How to Change a Glow Plug on a Mercedes 300SD

A diesel engine relies on the compression of fuel and air during the combustion cycle. Combustion efficiency is reduced when a diesel engine is cold, so glow plugs heat the engine block and cylinders before the engine cranks. When the ignition is turned on a diesel vehicle, there is usually a message on the instrument panel advising drivers to "wait to start." When the glow plugs have heated the engine sufficiently, the message turns to "start." The glow plugs are not required when the engine is running. Replacing a glow plug in a Mercedes 300SD is a simple and straightforward task.

• 1

  Raise the hood. At the top of the engine block, remove the glow plug cap from the glow plug you wish to replace. Pull the cap up and off the glow plug with your hand.

• 2

  Assemble a socket wrench with a deep extension and a Mercedes glow plug socket. Insert the socket onto the top of the glow plug and push down until the socket engages the glow plug.

• 3

  Turn the socket counterclockwise to remove the glow plug from the cylinder. The socket has a rubber gasket that will grab and hold the glow plug as you lift it out.

• 4

  Insert the new glow plug into the hole in the cylinder by hand. Turn the glow plug clockwise until finger tight. Push the socket onto the top of the glow plug and use the socket wrench to tighten the glow plug a little more. Do not over-tighten.

• 5

  Close the hood and run the engine to test the glow plug.