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Common Failures Of Glow Plugs Or Diesel Glow Plugs

Do you have a vehicle with a diesel engine? If so, it likely uses glow plugs to help you cold start. Over time, these glow plugs can wear out or burn out, which can cause hard starting and other undesirable symptoms. If you want to know about them, stick around and read all about common glow plug or diesel heater failures.

It’s a good idea to familiarize yourself with the common symptoms of faulty glow plugs. This way, you can immediately fix the problem in your vehicle.

Symptoms and common faults of glow plugs or diesel heaters

Diesel engines rely on the heat from air compression to ignite the fuel. For this reason, diesel engines can be difficult to start when cold. Heaters solve this problem by acting as heating elements to preheat the air in the combustion chambers. Diesel engines usually have one heater per cylinder.

Each glow plug or glow plug has a terminal, a threaded body, and a tip that protrudes into the engine’s combustion chamber. Applying voltage to the terminal end of the spark plug allows electrical current to pass through a heating element at the tip of the spark plug.

Modern vehicles have a control module that manages the glow plugs. The module takes into account inputs from various sensors, such as ambient air and coolant temperature, to determine when the heaters should be turned on. On many vehicles, the control module activates the glow plugs via a relay .

Glow plugs can wear out or burn out over time, often leading to one or more noticeable symptoms. Common diesel heater or glow plug failures include:

1- Problems starting

A couple of bad glow plugs may not cause a problem in hot weather, but will cause starting difficulties when the temperature starts to drop . What’s more, if multiple glow plugs burn out, the engine may refuse to start in the cold.

2- Bright warning lights

In modern vehicles, 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. On some vehicles, the glow plug indicator may also start blinking .

3- White smoke in the exhaust pipe when starting

White smoke coming from the exhaust of a diesel vehicle usually indicates that there is unburned fuel in the engine. The problem can sometimes be attributed to faulty glow plugs that are preventing the engine from getting hot enough to burn the fuel.

If the glow plugs are really to blame, the problem will be most noticeable at start-up or soon after . Late model diesel vehicles can run the glow plugs for several minutes after starting the vehicle. You may still see smoke during that period.

4- Poor engine performance and/or slowdown after starting

Glow plugs usually work for a certain time after starting. If one or more spark plugs are bad, you may experience rough idle and/or poor engine performance during that period. This is one of the common failures of diesel heaters or glow plugs.

How to know if the car heaters are bad?

There are some faults that can mimic the problems in the heaters. If your vehicle exhibits one or more of the above symptoms, you’ll want to be sure that the glow plugs are to blame. To do this, you will have to test the heaters thoroughly.

In a nutshell, the signs that you have a glow plug problem are a slow start and a complete fail to start. Once the vehicle starts you may notice rough idling, hesitation, and lots of white smoke until the engine warms up. If you have a four cylinder, a bad glow plug will cause a slightly hard start and misfire.

Two bad spark plugs will cause very hard starting and very rough running . Three faulty plugs make the car almost impossible to start and keep running. The same applies to larger engines. If your vehicle starts fine and then you plug in the block heater overnight, but it won’t start, then you probably have a glow problem unless your engine has very low compression.

How to check glow plugs?

Learning to test glow plugs is easy – anyone can do it! First, disconnect the wires from each of your glow plugs. Then clip the 12 volt test light clip to the POSITIVE battery terminal. Touch the probe of your test light to the glow plug terminal (NOT the wire harness!). Any glow plug that does not illuminate the test light is bad.

You can also use an ohmmeter – (Again, disconnect all wires from each glow plug to avoid parallel paths.) Measure the resistance between the terminal ground of each glow plug. Any glow plug that reads infinity or more than a few ohms is bad.

Another method is using an ammeter : it measures the current draw of each socket individually; with this method, you can catch glow plugs with slightly low current draw that are starting to fail.

Why do glow plugs burn?

Your engine likely has injectors that drip onto the glow plugs and erode them. Sometimes the vehicle’s glow plug driver will ” stick ” causing very rapid glow plug failure. Often the dash light will not come on to alert you that this is happening.

The Ford/International 6.9L diesel is famous for this; It is wise to consider replacing the controller on this engine when replacing the glow plugs. Another common cause of glow plug problems is the use of high-amp battery chargers to ” boost ” an engine to start.

Many of these chargers put out too high a voltage and will actually blow the tip of the glow plug ; Anything that causes a voltage spike in your vehicle’s electrical systems can damage the plugs. If all your glow plugs will fail at the same time.

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