Whether you’re a first-time diesel owner or have spent most of your life driving diesel trucks, you should have a basic understanding of the important role that glow plugs play in your engine.
A diesel engine cannot start and operate properly without the heat generated from this plug, especially when the outside temperatures start to dip.
While there’s a chance you may never have to worry about a glow plug failing (depending on how long you keep your vehicles), glow plugs will eventually wear out or get damaged, leading to symptoms that will impact your engine’s performance.
Recognizing the symptoms of a bad glow plug early on can save you time, frustration, and potential costly engine repairs down the road.
How a Glow Plug Works
Simply put, diesel glow plugs warm up the engine’s combustion chamber before the fuel is ignited. This will make it easier for the fuel to be ignited in the cylinder.
In cold temperatures, this process becomes even more critical, as diesel fuel can be challenging to ignite when it’s cold.
As opposed to gasoline engines, diesel engines rely on compression to generate heat within the combustion chamber. This is where glow plugs come in, providing that extra heat source to properly ignite the diesel fuel and get your engine running. When functioning correctly, glow plugs help make sure your diesel engine starts quickly and efficiently.
Glow Plugs vs Spark Plugs
Do not confuse glow plugs with spark plugs because they serve different functions in different types of engines. While both components help with the ignition process, they work in contrasting ways.
In gasoline engines, spark plugs are responsible for creating a spark that ignites the air and fuel mixture. This spark initiates the combustion process, enabling the engine to run.
On the other hand, glow plugs are used in diesel engines to heat up the combustion chamber. They don’t directly cause the combustion process, but they make it easier for the fuel to ignite within the chamber.
A glow plug features an electrode that generates heat when it is activated. This will cause the plug to “glow”, which is why it is named that. This is similar to how an incandescent light bulb will glow after it heats up too. This glowing effect is what heats the cylinder and allows for the successful ignition of the diesel fuel.
Here’s a good simple demonstration of how a diesel glow plug works:
See Also: Symptoms of a Bad Spark Plug
Top 4 Bad Glow Plug Symptoms
You now know that glow plugs are a necessity for a diesel engine to function properly. So, if the diesel glow plug were to become damaged or become faulty for some reason, the symptoms should be easy to spot. In fact, the whole car’s functionality will change because of it.
Here are four of the most common indicating a bad diesel glow plug.
1) Difficulty Starting the Engine
One of the most common symptoms of bad diesel glow plugs is difficulty in starting the engine, especially during cold weather. You may notice that it takes several attempts to get your vehicle going, which can be frustrating at best.
Since the pressures in the cylinder are mainly responsible for igniting the diesel fuel after it’s been injected into compressed air, a bad diesel glow plug will place more pressure on the engine for this ignition to be successful. That is why starting your vehicle becomes difficult.
2) Engine Misfires
If the glow plug is not functioning properly, the air inside the cylinder may not reach the required temperature, leading to incomplete combustion and a misfire. When the diesel fuel in a cylinder fails to ignite properly, it can lead to an engine misfire.
While you’ll still be able to drive your vehicle with a bad glow plug, you’re going to likely experience a decrease in normal performance.
3) Excessive Smoke From Exhaust
Another sign you have a problem with your glow plugs is excessive exhaust smoke. This can manifest as black, white, or blue smoke, depending on the cause.
Black smoke typically indicates incomplete combustion, often resulting from faulty glow plugs that don’t generate enough heat. On the other hand, white smoke may indicate that the glow plug is not heating the fuel properly, causing it to burn inefficiently.
Regardless of the smoke’s color, excessive exhaust emissions are a sign that you should get your glow plugs checked.
4) Check Engine Light On
Usually when a diesel glow plug malfunctions, the Check Engine light in the dashboard will turn on. To confirm the actual problem, you should use a diagnostic scan tool to read the specific trouble code(s) that have been stored by the vehicle’s ECM.
Commonly, the code showing will be P0380 which is defined as glow plug/heater circuit “A” malfunction.
Diagnosing a Faulty Diesel Glow Plug
Visual Inspection
Start by giving your diesel glow plugs a visual inspection. You can often spot signs of wear or damage on the surface of the glow plug.
Look for cracks, swelling, or discoloration. Sometimes, carbon buildup or a broken heating element can be visible. Keep in mind that some issues might not be apparent just by looking at the glow plug, so further diagnosis might be necessary.
Use of Scan Tool
As already mentioned, when your Check Engine light is on, it’s a good idea to use a scan tool to help identify the issue. Many times, faulty glow plugs will trigger a code in your vehicle’s computer system, pointing you in the right direction.
With your scan tool, you can retrieve these codes and determine if bad glow plugs are causing the problem. Remember, other factors could also cause the check engine light to come on, so don’t assume the glow plugs are the only problem.
Professional Diagnosis
If you’re unsure or unable to diagnose the issue yourself (or simply don’t have the time), it’s best to consult with a professional technician or your trusted mechanic. They have the tools, knowledge, and experience to assess your vehicle’s glow plug system thoroughly.
Glow Plug Replacement Cost
We recommend Parts Geek for the best prices and selection.
The typical replacement cost for a diesel glow plug will be anywhere from $95 to $210. The cost of an individual glow plug will $15 to $50 while the cost of the labor will be about $80 to $160.
