Tip & How-To about Toyota Corolla
Old spark plugs removed from your engine can tell a lot about the condition of the engine. Experienced mechanics can use this information to help diagnose several conditions simply by examining the state of the old plugs.
This tip is written as a useful guide to help diagnose engine problems and give a general indicator of the state of your engine. If you have ever wondered what you can tell from the condition of your old spark plugs please see the illustrations below.
As an example note that conditions such as over advanced ignition timing, lean fuel mixture or intake manifold leaks can often be determined from a plug that shows absence of deposits and a burnt or eroded electrode. However, sometimes this condition is caused by incorrect spark plugs having been fitted at some stage and in this case can be easily rectified by replacing the spark plugs with plugs that are in the correct heat range.
More serious conditions can be determined from oily deposits which indicate that oil is leaking past the valves guides or piston rings. In this case the mechanical fault will need to be rectified which will involve an engine strip down. These symptoms when seen on old spark plugs are a very significant diagnostic pointer as to how the engine has been performing up to this point and can help identify faults in the engine itself
The first picture in the illustration is of a healthy spark plug. Please follow through each of the images and you can compare your old spark plug against each of the images for a description of the engine problems that are associated with it.
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Normal: Here we see a normal used plug which should be a brown to greyish color with only slight electrode wear. This plug has been operating correctly and is the in correct heat range for the vehicle.
Worn: On this plug you can see a rounded electrode. The normal colour deposits are seen but this plug has simply been in the vehicle too long.
Plug Run Too Hot. The symptoms here are a blistered white insulator with an eroded electrode and an absence of deposits.
Causes: Incorrect heat range plug used, over advanced ignition timing, lean fuel mixture, intake manifold vaccuum leak, sticking valves or insufficient engine cooling.
Carbon Deposits: Dry sooty deposits like this indicate either a rich fuel mixture or a weak ignition. This is a good indicator of a clogged air filter or a problem in the fuel or engine management system. Also check for ignition problems.
Preignition: Here we see melted electrodes. You will note the insulator is white. This is an indication of severe misfiring and overheating. Can be an indicator of severe engine damage. Causes: Check for over advanced ignition timing, lean fuel mixture, insufficient engine cooling and lack of lubrication in the engine. Also check correct heat range plug has been used.
Ash Deposits: Encrusting of light brown deposits observed on the electrodes. Causes: Oil or fuel additives - try changing the gas brand that you are using. Severe cases often indicate engine valve seal problems whereby oil is seeping into the combustion chambers.
High Speed Glazing: Insulator looks glazed and yellow. Condition is associated with sudden temperature rises from hard accelleration of the engine. Plugs in this condition can cause misfires at high speeds. These plugs will need to be changed and consider replacement with a colder temperature range plug.
Oil Deposits: Indicative of oil leaking past the valve guides or piston rings and fouling the plug. Vehicle will run badly. Engine will need stripdown and repair.
Detonation: Insulator appears cracked or chipped. May have resulted from poor gapping technique which has damaged insulator. Can lead to piston damage. Replace plugs and ensure fuel anti knock values are correct. Ensure plugs are carefully and correctly gapped.
Gap Bridging: In this case deposits can clearly be seen lodged between the electrodes. The plug will not fire and this results in a dead cylinder. Clean or replace the plug.
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