help heat up the air in the cylinders so that the diesel fuel can combust properly.

Purpose of Glow Plugs

Unlike gasoline engines, diesels do not use spark plugs to ignite the fuel/air mixture in the cylinder. Instead, diesel engines rely on heat and compression to raise the temperature inside the cylinder high enough to ignite the diesel fuel when it is injected.

However, this presents a challenge in cold weather, when starting a cold diesel engine. The low ambient temperatures make it difficult to achieve the needed heat and compression. This is where glow plugs come in – they help “pre-heat” the combustion chamber so that the engine can start.

How Glow Plugs Provide Heat?

Glow plugs consist of a heating element housed in a metal tube. When powered on, the heating element gets hot through electrical resistance and reaches temperatures between 1000-1400°F. This heat is conducted through the walls of the glow plug into the air in the combustion chamber.

Most glow plugs turn on automatically when the vehicle is being started in cold temperatures. They heat up the air in the cylinders so that as soon as the diesel fuel is injected, ignition can take place. After the engine warms up and is running smoothly, the glow plugs turn off.

Now that we understand what glow plugs do, let’s look at why they are so important for diesel engine operation.

Why Diesel Engines Need Glow Plugs

There are two main reasons diesel engines rely on glow plugs: for cold starting and for reducing emissions.

Cold Starting in Diesel Engines

As mentioned earlier, diesel engines do not use spark plugs to ignite the fuel/air mixture. They rely solely on heat and compression.

In cold weather, it can be difficult for diesel engines to produce enough heat and pressure inside the cylinders for ignition. The engine block itself acts as a heat sink, absorbing heat from the cylinders. Plus, cold outside air being drawn into the cylinders further reduces the internal cylinder temperature.

This is why glow plugs are vital for cold starting. By preheating the air in the cylinders, they allow the diesel fuel to ignite immediately when injected into the cylinder. Without glow plugs, diesel engines would not start in cold climates.

Reducing Emissions

Glow plugs also help diesel engines reduce emissions during warm-up. The extra heat they provide allows more complete combustion of the fuel, decreasing the amount of unburnt fuel emitted from the tailpipe.

This benefit is especially important today, as emissions standards for diesel engines get stricter. Glow plugs play a role in helping engine manufacturers meet emissions requirements.

Now that we know why diesel engines need glow plugs, let’s look at how many are typically used.

How Many Glow Plugs Does a Diesel Engine Have?

The number of glow plugs in a diesel engine depends on several factors:

• Engine size and cylinder configuration
• Glow plug location
• Aftermarket modifications

Let’s look at how each of these affects glow plug quantities.

Engine Size and Configuration

In general, the more cylinders an engine has, the more glow plugs it will need. This is because each cylinder needs at least one glow plug to pre-heat the air during cold starts.

Here are some typical numbers by engine cylinder count:

4-Cylinder Engines

Most 4-cylinder diesel engines have 4 glow plugs, one per cylinder. Some smaller engines may have only 2 or 3 glow plugs.

6-Cylinder Engines

6-cylinder diesel engines typically have 6 glow plugs, again one per cylinder.

8-Cylinder Engines

8-cylinder diesels usually have 8 glow plugs, one for each cylinder.

Some large diesel pickup trucks and SUVs today use 8-cylinder diesel engines, like the Ford Super Duty with the 6.7L Powerstroke V8 diesel. These utilize 8 glow plugs.

Glow Plug Location

Where exactly are the glow plugs located in the diesel engine cylinder heads? This depends on the fuel injection system:

Direct Injection vs Indirect Injection

• In direct injected diesel engines, the glow plugs are located in the main combustion chamber, in the center of the cylinder head.
• In indirect injected diesels, the glow plugs are located in a smaller pre-combustion chamber in the cylinder head, separate from the main cylinder chamber.

This is because, in indirect injection engines, the diesel fuel is injected into the pre-combustion chamber first, where mixing occurs before the combustion event. The glow plugs heat this chamber so fuel ignition can take place there before the flame front spreads into the main cylinder.

Today, most modern diesel engines use direct injection, so glow plugs are commonly located directly inside the main combustion chamber.

Aftermarket Additions

Some diesel engine owners, especially those with vehicles used in very cold climates, choose to install additional glow plugs beyond the factory number.

This helps ensure positive cold weather starting by providing additional pre-heating capability. It’s fairly straightforward to add more glow plugs to a diesel engine.

The drawback is that too many glow plugs can reduce engine efficiency and fuel economy. Most owners find the stock number from the factory provides sufficient cold starting ability when the glow plugs are properly maintained.

