Symptoms of bad Glow Plugs: (Testing & Replacement cost)

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.

Symptoms of Bad Glow Plugs

Diesel Engine glow plugs working principles.

A glow plug is an essential diesel engine component coined with a piece of metal and a heating element on the tip. It has a pencil-like shape.

In petrol engines, the spark plugs ignite the air-fuel mixture in the combustion chamber. On the other hand, diesel engines use glow plugs to start. The principal job of the glow plugs is to heat the air in the combustion chamber to let it reach the required temperature.

When you turn on the ignition, the glow plugs get electrified. As a result, the heating element heats up and emits orange to red light. The air-fuel mixture in the combustion chamber ignites close to the heated glow plug and initiates combustion.

Like spark plugs in petrol engines, there is one glow plug per cylinder. A diesel engine can have four to ten glow plugs, depending on the number of cylinders.

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.

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.

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.

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.

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.

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.

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.

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.

can faulty glow plugs cause loss of power

How to test glow plugs

There are several ways on how to test glow plugs. Of all these methods, we’ll concentrate on testing glow plugs with battery and glow plugs resistance test. The most accurate among these two is the resistance test.

But before embarking on the resistance test, check the settings on your multimeter and see if it can measure low resistance enough to give accurate readings. Glow plugs’ resistance is typically between 1 to 6Ω. If your multimeter can’t get such small reading, you should get another one.

Let’s start with the resistance test.

  • Disconnect the power line that connects to the glow plugs. If not, you’ll be measuring all the glow plugs at once instead of doing it individually.
  • Grab your multimeter and set it to the lowest resistance possible
  • Place the black multimeter probe on the engine ground close to the glow plug
  • Connect the red probe to the power supply on the tip of the glow plug
  • Turn on the glow plugs
  • Record the readings on the multimeter. A good glow plug will not read more than 6Ω.
  • Switch off the glow plugs
  • Disconnect the red probe connected to the glow plug tip
  • Repeat these steps on all the glow plugs for an accurate reading.

The step above is a simplified process of testing glow plugs without removing them. If you want to remove the glow plugs before doing so, go ahead. With the glow plugs out, connect the black multimeter probe to the hex thread on the glow plug. Then connect the red probe to the tip of the plug.

Testing glow plugs with battery.

This is not a standalone test. Instead, it is a confirmation test to ensure the result from the above is as expected. I recommend removing the glow plugs before trying the battery test.

  • Remove the glow plugs from the engine.
  • Get two wires and connect one to the tip of the glow plug and the other wire to the hex.
  • Connect the hex wire to the negative battery terminal
  • Connect the tip wire to the positive battery terminal
  • A good glow plug should turn hot red within 10 seconds of testing.

How to buy glow plugs

If your diesel engine still has the OE glow plug, getting the right replacement plug is much easier. You must take the OE part number and search for it when placing your orders online or at a local part shop.

Some online stores have lockup search enabled on their website. All you have to do is to input the number plate to see which glow plug will fit your car. Stick to the recommended number plate for optimum performance.

If you have replaced the OE plug, copy the part number for the current plug and get another glow plug with the same number. This way, you’ll be sure of what you are getting.

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.

How much does it cost to replace new glow plugs?

Glow plugs are not as expensive as they seem. Based on the brand and quality, a new glow plug costs between $25 and $50. If you are a DIYer, you can replace the plugs yourself and save the labor charge. But if you need convenience, contact a mechanic to replace them. The mechanic will charge you $75 to $200 for labor.

Depending on the mechanic doing the job and your engine configuration, the job will take around two hours to complete. How much time it takes determines the labor cost.

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.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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