Keep in mind that each cylinder has its own glow plug so if you have a full size truck with a V8 engine, you may want to consider replacing all eight glow plugs even if only one plug is known to be bad. The labor cost isn’t going to be too much more whether one glow plug or many have failed.
It’s similar to when replacing spark plugs. Sure you can replace only the bad one but there’s a good chance another or all of the rest will fail soon anyway.
The exact cost will depend on the make and model of your diesel vehicle and whether you take it to a dealership or auto repair shop.
In general, the part itself isn’t too expensive but compared to gasoline engines, even the best spark plug is likely going to cost less than a cheap glow plug. If you can find a reasonably priced mechanic with a low hourly rate, then it will reduce your expense even more.
Frequently Asked Questions
How Long Do Glow Plugs Last?
The lifespan of glow plugs can vary depending on factors such as usage, maintenance, and the type of vehicle. However, on average, glow plugs can last anywhere from 100,000 to 200,000 miles.
How Many Glow Plugs Does a Diesel Engine Have?
The number of glow plugs in a diesel engine can vary depending on the number of cylinders and the type of engine. For example, an eight-cylinder engine using in-cylinder plugs would have eight glow plugs. A six-cylinder engine would have six. And so on.
That said, a small number of diesel engines, especially direct-injection engines, may not use starting-aid devices such as glow plugs.
Can Defective Glow Plugs Lead to Power Loss?
Yes, defective glow plugs can lead to power loss in your diesel engine. When the glow plugs are not functioning properly, the engine has trouble starting and maintaining its performance, especially in cold weather. This can result in decreased power output.
Do Bad Glow Plugs Trigger Limp Mode?
Bad glow plugs themselves will not directly trigger limp mode in your vehicle. However, they can contribute to other issues that may ultimately lead to limp mode.
Can a Diesel Engine Operate With Faulty Glow Plugs?
A diesel engine may still run with faulty glow plugs, but it will be much more challenging to start, especially in cold weather. It might take several attempts to get the vehicle to start, and you may notice rough idling and diminished power once it is running.
So, while you will probably be able to continue driving, it’s not advisable to continually run a diesel engine with faulty glow plugs.
Does a Broken Glow Plug Harm the Engine?
A broken glow plug can potentially harm your diesel engine, especially if it remains inside the cylinder. The glowing tip could break off and damage the engine components, such as the piston or cylinder walls. This could lead to costly repairs or even engine replacement.
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I am interested in Diesel Engine technology and car Electrics. I have learnt a lot here.
Kindly may you send me related PDFs .
Thank you very much.
Yes there is a danger of plug snapping , if you don’t fancy a gamble lay your car up until warmer weather that’s what I was going to do , found a local garage with a diesel mechanic who had the confidence to do the job all went well I am no longer afraid of those frosty mornings. Depends on your car’s age , value etc.
good day. may you please give effects if any if you replace short electrode glow plugs with longer ones. I have an Isuzu bighorn 4JG2 with all 4 glow plugs blown
What this article doesn’t tell you is the high risk of glow plug breaking when you try to change it – this is a delicate procedure and it shouldn’t be rushed – if a spark plug breaks , the engine must come out, the engine head must be taken down , injectors removed – which on a cold engine might not come out willingly and might break and need replacement, machine the broken glow plug out, resurface the head, put a new head gasket, new head bolts, new injector seals and hope the work is done right otherwise the head gasket might fail.
This has the potential to bring a world of hurt to your bank account plus it will leave your car not functional for some time.
If a glow plug doesn’t come out, better not force it unless you are prepared for these consequences, the car can still run, maybe not perfect but the alternative is horrific.
Can you drive a tdi diesiel car jetta 4 with only one glow plug working?Is it safe to drive it or bad?
How much should it cost to replace glow plugs on a citroen despatch reg mf54psx been told this is an awkward job
Hello. F350 diesel starts instantly, even though Heavy black smoke is coming out of the tailpipe! My scanner says Po677-cylinder #7-glowplug open.
Would replacing the glowplug stop the truck from smoking? I’m no mechanic, so “baby steps” please.
I’ve Swift Dzire Tours , When going to start Car….it take long self but not start engine..Is this problem relates to Glow plug or other than it.
Hi, thanks for your response.tdi glow plugs is something new,fist time l use diesel engine.
Hey chief advisers , Am student in automotive mechanics and I need some ideas on diesel engines can I get them from u please, thanks for supporting this web you are rely good teachers.
Hi there I have a problem I’ve got a Vauxhall zafira 1.7 diesel change glow plugs light still comes on dashboard and it blows white smoke from exhaust
I have a Land Rover Freelander TD4 2006 model discharging thick black smoke especially in the morning also struggle to start where it used to start with one kick but now takes three kicks. Struggle to start up the engine if parked for a long time. Please advise if it could be caused by the glow plug.
Glow Plug
Hey guy i have a nissan ud 2300
I’m trying to change the glow plugs but I can’t find the location
Could you give me a hand with that
I’m not familiar with that engine but they’ll be located on top of the cylinders. I’d recommend Youtube so you can visually see the general location, even if it’s not your vehicle they’re showing.
Thanks, your write up also help.
Always heat the engine up before removing glow plugs, less chance of breaking any then.