Home Diagnosis and TroubleshootingEngine Problems Diesel Glow Plugs: Symptoms Of Failure And How To Fix Them

Diesel Glow Plugs: Symptoms Of Failure And How To Fix Them

by Jordan Harris
Diesel Glow Plugs

Diesel engines are not equipped with ignition systems or spark plugs of any kind, so it is the job of glow plugs to create a spark in the engine when it’s too cold. Diesel glow plugs are constantly subjected to high temperatures and sustain wear and tear quite often. Thus, you have to replace them often.

Due to extensive exposure to high pressures and extreme temperature, a diesel engine may feature up to 10 glow plugs, 1 for every cylinder. You might not understand if only one of them goes bad. However, when 3 or more are faulty, the engine will be hard to start. Plus, bad glow plugs reduce diesel engine lifespan.

Some vehicles come with PCM’s which observe glow plug function and report the whole operations of every plug; however, as most also have glow plug relays, you may not be aware that any glow plug is bad.

What Are Glow Plugs?

Think of glow plugs as little heaters under the hood of your car. These heating elements warm the diesel and air in your vehicle’s engine for proper combustion. Glow plugs and spark plugs have similar functionalities, but the primary difference is that glow plugs heat fuel and air to start diesel-powered engines.

Every diesel engine is equipped with a glow plug for each cylinder, which answers why different models and makes require different counts of plugs.

Anatomy Of A Glow Plug

A glow plug starts as a heating coil to be entered into a metal tube. This tube contains electrically insulating powder (ceramic) and is sealed at an end. Once energized electrically, the heated tip of the glow tube can cross 1000°C quickly.

Ceramic glow plugs can be substituted for standard plugs. A heating element is put inside them which is then encased in Silicon Nitride, or ceramic. Thanks to the casing, the glow plugs can heat rather quickly, reaching greater temperatures for prolonged periods.

Good-condition, top-notch glow plugs decrease the emission of exhaust gases heavily, maintaining minimum ecological impact.

How Do Glow Plugs Work?

Glow plugs have to be hot enough under pressure to self-combust, which is not the case with spark plugs. In warm weather, fuel requires little help from other vehicle components for this to happen. However, it is often a struggle for diesel engines to combust fuel in cooler climates. The glow plugs warm the fuel so the engine can start.

The intake air is compressed to start the engine, and the engine pushes diesel into the ignition chamber as the piston rises. Combined with air, the fuel evaporates and combusts. As the ignition starts almost instantly after evaporation, many consider these processes simultaneous.

Glow plugs must go through a 3-phase procedure when an engine is started.

1) Pre-Heating: The diesel glow plugs warm up quickly to push the engine to its feet.

2) Temperature Maintenance: For optimal combustion, the glow plugs must maintain their internal temperature.

3) Post-Heating: The diesel glow plugs stay heated for some minutes once the engine has been ignited to keep the combustion process going.

Glow Plug Vs. Spark Plug

Although spark plugs and glow plugs perform the same tasks as combustion engines, their additional responsibilities are quite different from each other. Apart from their appearance, other significant variation includes:

1. Fuel

You will find glow plugs inside diesel engines while spark plugs can be found inside gasoline-powered engines. In a gas engine, sparks are generated by the spark plugs to ignite the blend of air and fuel in the ignition chamber. The same thing is done by glow plugs but in diesel-powered engines.

2. Heat Difference

Since gasoline is extremely volatile, it only takes one spark to activate the fumes. But as diesel isn’t as volatile in comparison, the diesel engine needs more intense compressions from the cylinder and higher heat levels to be ignited. Only diesel glow plugs can create these levels to ignite the diesel and air mixture within the combustion chamber of the engine.

3. Durability

Glow plugs are more durable than spark plugs because the latter works continuously when a car is in motion. On the other hand, glow plugs only function during the combustion process.

4. Ignition Process

Spark plugs need electrical energy to create sparks that combust the air and fuel blend in the engine. Glow plugs rely on electricity to warm up a heating element and that then creates enough heat to ignite the air and diesel mixtures.

