A diesel engine's glow plugs are often said to be similar to the spark plugs in a gasoline engine. The glow plugs heat up, causing the air inside the combustion chamber to heat up. The pressure and heat combine with diesel fuel to cause a small explosion in the combustion chamber. This pushes the piston down and starts the motion that turns the diesel engine. If something goes wrong in a diesel engine, the glow plugs are often the first item you should check.
Put the key into the ignition and turn it as if you were starting the engine. If the engine does not start, turn the key all the way to the off position. Repeat this step five or six times. Cycling the engine may cause the engine to start. If it does start, it means your glow plugs are not getting enough heat the first time you cycle the engine. By turning the key five or six times, you are causing the glow plugs to heat up a little more each time.
Open the power relay. If the engine does start after cycling, check the power relay, which is normally located under the hood and is usually affixed to the frame.
Pull firmly on the square metal fuses until they are detached from the power relay box.
Examine the tips of the power relay fuses. Look for any signs of damage. This can include corroded metal tips, charred tips or mangled tips. Replace the fuses if they are damaged.
Check the wires that run from the power relay box to the glow plugs, if the power relay fuses did not have any obvious damage.
Pull firmly on the wires. Do not attempt to disconnect them. The point is to make sure that none of the power relay wires are loose.
Examine the power relay wires for signs of damage. With a flashlight, follow the wires all the way from the power relay box to the glow plugs. Check for any cuts, slashes, signs of water or fire damage.
Loosen the bolt around the first glow plug with your 18mm wrench, if the engine started and there was no damage to the fuses or the power relay wires.
Continue loosening the bolt until it can be removed by hand. You will want to remove it by hand so the bolt does not fall into the engine.
Unscrew the glow plug and examine it for any signs of damage. If there are obvious signs of damage to even one of the glow plugs, it is best to replace all of them.
Grip the glow plug with your vice grips. Be sure that you have a firm grip on the vice grips so that you do not drop either object.
Carefully place the screw-end of the glow plug on the positive cell of your car battery. Be sure to wear gloves and safety goggles during this procedure.
Watch the glow plug to determine if it heats up. Do not touch the glow plug with your hands during this procedure. You will burn yourself. If the glow plug does not heat up, it is not working. If the glow plug does heat up, the glow plug itself is working and you will need to check other possibilities for your vehicle's problems.
Be sure to wear your mechanic's gloves during this entire troubleshooting procedure. Allow the engine to cool before troubleshooting the glow plugs.
Glow plugs will burn you if they are receiving power or have recently received power.