Signs & Symptoms of a Bad Glow Plug

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Signs & Symptoms of a Bad Glow Plug
A faulty glow plug can cause serious damage to your engine. (Jupiterimages/Comstock/Getty Images)

The glow plug on a vehicle is a small electrical device used to heat the combustion chamber in a diesel engine. This helps ease the ignition of a cold engine. A bad glow plug can cause several starting problems and, over time, can wear down your engine. To protect the life of your diesel engine, diagnose and fix a bad glow plug early.

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Indicator Light

Some vehicles are equipped with an on-board diagnostic system that will alert you of problems with your glow plug. When you start your vehicle, pay close attention to the LED lights on your dashboard. If any warning lights are on, consult your owner's manual to identify the problem. A glow plug warning light can indicate a faulty temperature sensor, a disconnected glow plug relay or a corroded glow plug.

Hard Starting

Second to an indicator light, a hard-starting engine usually is the most common symptom of a bad glow plug. Without a working glow plug, the combustion chamber might have difficulty igniting the fuel. If on checking your fuelling system and battery, both appear in good condition, the glow plug should be your next step.

Irregular Engine Behavior

Even after you have your vehicle started, your glow plug can still affect your engine until it becomes properly warmed. You might notice your engine misfiring as a result of a bad glow plug. Listen to your engine as you shift into gear. If you detect the sound of rough idling or a piston misfiring that stops after about 10 minutes of driving, check your glow plugs.

White Smoke In Exhaust

Look for white smoke coming from the exhaust pipe when you start your engine. Diesel fuel is pumped into the combustion chamber to ignite the engine. A disconnected glow plug allows diesel fuel to leak from the chamber into the exhaust, where it is ignited in the exhaust pipe once the vehicle starts. The ignition of diesel in the exhaust produces white smoke, which exits through the exhaust pipe. The smoke should stop after about five minutes of the engine running. Still, it indicates something is wrong.

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