Checking and Replacing Glow Plugs

To keep a diesel engine starting properly in cold weather, it’s important to make sure the glow plugs are in good working order. Let’s look at how to check glow plug operation and determine when to replace worn-out plugs.

Testing Glow Plug Operation

The simplest way to test if a glow plug is working correctly is to use a multimeter to check the electrical resistance. Resistance should be between 0.4-2 ohms when the plug is at room temperature. Much higher resistance means the heating element has worn out.

You can also use a specialist diesel glow plug tester. These apply voltage to the plug and measure how fast it heats up. Slow heating indicates a bad glow plug.

On some vehicles, there is a glow plug indicator light on the dashboard that will alert you to any failures. However, sometimes this light only verifies the control circuit, not the plugs themselves. Manual testing is best.

To check all the glow plugs on a diesel vehicle:

• Turn the ignition key to the “on” position but do not start the engine. This activates the glow plugs.
• Wait 1-2 minutes for the plugs to heat up.
• One by one, disconnect each plug wire and measure resistance with a multimeter. Do this quickly before they cool down.
• Check that each glow plug shows the expected low resistance value in the hot state. Higher than normal means it’s worn out.
• Reconnect each plug wire before moving to the next one to test.

This procedure will verify if any glow plugs have failed and need to be replaced. Catching bad plugs early prevents difficult cold starting.

When to Replace Glow Plugs?

As with any vehicle component, glow plugs eventually wear out over time. Here are some 

signs it’s time to replace your diesel engine’s glow plugs:

• Hard starting in cold weather – if the engine cranks over slowly or takes longer to start on cold mornings, faulty glow plugs could be the culprit. They may not be heating up fast enough.
• Check Engine light – the ECU may detect a bad glow plug and set a trouble code. Have the vehicle scanned to pinpoint which cylinder’s glow plug needs attention.
• Failed manual testing – if glow plug resistance measures higher than specifications during manual testing, replacement is needed.

Most manufacturers recommend replacing glow plugs between 60,000 – 100,000 miles. The exact interval depends on driving conditions and engine maintenance. Short trip driving and frequent cold starts accelerate wear.

Replacing glow plugs requires removing the cylinder head in most diesel engines, so it’s labor-intensive. Have this performed by an experienced diesel mechanic. They can also check for carbon deposits around the plugs that could cause premature failures.

Here are a few tips for new glow plug installation:

• Only use original equipment or high-quality aftermarket glow plugs from a reputable brand. Inferior plugs won’t last.
• Apply anti-seize compound to the threads to make future removal easier.
• Tighten plugs to the specified torque setting in the factory service manual. Overtightening can damage the housing.
• Be extremely careful not to drop anything into the open cylinders during service, as this can cause internal engine damage.

Proper maintenance and replacement of glow plugs is essential for keeping your diesel engine running smoothly in cold weather. Catching problems early prevents being left stranded by a no-start situation.

Frequently Asked Questions

How often should glow plugs be replaced?

Most manufacturers recommend replacing glow plugs every 60,000-100,000 miles. Short-trip driving or frequent cold starts may require earlier replacement.

What happens if a glow plug fails?

A failed glow plug will make cold starting difficult. There may be extended cranking time, roughness, misfires, or a lack of startup when temperatures are low.

Can a diesel engine run with a bad glow plug?

Yes, it may be possible once the engine is warm. But cold starting will become progressively harder as more glow plugs fail. It’s best to replace bad plugs promptly.

Do gas engines have glow plugs?

No, glow plugs are only used in diesel engines. Gasoline engines use spark plugs instead to ignite the air/fuel mixture.

How much does it cost to replace glow plugs?

Glow plug replacement cost can range from $200-$1000 including labor, depending on the number of plugs and difficulty of access on the engine. Mercedes-Benz models tend to be most expensive.

Conclusion

Glow plugs play an integral role in cold starting for diesel engines. By electrically pre-heating the air in the cylinders, they allow the diesel fuel to ignite even in low ambient temperatures.

The number of glow plugs in a diesel engine can range from as few as 2 up to 8 or more, depending on factors like engine size, cylinder count, and aftermarket additions. Most direct-injected 4 to 8-cylinder diesel engines have one glow plug per cylinder.

It’s important to periodically check glow plug operation once a diesel engine surpasses 60,000 miles. Simple resistance testing can identify any failed plugs that should be replaced.

Maintaining the glow plug system keeps diesel engines starting smoothly even in frigid winter conditions. With proper care, your diesel will provide years of reliable service.