Glow plugs are essential engine management system components found in diesel-powered engines. They replace the spark plugs in gasoline engines.
Glow plugs ensure your vehicle starts with ease even in cold weather conditions, where it is harder to start the engine. Their primary function is to preheat and warm the engine cylinders for easy combustion.
They are essential components that last long in the engine. But they do fail over time due to regular use. In this article, I’ll discuss the symptoms of bad glow plugs. But first, let’s look at their working principles.
What is a Glow Plug?
Gas-powered engines use spark plugs to ignite the air-fuel mixture inside the combustion chambers. On the other hand, Diesel engines rely on the heat from the compression of air to ignite fuel. Because of this, diesel engines can be difficult to start when cold.
Many diesel-powered vehicles use glow plugs to address this issue. The glow plugs act as heating elements to preheat the air in the combustion chambers, making the vehicle easier to start. Diesel engines typically have one glow plug per cylinder.
Each glow plug has a terminal, a threaded body, and a tip the protrudes into the engine’s combustion chamber.
Applying voltage to the terminal end of the plug allows electrical current to pass through a heating element in the plug’s tip. The plug’s body, which is threaded into the cylinder head, acts as an electrical ground for the heating element.
Modern vehicles have a control module that manages the glow plugs. The module looks at inputs from various sensors, such as the coolant temperature and oil temperature sensors, when determining when to turn on the glow plugs. On many vehicles, the control module activates the glow plugs via a relay.
How do glow plugs work?
Glow plugs work on a simple yet effective scientific principle. Electrical energy is converted into heat. This heat then increases the temperature inside a diesel engine’s combustion chambers.
A glow plug’s operation begins when the vehicle’s ignition is turned on. Electrical current from the vehicle’s battery is supplied to the glow plug, focusing on the heating element at the plug’s tip. This element quickly heats up due to the electrical resistance it provides.
The heating element warms up and raises the temperature in the combustion chamber. This helps to ignite diesel fuel more effectively for ideal combustion. Once the engine starts running smoothly, the glow plugs turn off since they have completed their task.
Types of glow plugs
A few types of glow plugs are available in the market, catering to specific engine requirements and conditions.
#1. Pressure Sensor Glow Plugs.
Pressure Sensor Glow Plugs (PSG) are “smart” plugs with a sensor in the tip that measures combustion chamber pressure.
This information is sent to the Engine Control Unit (ECU) to adjust the heating pattern accordingly. They provide precise control for optimizing engine performance.
#2. Ceramic Glow Plugs.
Designed for extreme temperatures, Ceramic Glow Plugs (CGP) can reach up to 1500°C in under three seconds. This rapid heating helps start engines quickly. It’s beneficial in low-compression diesel engines facing Environmental Protection Agency (EPA) restrictions.
#3. High-Tech Steel Glow Plugs.
They come in two kinds: GE and GN. GN plugs are modern three-phase glow plugs that heat up quickly, remain hot during ignition, and cool down after three minutes. The plugs are long, pencil-like.
Older diesel engines used GE-type plugs, which could take up to 30 seconds to heat, but modern high-tech steel allows them to heat in just seven seconds. GE plugs are typically shorter and more similar in size to spark plugs.
What are the symptoms of bad glow plugs?
The common signs of failing glow plugs are engine warning lights, hard starting, engine misfiring, rough idling, decreased fuel efficiency, white smoke, and black smoke from the exhaust tailpipe.
#1. The check engine light will come on.
The check engine light is the first sign that shows something is wrong with the engine. In case of a failing glow plug, the car computer will trigger the check engine light to notify you that the glow plugs are bad.
However, several factors can trigger the check engine light. Hence, you must diagnose the vehicle once the warning light appears on the dashboard.
#2. The engine will experience hard starting.
The most prominent symptom of a bad glow plug is hard starting. If the glow plug is faulty, the combustion chamber will not heat up to the required temperature to burn the diesel and start the engine. Starting a diesel with bad glow plugs will relentlessly cause hard starting.
#3. Engine is misfiring.
The diesel engine combustion process requires a high amount of compression, diesel injected into the engine, and at the right amount. With all this in place, combustion still won’t take place if the combustion chamber doesn’t reach the required temperature.
Can faulty glow plugs cause loss of power? You guessed it right! The diesel engine will misfire if there are issues with fuel injection, fuel quality, and compression.
The heat required for combustion comes from the glow plugs and compression pressure. The temperature in the combustion chamber needs to get to 232 degrees Centigrade or 450 degrees Fahrenheit for the combustion to occur.
If the glow plugs are bad, the combustion chamber will never reach the necessary temperature to keep the engine running optimally.
