Fouled Spark Plug Defined: How to Clean and Replace a Corroded Plug

January 27, 2012

A fouled spark plug refers to the condition when deposits of carbon or a coat of oil or fuel can be found on the electrodes at the firing tip. These foreign materials cause the spark plug current to be diverted elsewhere and ground out via the nearest metal surface. Normal firing will cause current to jump across the electrodes at the firing tip, providing the spark that's needed to ignite the air-fuel mixture inside the combustion chamber.

Obviously, once a spark plug is fouled, it will not be able to deliver the correct voltage and produce the spark needed for combustion. Fouled spark plug symptoms may include engine misfiring, loss of power and rough engine performance.

Cleaning and Replacing Spark Plugs

As an alternative to changing fouled spark plugs, try cleaning them first to remove any buildup.
Before doing so, make sure that the engine is off and has is at a comfortable temperature to avoid any injury.

  1. Pull off the cables connected to each spark plug by gripping each by the boot, not by the wire, to prevent damage. On some engines, you may need to use a special set of angled long-nose pliers to pull off each cable.
  2. Use a ratchet wrench to unscrew each spark plug. In most cases a wrench extension has to be used to reach each plug.
  3. After removing all the spark plugs, check to see if cleaning or total replacement is what's needed. Note that spark plugs that have damaged electrodes need to be replaced. For carbon fouling, a steel brush and sandpaper are very ideal. Use a tool, such as a bench vise, to hold the spark plug steady as you're cleaning off the buildup. Brush and sand the electrodes until the metal is exposed. Use light grade sandpaper for the inner sections in the electrode gap. For oil and fuel fouling, use a solvent for cleaning. Wipe dry any excess solvent before reinstalling each spark plug.
  4. Use your ratchet wrench for screwing back in all spark plugs. Reconnect all cables, taking note of the proper sequence.

After cleaning or doing a total replacement of your spark plugs, the engine should start and run smoothly.

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