How To Clean Your Spark Plugs and Diagnose Problems

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    In this how to we will be looking at a few problems that you might have with your lawnmower or vehicle’s spark plugs. Although eventually you may need to replace your plugs you normally shouldn’t have to do it due to fouling or physical damage. If you do have fowling or physical damage to your plugs you will need to cure that problem or your new plugs will have the same problem.

    There are a few conditions that happen to plugs that you need to be aware of. The first is fowling. This is when the plug gets so dirty that it does not operate correctly. Fowling can happen from a few things. You can have oil in the combustion chamber that will make the plug wet or you can have a too rich fuel mixture that deposits a black chalky coating on the inside of the plug.

    Wet conditions may or may not mean physical damage has occurred. It could mean work valve guides which let oil past the valves or it could mean worn piston rings that let oil past the piston. It could also be a temporary situation because you over filled your engine with oil. Additional wet conditions that are not as common include moisture from the cooling system getting into the combustion chamber or fuel in the combustion chamber.

    When your plugs foul because of a fuel problem it is due to a rich fuel mixture. The presence of oil might also cause dark soot like deposits but normally that is a wet condition as oil does not normally burn off. This condition can also happen if the plug’s heat range is too cool. To burn off soot the plug must run hot enough. Increasing the heat range is not normally recommended or necessary to do on your own. You should just make sure the plugs are the correct rating for your engine.

    Other conditions can be physical damage such as a cracked cone or a burnt electrode. This often means overly hot conditions or electrical system problems.


    Curing Plug Fowling or Damage Problems

    To cure a plug that is fowled or damaged you should consult your engine diagnostic guide.

    The first thing you want to do is make sure that everything is running optimal so this means check the spark plug number and change it if its wrong. Read your engine guide about how to diagnose electrical problems with your engine. On small engines you can do this with a volt meter on vehicles with computers you will need a special device to read the codes.

    Make repairs as needed. Additionally if you have fowling problems from the fuel system you will want to replace your air filter which may be choking off fresh air and increasing the fuel ratio. You will also want to change your oil and look for any metallics which would lead to a diagnosis of mechanical damage to the engine.

    A Compression Tester can tell you if your engine Rings or Valves need servicing.

    If you have simple fowling of the plugs you can also clean the plugs. This is not a perfect solution but it can do quite well in restoring spark to your engine enough to diagnose and make repairs.

    In our case we have a black soot fowled plug which we cleaned with a spark plug cleaner and then followed that up with carburetor cleaner to remove the abrasive before reinstalling it in our engine. The job is not perfect but you can see the dramatic difference between the black plug and the finished plug.

    In the side by side comparison you see one slightly dirty but functioning plug and another that is fowled beyond reasonable use.

    Final Note

    In our example we believe that the problem is due to a clogged air filter that needed to be replaced.

    The plug was cleaned and a new air filter will be installed for testing.

    If the condition is cured by a new air filter then the plug should get hot enough to burn off the remaining residual soot. However since plugs are not that expensive we will order one to replace the one we have.

    Remember oily wet means internal problems like valves or piston rings, Dry fowling means fuel. Eroded can be from electrical or chemical such as coolant in your combustion chamber. Cracks are from overheating.

    Having a Spark Plug cleaner and gapping tools can really help in a pinch and they are probably worth the investment if you normally do your own work.

    I also strongly suggest that you keep old plugs for situations like this. Its better than running out to the store in the middle of the night if they are even open.


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