How to Troubleshoot a Spark Plug

Spark plugs are a crucial part of ignition. Without properly functioning spark plugs, an engine may run poorly or not at all. It is simple to troubleshoot spark plugs, based on visual inspection and analyzing any symptoms the engine or vehicle is experiencing. Many spark plug problems can be remedied by removing the damaged spark plug and replacing it with a new plug purchased from an automotive supply store. Spark plugs are inexpensive to replace.

  • Open the hood of the vehicle, or otherwise gain access to the engine so that you can view the spark plugs.

  • Inspect the spark plugs for a thick black coating. This layer of carbon occurs when the vehicle has been stopped and started repeatedly, the engine has a dirty air filter or the air-to-fuel ratio is off. Correct by replacing the spark plug, replacing the air filter and using the correct type of gasoline.

  • Feel the spark plug to determine whether it is coated in oil, which will look wet, slick and black. Oil on a spark plug is often associated with engine misfiring. Check the engine for oil leaks and replace the plugs.

  • Check the plugs for an ash-like substance covering the plug. A normal spark plug is smooth and white, gray or brown without a powder coating like ash. The ash can be present due to additives in the fuel or oil. Do not use additives unless they are recommended by the manufacturer. Replace the plugs.

  • Adjust the gap between the two electrodes, if necessary, using a spark plug gapping tool. How much voltage the plug needs is directly related to the gap size. The vehicle's service manual will show the proper size, usually listed in thousandths of an inch and hundredths of a millimeter. Replace the spark plugs with plugs of the correct size, referring to the manual, if necessary.

  • Look at the electrode portion of the spark plug. If the electrode is melted or otherwise damaged, the spark plug may have the incorrect heat range for the engine. The engine may overheat or not work properly at high speeds. Replace the spark plug using a plug in the heat range suggested by the manufacturer.

  • Remove any damaged or dirty spark plug and insert a new plug, tightening by hand. Clean the spark plug holes, then use a torque wrench to tighten the plug to the manufacturer's specifications.