Vehicles using diesel oil as fuel use glow plugs within the engine’s combustion chamber to heat the engine’s cylinder and ignite the fuel.
Furthermore, glow plugs maintain smooth engine performance once the engine is running.
Over time, and for various reasons, glow plugs lose their efficiency. So, you must replace them to maintain performance. But, don’t worry, they are very affordable and cost-effective.
Depending on the repair shop, glow plug manufacturer, and its quality, the typical cost of replacing glow plugs is between $25-$50 per plug and $90-$200 for a mechanic’s labor to change a glow plug set.
Usually, the job takes a qualified mechanic up to two hours to complete depending on the engine’s design. And, of course, the longer it takes to finish the job, the more you will pay.
Glow Plug Replacement Cost*
You will find that the cost of replacing glow plugs varies with the make, model, and age of your vehicle.
The table below gives you an idea of how much the price changes with vehicle type.
|Vehicle||Year Model||Average Dealer Price||Parts Cost||Labor Cost|
|Volkswagen Jetta 1.6L Diesel||1989||$185||N/A||N/A|
|Volkswagen Jetta 2.0L Turbo Diesel||2013||$510||N/A||N/A|
*Note: Prices are estimates and were correct at the time of writing (February 2023). Since then, cost estimates may have changed. Therefore, use our figures only as a starting point for your own research.
What Are Glow Plugs?
Diesel fuel in the presence of oxygen self-ignites under pressure. However, cold ambient temperatures prevent the fuel/air mix from igniting until the engine has warmed up. That’s what glow plugs are for.
Unlike spark plugs that ignite gasoline fuelled cars, glow plugs warm the cylinder, so the diesel oil ignites more easily. You can liken the effect to an incandescent light bulb filament that glows and heats up using an electrical current.
Then, the glowing plug helps to ignite the fuel and start the piston’s compression cycle. Without the glow plugs, you won’t be able to start the engine.
Once the cylinder has warmed and the fuel/air mixture ignites, the glow plug will keep the engine running smoothly. However, your engine will have difficulty starting if the glow plug gets damaged or otherwise becomes faulty.
Furthermore, if it’s cold outside, the defective plug won’t be able to start the engine at all.
Advantages of Replacement Glow Plugs
- Efficient ignition during cold weather
- Gas-tight seal around the plug’s thread
- During idling and the engine warm-up phase, the car has reduced emissions.
- During idling, the engine runs smoothly.
- Modern glow plugs have high corrosion resistance and excellent heat transfer.
Glow Plug Replacement Pricing Factors
Several factors affect the price of replacement glow plugs.
Here are a few of the most significant.
Finding the correct glow plugs for your vehicle can be difficult as many different kinds are available, and some designs are for specific car models.
Furthermore, the glow plug manufacturers also offer various quality levels of glow plug for you to choose from. Some of the best manufacturers include:
- Standard Motor Products
Named brands like these will cost more but have longer lifespans and are usually better quality than budget glow plugs.
Always get advice from qualified diesel mechanics, specialist retailers, or your vehicle’s manufacturer’s handbook before fitting new glow plugs.
Glow Plug Lifetime
The average expected lifetime of your glow plugs determines how many you must fit and how much you pay over your car’s lifetime. However, there’s some good news as glow plugs are incredibly long-lasting.
Also, they’re one of the most cost-effective car components, making replacing them less expensive than you might first expect.
Typically, under everyday use, glow plugs last around 100,000 miles, after which they will gradually deteriorate because of natural wear and tear.
Type of Vehicle
Usually, changing the glow plugs on a standard engine is relatively straightforward if you have the tools and knowledge. However, for some reason, some vehicle manufacturers make replacing glow plugs very difficult.
You might have to remove hoses, covers, and many other components before you have enough room to access and replace them. Difficult access extends the average time of the job and increases the mechanic’s labor charges.
Type of Glow Plug
It’s essential to use the type of glow plug recommended in your car manual.