Diesel glow plugs will signal the end of their service life in many ways. If you can notice any of these issues in your vehicle, you must get in touch with a professional technician immediately. Alternatively, we will also talk about the DIY aspect of it later on in this article.

Symptoms Of Failing Diesel Glow Plugs

  • Generally, one of the first symptoms of issues with glow plugs is a misfiring engine. The diesel engine does not get the heat it requires to work, thus causing the engine to misfire.
  • Decreased engine performance is another common sign. When the compressed air and fuel aren’t ignited properly and the misfiring starts, the overall engine performance of the vehicle takes a hit. You will notice low acceleration when you press on the gas pedal. With poor glow plugs, the engine will refuse to accelerate even when you step on the gas pedal.
  • A power reduction can be the result of misfires in the engine – a sign of bad glow plugs. The power decrease can get so bad that the car will not even move.
  • Hard starting can be noticed in cars with bad diesel glow plugs. While gas-powered engines need sparks to ignite the fuel blend, diesel engines have to work with cylinder pressures for the same purpose. The engine has to overcome the extra pressure of igniting the mix if the diesel glow plugs don’t work.
  • Black smoke may be released by the exhaust. For faulty glow plugs, they may disturb the delicate ignition process, which in turn may make the engine produce black smoke which is then emitted from the tailpipe. Black smoke from the car has to be diagnosed properly as there can be many other reasons behind this issue.
  • Decreased fuel efficiency is one thing to keep an eye out for. After the performance of the engine is weakened, it will have to work harder to meet the road’s demands. Your vehicle will consume more fuel to perform the same tasks it used to on a lower scale before. Miles per gallon is reduced, so you end up spending more to keep driving.

Why Do Glow Plugs Fail?

If your vehicle has displayed other signs and then starts giving lesser mileage for the fuel, it shouldn’t be a surprise to you.

Although glow plugs usually have long lifetimes, there will be instances in which they will fail much before their designated times. These are some of the main causes of glow plug failure:

  • You used the wrong kind of glow plugs
  • Your vehicle’s engine overheats excessively
  • The power supply is lengthy (perhaps because of a stuck relay)
  • Your engine works at extremely high voltages
  • The pistons have gone bad
  • The valves are damaged
  • The piston ring is seized
  • Eroded glow plugs due to dripping injectors

One other common reason behind glow plugs failure is high-amperage battery chargers, components used to elevate engine starting. These chargers are capable of producing voltages high enough for the glow plug tips to explode.

If you want to get a battery charger, select an intelligent model to avoid this issue. Battery chargers are quite a financial investment so make sure to have adequate knowledge on this topic first.

Pro tips: If all the glow plugs fail at the same time, it’s not because of natural wear and tear. Something specific damaged them.

Is It Okay To Drive With A Poor Glow Plug?

Glow plugs wear out over time and that will negatively impact your vehicle’s performance. The car may still start and allow you to drive it, but there will be a significant reduction in efficiency without the good glow plugs. As diesel glow plugs deteriorate, starting the car will take more attempts, and crossing higher speeds will have you crashing your feet against the gas pedal numerous times.

Diesel Glow Plugs

Your vehicle might not start at all if you live in cold regions, as the glow plugs fail to produce enough heat for the combustion process. Apart from all that, bad glow plugs will decrease fuel efficiency as well as increase your vehicle’s carbon footprint. Replace the diesel glow plugs as soon as they go bad to avoid inconvenience and these costs.

How To Test Bad Diesel Glow Plugs

Materials Needed

Step 1:

Find out the resistance value of the multimeter. Before you test the terminals, you have to determine the digital multimeter’s resistance value. For this, turn on the multimeter and set the settings to read in Ohms.

Pro tip: Ohms is shown by the symbol “omega.” It’s the symbol that resembles a horseshoe upside down (Ω).

When the multimeter has been set to Ohms, connect the multimeter’s two leads and check the resistance reading it is showing. If you get zero, try adjusting the multimeter’s setting and set it to a higher sensitivity. Keep doing that till you get a reading.

