Diesel engines naturally run a little dirtier than standard gasoline engines. Diesel fuel is less refined than gasoline. This can produce impurities in the fuel. If there is an oil leak in the engine, it can also foul the glow plugs. Additionally, the plugs can become dirty through carbon build-up from the normal heat cycle of the plug. When this happens, the glow plugs will need cleaning.
Pull the glow plug wire out of the engine. You will be cleaning all of the plugs, but you should start and work on only one plug at a time. Pull the top of the plug wire off the top of the engine. You may need to twist the plug wire gently back and forth while you are pulling up on it, but it should come right out.
Attach the glow plug socket to the socket extension and lower the socket into the plug well.
Orient the wrench so that the socket grabs onto the top of the plug.
Turn the socket (and plug) counterclockwise with a socket wrench to remove it.
Examine the electrode on the bottom of the plug and make sure there is no damage to the plug. Remove the plug from the plug socket and spray it thoroughly with brake parts cleaner. This will remove any loose carbon deposits and oil from the bottom of the plug. Allow the plug to air dry.
Reinsert the plug, tighten it, and secure the plug wire back over the glow plug.
Repeat steps 1 through 6 for each glow plug.
For specific information about your diesel engine, consult the particular vehicle's manual (See Resources).