Unfortunately, some vehicles require expensive glow plugs.
Premium or Budget Brands
Generally, premium glow plug brands cost more to buy, but have a longer lifespan and operate more efficiently over a broader range of conditions.
In contrast, you might find a set of budget glow plugs don’t last as long and are less efficient.
7 Signs Of Bad Glow Plugs
When glow plugs become faulty or damaged, you will notice a few difficulties when starting and running your engine. The most obvious sign is an illuminated warning light on your dashboard.
When this occurs, it’s essential to take your car to a repair shop specializing in diesel motors or change them yourself if you have the tools and experience.
Here are a few signs to watch out for, so you can get your glow plugs replaced as soon as possible.
1. Slow Engine Starting or Failure to Start
A faulty glow plug won’t generate enough energy to heat the engine’s cylinder and to help ignite the fuel.
If it’s not producing the required heat fast enough, you might need several attempts before the engine starts. And, even worse, if you have lousy glow plugs and it’s freezing outside, you won’t get the engine started at all.
2. Engine Misfiring
When your exhaust backfires, you know there’s something wrong. Often, this happens when the diesel fuel partially ignites in the compression chamber, and unburnt oil ends up in the exhaust.
If you experience backfires (or a misfire), it’s probably something to do with your glow plugs.
3. Dashboard Warning Lights Illuminate
You have probably seen the pretty lights on your car dashboard. Generally, those tell you when something goes wrong or isn’t working as it should.
Look out for the faulty glow plug indicator; if it’s lit, you know you must do something about it because something’s wrong. Look in your car’s manual for the symbol description and location of the glow plug indicator light if you aren’t sure where it is.
4. Rough Idling
Part of the glow plug set’s job is maintaining a smooth-running engine after it’s started.
If your car’s idling sounds rough and uneven, you know it’s probably the glow plugs malfunctioning.
5. Decreased Fuel Efficiency
When your glow plugs don’t operate properly, the diesel oil doesn’t get burned efficiently. Then, unburnt fuel ends up in the exhaust and drips onto the road or causes backfires.
You are wasting fuel that otherwise would power your vehicle, which will eventually cost a lot of money over the car’s lifetime.
Furthermore, you will increase your vehicle’s carbon footprint and damage the environment if you let oil go to waste like this.
6. Too Much Exhaust Smoke
Several issues cause too much grey or black exhaust smoke. One of these issues might be faulty glow plugs.
If you experience smoke and other problems mentioned on this list, you know it’s glow plugs.
7. Poor Acceleration
No matter how hard you try, you can’t get a vehicle with faulty glow plugs to perform as it should. Even after you eventually get it started, you’ll notice problems when you press the accelerator and don’t generate much speed.
However, other engine problems can cause power loss, so don’t automatically assume it’s glow plugs unless you have another symptom from this list.
How Do You Replace Glow Plugs? (Video)
If you suspect you need new glow plugs, you might also need to replace the wiring harness and the glow plug relay. Therefore, unless you know what you’re doing, ask a professional to diagnose the problem and replace the faulty components.
However, it’s useful for non-professional mechanics to know the general procedure for replacing glow plugs, as it gives them an idea of the project’s difficulty and what tools they need.
Before we go any further, you must understand that this description is for general information, as repair procedures and safety hazards vary according to the vehicle’s make and model.
To overcome this problem, mechanics should reference an official factory service manual or use an appropriate Chilton or Haynes manual, which is more affordable.
- Ensure the ignition is turned off and the engine is cold. Remove all covers to allow access to the glow plugs. Sometimes, depending on the vehicle, you might have to remove the valve cover.
- Some vehicles have a bus bar that connects the glow plugs. If yours does, remove the small fastening nuts and the bus bar. Other cars use leads similar to spark plug leads. Only unclip these from each plug, noting which lead connects to its corresponding plug. This information will help with reconnection.