Jot down this value on paper as you will be needing this for some important calculations later on.

Step 2:

Find the glow plugs within the engine. Glow plugs are set inside cylinder heads and come with a heavy gauge wire attached to them – similar to a standard spark plug wire. Take off any covers as that may be stopping your path to the glow plugs. Use a flashlight (if needed) for additional illumination.

Step 3:

Detach the wires of the glow plugs. When you have located all the glow plugs, detach any caps or wires linked to them.

Step 4:

Take the multimeter and connect the negative terminal of the car’s battery to the negative leads. If possible, tuck the lead inside to secure it to the terminal. Or, you can secure it by tucking it under the clamping mechanism.

Step 5:

Repeat step 4 but this time with the positive terminal. Take the multimeter’s positive lead and touch it to the glow plug’s terminal.

Step 6:

Record the resistance of the glow plug. When both leads are connected to the terminals, jot down the resistance reading you see on the multimeter. Again, you should measure the reading in Ohms (Ω).

If there is no reading when the glow plug is touched, ensure the negative lead is still connected to the negative battery terminal.

Step 7:

Measure the true resistance value. You can do that by subtracting. A glow plug’s true resistance can be determined by obtaining the multimeter’s resistance value on Step 2 and subtracting it from the resistance value of the glow plugs.

Step 8:

Analyze the resistance value. Evaluate the measured true resistance value against the factory specification. If the acquired resistance of the diesel glow plug exceeds or falls short of the acceptable specification, the glow plug needs a replacement.

Pro tip: For the majority of glow plugs, the accepted range for resistance value lies between 0.1 to 0.6 Ohms.

Step 9:

Repeat for the leftover glow plugs until all of them have been tested. Even if one of the glow plugs doesn’t pass the test, it is best to replace the entire set. Changing only one, or just a few of the diesel glow plugs might cause problems with the engine similar to a bad glow plug if there is an excessive deviation between the resistance readings.

For most vehicles, checking a glow plug’s resistance is a fairly straightforward chore, granted you have unobstructed access to the glow plugs. However, if they are in a more discreet location, or you do not feel comfortable with the task, this is a service that any good auto mechanic will be able to do. If needed, they can also change the glow plugs so your car can go back to full functionality.

Given you have some technical know-how and want to replace the diesel glow plugs yourself, you will require a few tools such as a universal joint and a ratchet wrench set, deep sockets, screwdrivers, a glow plug connector remover and installer, 6-point combination wrenches, valve cover gaskets, a glow plug chamber reaming tool, and some penetrating lubricant.

Before starting, gather all necessary tools and supplies in one place and make sure to go through the instructions carefully. Don’t rush and miss and/or skip any of the steps because this is delicate work. Also, remember that instructions are generalized and should apply to most diesel engines. It’s best to consult with your repair manual for detailed instructions regarding your particular vehicle.

How To Replace Diesel Glow Plugs

Safety should always be your first concern when working with machinery; maintain caution around sharp instruments, hazardous materials, and hot objects. Don’t substitute equipment unless you know for sure that it won’t compromise the safety or performance of your car.

Moreover, since fuel and vapors are associated with vehicles, do not allow open flames or smoke in the work area; best to have a fire extinguisher nearby for gasoline fire mishaps.

Now that we have gone over the primary safety instructions, make sure to check your vehicle’s owner’s manual. That should point you in the direction of the diesel glow plugs’ location. Follow these steps to change them:

  • Take off the valve cover.
  • Remove anything that is obstructing your way to the glow plugs.
  • Detach the electrical connector and take off the intake manifold glow plug.
  • Use a combination wrench or deep socket to remove the glow plug attached to the cylinder head. Do not snap it!
  • Fasten the glow plug reamer to the glow plug opening fully.
  • Install the replacement glow plugs.
  • Reattach the glow plug terminal and the connector.
  • If required, change the valve cover too.
  • Reinstall any components that had been removed for access to the glow plug.