#4. The engine is rough idling.
If any glow plug is damaged or carbon fouled, it will be difficult to start the engine. And when it finally starts, it will struggle to stay running.
This is known as rough idling. This can also be one of the symptoms of a bad glow plug relay. The rough idling will cause white smoke.
#5. The exhaust is emitting white smoke.
White smoke is a common issue when there’s not enough heat to burn fuel properly. The engine will export the unburnt fuel particles through the exhaust.
In such cases, you’ll perceive fuel smell from the tailpipe. It is not uncommon to see white smoke from diesel exhaust during cold weather.
The common reasons for white smoke from the exhaust are lousy glow plugs and damaged glow plugs control module.
Though, your engine can emit excessive white smoke due to other issues like air in the fuel system, low compression, and low engine cranking speed.
If the white smoke continues even after the engine gets hot, you likely have a worn injector pump, retarded injection timing, or one or more bad injectors.
#6. Black smoke Coming from the exhaust.
Generally, black smoke happens due to rich air-fuel ratio. That is too much fuel and too little air. The thing is, lousy glow plugs will not burn all the fuel in the combustion chamber. This will make the O2 sensor think there’s too much fuel entering the engine.
Ordinarily, the unburnt fuel will produce black smoke from the tailpipe. Black smoke is, therefore, a sign that the glow plugs are faulty or there’s something wrong with any other vital engine component.
#7. There is poor fuel efficiency.
In diesel engines, fuel and temperature strictly determine fuel efficiency and not the plugs. However, glow plugs influence the engine temperature.
Since diesel engines must get to the required temperature before combusting the fuel, lousy glow plugs will weaken the engine. As a result, the engine will work harder to meet the driver’s demand. This will invariably lead to poor gas mileage.
But wait a second! Are glow plugs only used for starting? The glow plugs are primarily designed to help the engine start. But now, glow plugs are used to measure the temperature in the combustion chamber.
Car manufacturers are advancing glow plug technology. Glow plugs now feature a pressure transducer called Glow Combustion Sensor (GCS).
The primary function of this technology is to measure the temperature in the combustion chamber and relate the information to the car computer. The PCM will use this information to decide on air-fuel input.
Potential Causes of Bad Glow Plugs
When it comes to the intricate world of diesel engines, understanding the potential causes behind bad glow plugs is essential. Glow plugs are the unsung heroes responsible for heating the combustion chamber, ensuring a smooth start in cold conditions.
However, like all components, they can sometimes fail. Let’s delve into the possible culprits that can lead to the demise of these critical engine elements.
#1. Normal Wear and Tear.
One of the most common causes of bad glow plugs is normal wear and tear. Just like any part of your vehicle, glow plugs have a finite lifespan.
Over time and extensive use, they can wear out and lose their effectiveness. High-mileage vehicles are particularly susceptible to this issue.
#2. Carbon Buildup.
Carbon deposits can accumulate on the surface of glow plugs over time, negatively impacting their performance.
These deposits can insulate the plugs, preventing them from reaching the necessary temperature for proper ignition. Carbon buildup can be exacerbated by poor-quality diesel fuel or irregular maintenance.
#3. Faulty Wiring.
The electrical components that power the glow plugs are equally important. Faulty or damaged wiring can disrupt the electrical flow to the glow plugs, rendering them ineffective. Corroded or broken connections can lead to inconsistent heating and starting problems.
#4. Temperature Sensor Malfunction.
Diesel engines rely on temperature sensors to determine when to activate the glow plugs. If these sensors malfunction or provide inaccurate readings, it can lead to starting problems.
A malfunctioning sensor may fail to activate the glow plugs when they are needed, particularly in cold weather.
#5. Poor Maintenance.
Regular maintenance is crucial for the longevity and performance of glow plugs. Failure to follow the manufacturer’s recommended maintenance schedule can lead to premature glow plug failure.
Routine inspections and replacements as needed should be a part of any diesel engine owner’s maintenance routine.
Diesel Glow Plugs vs. Spark Plugs
The glow plugs and spark plugs are very similar in nature, but do have some differences. Here are a few you will find when you examine the two.
#1. Fuel Type.
First, glow plugs are installed in diesel engines, while gasoline engines are outfitted with spark plugs. With gas engines, spark plugs are needed to create the spark that ignites air and fuel inside the combustion chamber.
However, this isn’t the case with diesel engines. The diesel engine doesn’t need a spark for combustion; it only requires heat, which is increased with the glow plug.
#2. Heat.
Gasoline is a volatile substance. It only requires a small spark to trigger combustion. However, diesel fuel requires compression and heat to combust.