- Using a suitably sized socket wrench, unscrew one glow plug and replace it with a new one. When fitting the new plug, use your fingers to rotate the plug and start the thread. Otherwise, you risk cross-threading the plug and damaging the cylinder head. Once the glow plug is screwed in finger tight, use a torque wrench set to the recommended torque. The torque value and other relevant information should be on the glow plug packaging. Otherwise, consult the vehicle service manual.
- Either replace the bus bar, ensuring you don’t tighten the retaining nuts too much. Or replace the leads, ensuring each lead connects to its appropriate plug.
- Verify that everything is installed correctly and replace the engine covers.
There are many online tutorials dealing with various aspects of glow plug replacement. You’ll probably find instructions to suit your vehicle if you search in YouTube.
In the meantime, here is a video from NGK Spark Plugs entitled Glow Plug – Tips and Tricks that you will find helpful:
Are glow plugs the same as spark plugs?
Glow plugs in diesel engines perform the same function as spark plugs in gasoline engines; they ignite the fuel.
However, there are significant differences between the two:
- Spark plugs generate sparks in a gasoline fuelled engine to ignite the fuel/air mix within the combustion chamber. In a diesel engine, glow plugs heat the oil/air mixture to assist the combustion under pressure.
- Gasoline is very volatile, so it only takes a spark to ignite the vapor. In contrast, because diesel oil is less volatile and the engine needs more intense cylinder compression, the temperature required for ignition is much higher. Glow plugs supply this intense heat.
- Glow plugs have a longer lifetime than spark plugs because spark plugs work non-stop on every piston cycle whenever the engine is running. On the other hand, compare this to glow plugs, which mainly work during initial ignition.
- Spark plugs use electricity to generate a spark, which ignites the fuel/air mixture. In contrast, glow plugs use electricity to warm a heating element. This creates enough heat to ignite the diesel/air mixture.
Why do glow plugs fail?
Typically, glow plugs last for around 100,000 miles.
However, sometimes they fail before this, caused by one or more of the following reasons:
- Have you installed the incorrect glow plugs in the engine?
- Does the engine suffer from overheating?
- Perhaps the engine operates with voltages that are too high for the glow plugs.
- Your engine has damaged pistons.
- Is the piston ring seized?
- Are the engine’s valves damaged?
- The glow plugs become eroded because the injectors have dripped onto them.
- Sometimes vehicle owners use high-current battery chargers to improve engine starting. This practice damages the glow plug tips.
If your glow plugs fail before the 100,000 miles mark, get them checked out by a professional. You could have an underlying problem that causes the symptoms.
How long do glow plugs last?
If your four-cylinder vehicle has one damaged glow plug, you will have problems starting the engine. Two faulty glow plugs will make it even harder to start.
However, three will make it impossible to ignite the fuel mix. Therefore, replacing faulty glow plugs is essential.
But, how often do you need to do this? Typically, glow plugs should last for around 100,000 miles, but they will gradually erode and deteriorate over time.
When you decide to replace the glow plugs, the cost will depend on whether you want a premium or budget set of plugs and the make, model, and age of your car.
Get professional advice on which glow plugs to choose.
Can I drive with faulty glow plugs?
By now, you should understand that bad glow plugs detrimentally affect your car’s performance.
You will have issues with starting the engine, it will probably take 2 or 3 attempts, and if you live in cold regions, it probably won’t start at all because the glow plugs can’t produce enough heat to trigger ignition.
But, once it’s running, you can still drive it. However, the vehicle won’t work efficiently without the correct, undamaged glow plugs, your fuel efficiency will be down, and the vehicle will operate at reduced power.
What Do I Do Next?
If you are experiencing power loss or reduced speed issues, using more fuel than expected, or notice any other symptoms we’ve listed in this guide, it’s time to replace your glow plugs. Over time, diesel engine glow plugs lose their efficiency and must be replaced. Typically, depending on your vehicle, the repair shop, glow plug manufacturer, and plug quality, the average retail price range for glow plugs is $25-$50 each, with labor between $90-$200 to change a set of plugs.