You’re done! The process is just as simple as changing a spark plug. It will take approximately an hour on some engines while it might be up to five on a few, depending on what is hindering access to the glow plugs and valve cover removal.

Glow Plug Replacement Cost

Typically, glow plug replacement will cost you anywhere between $95 and $210. Individual diesel glow plugs cost around $15 to $50 whereas the labor cost can be up to $80 to $160.

Know that every cylinder comes with its glow plug so if your vehicle has a V8 engine, it’s best to consider changing all 8 glow plugs despite only one of them going bad. You will not have to spend a lot on labor costs regardless of whether one of the glow plugs has gone bad or multiple.

The pricing is similar to when changing spark plugs. Granted you can just replace the faulty one but beware – the rest of the glow plugs are likely to follow suit soon.

The exact price will depend on the model and make of your diesel-powered vehicle and whether you get it from an auto repair shop or a dealership.

In general, the component itself isn’t too pricey but in comparison to gas engines, even a cheap glow plug is going to cost less than the best spark plugs. Try to find a reasonably priced technician with a decent hourly rate – that should reduce the expense a bit more.

Six signs it’s time to replace your glow plugs:

  1. The engine warning light on the dashboard is often the first indication that something is wrong with the glow plugs.
  2. Hard starting in cold conditions is the most common sign of damaged glow plugs.
  3. Misfires can occur due to faulty glow plugs, as the combustion chamber may not reach the required temperature to ignite the fuel.
  4. Rough idling, progressively harder starting, and white smoke from the exhaust can occur if one or more glow plugs are burned out, carbon fouled, or damaged.
  5. Glow plugs can affect fuel efficiency, as they provide additional heat in the combustion chamber, and if not functioning correctly, it can result in reduced fuel efficiency.
  6. Black smoke from the tailpipe can be a warning sign of something wrong either with the glow plugs or other vital components of the combustion process.

Final Thoughts

Given how important diesel glow plugs are to the proper functioning of an engine, it would be foolish to ignore the symptoms that they are going bad. Always try to maintain a routine when it comes to car maintenance and gets any abnormalities checked.


Here are some popular FAQs:

1. What Is A “Dual-Coil” Glow Plug?

As their name suggests, dual coil glow plugs comprise 2 heating elements in place of the usual one. The inclusion of an additional coil lets the glow plug have a unique “self-regulating” property. Glow plug designs of the past were a balancing act.

When the resistance was too high, the plug would increase in temperature slowly without getting burnt, lasting virtually forever. Similarly, if the resistance was low, the glow plug would heat up fast but burn out at the same pace.

Dual coil designs come with an interesting consideration: positive temperature coefficient. The resistance gets higher with increasing temperature in the glow plug. This lets the glow plug get hot very fast for quicker starts, as well as add life to it.

2. What Difference Does My Choice In Glow Plugs Make?

It makes quite a big difference, we would say. Yes, there are many places where you can be frugal. But, spend a bit more and get high-quality diesel glow plugs. No-name glow plugs in white boxes or cheap aftermarket ones can swell up after installing, making them impossible to remove. Worst case scenario: the tip will swell and burst in the cylinder, doing damage to the piston, valve, cylinder, and head. You will have to spend a hefty amount on engine rebuild.

3. Can I Replace Glow Plugs On My Own?

Sure! If you can replace spark plugs comfortably, you can do the same for glow plugs. Detach the wire, loosen the old plug, and thread in the new one! Try using a bit of anti-seize compounds and be careful not to damage the plug. Remember, you cannot over-torque them – they are hollow just like spark plugs.

4. How Long Do Glow Plugs Last?

If your vehicle is powered by a four-cylinder engine, one bad flow plug will make the process of starting your car hard. 2 faulty glow plugs will add to the struggle while 3 or more will make the job impossible.

On average, a diesel glow plug will last roughly 100,000 miles. So, you don’t have to worry about this situation often. However, they may fail before that, particularly if you frequently hard start, or use poor quality glow plugs.

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