There must be a high level of heat within the cylinders to produce combustion, which is where the glow plug comes into play. Glow plugs produce an intense heat that helps the engine ignite the fuel/air mixture in the combustion chamber.
#3. Durability.
Glow plugs last far longer than most spark plugs. Glow plugs are only used to warm the cylinders for ignition. After that, they are barely needed.
On the other hand, spark plugs work continuously to keep the engine working. Cheap spark plugs might only have a lifetime of about 30,000 miles.
#4. Ignition Difference.
Spark plugs take electrical energy to create a spark. This spark ignites the air and fuel mixture in the combustion chamber.
Diesel glow plugs don’t use a spark. They turn electricity into heat, which helps to ignite the fuel and air mixture.
How to test glow plugs?
Over time, glow plugs wear out, and their internal resistance changes. When this happens, the plugs may cause hard starting and other issues. Fortunately, testing the glow plugs to see if they require replacement is a fairly straightforward affair.
Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.
#1. Perform a Preliminary Inspection.
If you’re diagnosing the glow plug system on a modern vehicle and the check engine light is on, the first thing you’ll want to do is check for diagnostic trouble codes (DTCs). A control module monitors the glow plug system.
When the device detects a problem with one or more of the glow plug circuits, it will turn on the check engine light and store a diagnostic trouble code in its memory.
Note: If you have a scanner with OEM enhanced data, there is also a “key on engine running (KOER)” glow plug test you can perform on select Ford vehicles.
Next, you’ll want to perform a visual inspection of the glow plugs. Look for issues, such as damaged wires and poor connections. Make sure that the electrical connectors going to the glow plugs are clean and tight.
Repair any issues found during the visual inspection, clear the DTCs, and see whether the problem returns.
From there, you can move on to testing the glow plugs as necessary.
#2. Test the Glow Plugs with an Infrared Thermometer.
Before you whip out your digital multimeter, you might want to use an infrared thermometer to check for inoperative glow plugs.
To do this, first, make sure the engine is cold. Then, cycle the ignition a few times to heat up the glow plugs, and point the thermometer at each plug.
If you find a plug that has a much lower temperature than the others, that plug isn’t working properly, either due to an internal failure or a damaged electrical connector.
Note: This test is only used to locate plugs that aren’t working at all. A glow plug may pass this test and still have carbon buildup or be out of specification resistance-wise.
#3. Test the Glow Plugs with a Multimeter.
As was mentioned, the internal resistance of the glow plugs changes as the plugs begin to degrade. You can test the resistance of the plugs with a digital multimeter (DMM).
The process for testing glow plugs may vary depending on the type of vehicle you have. For example, if the plugs are difficult to access, you may want to check the resistance at the wiring harness rather than at the plugs themselves. It’s a good idea to consult a repair manual or repair database for the proper test procedure.
In general, however, you can test the resistance of a glow plug by doing the following:
- Make sure the vehicle’s ignition is in the OFF position.
- Disconnect the electrical connector from one of the glow plugs.
- Set your digital multimeter (DMM) to the ohms setting.
- Connect the red meter lead to the glow plug’s terminal.
- Connect the black meter lead to a good ground, such as the engine block.
- Note the resistance reading on your meter. If the meter reads out of limits (OL), that indicates an open circuit and a faulty glow plug. Likewise, if one glow plug has a significantly different internal resistance than the others, that plug is likely faulty and should be replaced.
How To Fix Bad Glow Plugs?
When your diesel engine’s glow plugs start to show signs of wear or malfunction, it’s essential to address the issue promptly to ensure reliable starts and optimal engine performance.
Fortunately, there are several effective fixes for bad glow plugs, ranging from simple maintenance tasks to component replacements. Let’s explore these illuminating solutions:
#1. Replacing Glow Plugs.
In many cases, the most effective solution for bad glow plugs is to replace them with new ones.
Glow plugs have a limited lifespan, and if they’ve worn out or become damaged, new plugs are the best way to restore proper function. Consult your vehicle’s service manual or a mechanic for the correct replacement parts and specifications.
DIY Tip: If you’re a skilled DIYer with the right tools, you may be able to replace glow plugs yourself. However, if you’re unsure or inexperienced, it’s advisable to seek professional assistance to avoid damaging other engine components.
How to replace glow plugs?
If, after testing glow plugs with test light or by any approach, and find that the test light is bad, the best solution is to replace the faulty plugs.
Here’s an easy-to-understand way of replacing a glow plug:
- Disconnect the negative battery terminal
- Remove the valve cover if necessary
- Locate the glow plugs on the engine
- Disconnect the power supply
- Take off the glow plugs
- Clean the plug surrounding and the plug hole
- Install the new glow plugs
- Reconnect the power supply
- Reinstall the valve cover if you removed it earlier
- Connect the negative battery terminal.
