Glow plugs are not like spark plugs in the sense they create a spark. They are used to create heat in a diesel engine. To understand why there is a need for a glow plug you need to understand the workings of a diesel engine. The diesel engine, named for its inventor Rudolf Diesel in 1892, is a type of internal combustion engine that uses compression to create the combustion of the fuel. The compression of any gas raises its temperature. The air is pulled into the cylinder at a much higher compression rate than spark induced combustion engines. At the end of the compression stroke of the cylinder, diesel fuel is injected into the chamber. The contact with the air (which through compression is around 1300 to 1600 degrees) causes the fuel to combust and pushes the piston down. In cold weather diesel engines can be difficult to start. The cold cylinder block and cylinder head draw out the heat in the cylinder during the compression stroke. This prevents ignition. This is where a glow plug comes into play.

When starting a diesel engine you do not crank the key all the way the first time. The key is just to right before ignition to start the glow plugs. This is called glowing or pre heating. An indicator panel will light up with (wait to start) on the display until the glow plugs have sufficiently heated the cylinder. When the temperature is high enough the (wait to start) light will go off and the (start) light will come on. At this point you can start the vehicle. If you stop the vehicle for a short time and turn the key you will usually get the (start) light as there is enough ambient heat from the previous running.

The glow plug resembles a spark plug in size and shape. They come in two types, quick-start pencil elements and slower pencil elements. It is a pencil-shaped piece with a heating element at the tip. They are housing with a screw in thread with the pencil element pushed in. When electricity is applied to the glow plug, it takes on the characteristics of its name and glows bright orange and put out a large amount of heat. The element is designed for a 12 volt current. A quick start pencil element can reach a temperature of 1625 degrees while a slow pencil element can attain a temperature of nearly 2000 degrees after 30 seconds. Quick start glow plugs are usually used in passenger vehicles while slow glow plugs are for more industrial type vehicles like semis and delivery trucks.

This heat is focused on the cylinders and the engine block surrounding the cylinders. This heat keeps the block from suffering from thermal diffusion, meaning the block's heat won't dissipate. There are internal sensors that let the relay to the "wait to start" when to go off. In some vehicles it is a time frame that is reached like 10 to 20 seconds then the glow plugs will turn off and you can start the ignition. To meet emissions rating some vehicles leave the glow plugs on for as much as 180 seconds to properly burn the starting fuel. Combustion efficiency is greatly reduced when the engine is cold. A glow plug is made from such metals as platinum and iridium because of these metals, resistance to oxidation and heat.

The main enemy of a glow plug is wear and tear. It is possible to start a diesel engine while the glow plugs are still glowing, but this is harmful to the plug. The heat of combustion added to the glow plug's own heat from being electrified can cause the plugs to overheat. This overheating causes the glow plug to lose some of its already limited lifespan. Since glow plugs are inexpensive it is a good idea to change them out every year or every six months or 60,000 miles in colder climates as worn out plugs do not deliver the needed heat to your engine and makes it more and more difficult to start. There is no set schedule for replacing glow plugs and most are usually forgotten until they go black.

How to Replace Glow Plugs?

First remove the valve cover. Whatever cover is on the glow plugs, remove that. Disconnect the connector and remove the intake manifold glow plug from the cylinder head. Use a deep socket wrench and remove the glow plug from the cylinder head. Screw the glow plug into the glow plug opening all the way in the out. Now put in the new glow plug. Then reconnect the connector to the glow plug terminal. Replace the valve cover with a new gasket if it is needed. Some glow plugs require a gasket. Now put whatever you took off to get to your glow plug back on. It is pretty easy to install a glow plug especially if it in a larger vehicle, like a truck.

It is good to refer to your owner's manual before you begin installing or removing anything on a vehicle. These instructions are general guidelines on how to install the glow plug. Each vehicle is different and requires specific techniques to install their plugs. Refer to the proper repair manual. Always be safe when working on your vehicle. Try not to replace a glow plug just after running your vehicle. Let it set overnight or wait until it cools down. Make sure you follow your tool guidelines and do not substitute tools. Try not to smoke while you are doing your repair work. Fuel and vapors are present and will cause an explosion.

Testing glow plugs can be done while they are still in the engine. You need to disconnect the wire attached to each glow plug. Connect a test light to the positive battery terminal and touch the glow plug if the test light lights up, then the glow plug is still good. If it doesn't the plug is bad needs to be replaced.