Symptoms of a Bad Glow Plug

The glow plug is an electrically heated, thin metal probe that warms the air and diesel fuel mixture in the combustion chambers. This helps in the initial start-up of a cold diesel engine.

Diesel engines are different in design from gasoline engines. Instead of using a spark plug to ignite the fuel-air mixture in the cylinders like a gasoline engine does, a diesel engine uses very high compression of the mixture to produce the heat required to ignite it. The only problem with this system is that it does not work when the diesel engine is cold, because there is not yet enough residual heat in the engine to ignite the fuel and air when it is compressed. This is where the glow plug comes in.

A glow plug is a heating device used to help diesel engines get started at low temperatures. There is usually one glow plug per cylinder. The glow plugs are powered by the vehicle’s electrical system, and operated by an electronic control module that is part of the engine computer.

Each glow plug has a filament, which heats up and “glows” like the filament in an incandescent light bulb or a toaster. The glow plugs are activated as part of a diesel engine’s cold starting procedure, providing enough heat to ignite the fuel and air mixture in the cylinders until the engine warms up. At that point, the glow plugs shut off and the normal diesel combustion cycle takes over.

What are the signs of a bad glow plug?

If you drive a diesel-powered car with glow plugs, there are numerous symptoms of a bad glow plug. Let’s check the list:

• Your car is hard to start when it’s cold out: A bad glow plug can keep the fuel-air mixture in that cylinder from warming up enough to ignite. While your engine may eventually start with only one bad glow plug, this indicates a problem that could get worse if another bad glow plug appears. Call your mechanic and have the problem fixed now.

• Your glow plug indicator light is not working: Most diesel-powered cars require you to turn the power on to light up the glow plug indicator light, wait a few seconds until the glow plugs warm the engine enough and the light goes off, and then start the engine. A bad glow plug can cause your indicator light to not come on, flash, or remain on. Whichever one it is, it is time to have a mechanic check it out and repair it.

• You experience poor idling, driving performance or fuel economy: You can confirm the likelihood that these issues are caused by a bad glow plug if they go away after 10-15 minutes of driving, when the engine is fully warmed and the glow plugs are not needed.

• Black or white smoke comes out of your car’s exhaust: A bad glow plug can result in exhaust smoke that can be either black or white in color. This smoke indicates that the diesel fuel has either not been burned completely inside the cylinders (if it’s black), or it has not been burned at all (if it’s white). In both cases, get your car to a mechanic’s repair shop immediately.

• Your check engine light comes on: A bad glow plug will throw your car’s emissions completely out of whack. This situation can definitely cause your engine computer to trigger the check engine light on your dashboard.

Should you drive your car with a bad glow plug?

Because your glow plug system is only used for starting and warmup, you can technically drive your car, as long as you can get it started, and any problems do not cause any safety concerns. To avoid any serious issues (like hesitation in the middle of an intersection) until you get it fixed, you should let the car warm up fully before you drive it.

On the other hand, prolonged use of a car with a bad glow plug can also cause carbon buildup in the affected cylinder, as well as potential damage to your emissions control equipment, and that can mean expensive repairs. Get it fixed ASAP!

How do you fix a bad glow plug?

Your mechanic’s first step in fixing a bad glow plug is likely to be the use of a scan tool to detect any codes that may be in the engine computer. This can indicate precisely which glow plug is causing the problem. The other glow plugs, as well as related components like relays and the control module, can also be checked for proper operation, and replaced as necessary.

Once the bad glow plug has been replaced, your vehicle will be allowed to sit until it is completely cold, so that the glow plug system can be tested and verified to work correctly.

Can you replace a bad glow plug by yourself?

If you have basic mechanical tune-up skills, like changing your oil and your spark plugs, replacing a bad glow plug should be doable. Glow plugs have many similarities to spark plugs, so the skills should be transferable. As long as you can access the glow plugs without removing half of the engine, this should be a manageable task. Be careful not to break the old glow plug off while removing it, and make sure not to overtighten the new one. If you run into any trouble, call a mechanic for assistance.

Just like spark plugs, glow plugs eventually wear out and need replacement. Check your owner’s manual for the recommended replacement interval on your glow plugs, and replace them all if it’s time for it.