#2. Cleaning Carbon Deposits.
If carbon deposits have accumulated on the surface of the glow plugs, cleaning them can often improve their performance. To do this:
- Remove the glow plugs: Carefully remove the glow plugs from the engine using the appropriate tools.
- Clean the plugs: Use a suitable cleaning agent or a wire brush to gently remove the carbon deposits from the tips of the glow plugs.
- Inspect and reinstall: After cleaning, inspect the plugs for any visible damage. If they appear to be in good condition, reinstall them in the engine.
#3. Repairing Wiring.
Damaged or corroded wiring can disrupt the electrical flow to the glow plugs, rendering them ineffective. If you discover faulty wiring, repairing or replacing it is essential. Make sure to use high-quality wiring and connectors to ensure a reliable electrical connection.
#4. Addressing Sensor Issues.
If the temperature sensors responsible for activating the glow plugs are malfunctioning, it’s crucial to address and repair or replace them. Malfunctioning sensors can lead to starting problems, especially in cold weather.
#5. Regular Maintenance.
Preventing glow plug issues is often a matter of routine maintenance. Follow the manufacturer’s recommended maintenance schedule, which typically includes regular inspections and replacements of glow plugs as needed.
By staying on top of maintenance, you can prolong the life of your glow plugs and ensure they function optimally.
When to replace glow plugs?
If you are experiencing any of the issues listed above, replace the glow plugs as soon as possible. Not only will this help to solve the listed issues, but it will also improve the efficiency of your engine.
If you haven’t noticed any problems, the 150,000 km (100,000 miles) mark is still a good point to replace the glow plug.
Replacing them at this point helps to prevent future problems. It improves engine efficiency, ensuring smooth starts and operation under various conditions.
Is It Safe to Drive a Car with Bad Glow Plug Symptoms?
Even while a bad glow plug won’t stop your car from starting or moving, it’s never a good idea to overlook the problem because it might lead to additional issues that you’d rather avoid.
Because your car won’t start if you have a lot of bad glow plugs, things become more difficult. This is a crucial circumstance, particularly if you’re starting on a chilly day or in a place where there isn’t any local aid.
How much does it cost to replace new glow plugs?
The typical replacement cost for diesel glow plugs generally ranges from $300 to $800, depending on the engine size and labor rates.
Individual glow plugs usually cost between $25 and $70 each, with labor costs varying based on the vehicle and shop but often ranging from $200 to $500 for a full set.
Keep in mind that each cylinder has its own glow plug, so for a full-size truck with a V6 or V8 diesel engine, you may want to replace all the glow plugs even if only one is known to be bad.
Labor costs don’t increase significantly whether you replace one glow plug or all of them, since much of the work overlaps.
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 after. Replacing them all at once is simply more cost effective long term.
In general, a glow plug isn’t too expensive, but compared to gasoline engines, even the best spark plug is likely going to cost less than a low-end glow plug. Finding a reasonably priced mechanic with a fair hourly rate can help reduce your overall expense.
FAQs.
Q: How do I test if my glow plugs are bad?
The easiest free test is the “warm-up test.” Note if symptoms like hard starting and smoke disappear within 15 minutes of driving.
If they do, test the glow plugs. You can also use a multimeter. Set it to Ohms and test the resistance between the plug terminal and ground. A reading of 0.1–6 Ohms means it works; an “OL” reading means it has failed.
Q: What OBD codes indicate bad glow plugs?
Common codes include P0380 (general circuit malfunction) and P0670 (module control circuit).
Codes P0671 through P0678 indicate a specific cylinder circuit is open. If you get a specific code like P0671, test that single plug. If you get codes for all cylinders at once, check the relay or module.
Q: Should I replace all glow plugs at once?
Yes. Glow plugs wear at the same rate. If one fails, the others will likely fail within months. Replacing them all at once saves you from paying the labor charge twice.
Q: What is the difference between a glow plug and a spark plug?
A spark plug creates a spark to ignite gasoline in a gas engine. It fires every time the cylinder fires. A glow plug is a heater used in diesel engines. It only warms the combustion chamber for cold starts. Diesel engines rely on compression heat, not sparks, to run.
Final Words
If you experience any of the symptoms of bad glow plugs above, have your mechanic test them to confirm if they are truly faulty. Of course, you can run the test yourself if you are a DIYer. Do not ignore replacing them once the test confirms they are damaged.
Most people postpone replacing glow plugs, thinking it’ll dig a hole in their pocket. Now you know it won’t, replace those pencil-like plugs and get your car rolling on the road like a bad out of the hell.