Reading a Used Spark Plug

 

    A lot can be learned about your engine condition by properly reading and "understanding" your spark plugs. This guide will show you pictures as well as explanations on cause and affect of what you see when viewing you spark plugs.

 

NORMAL CONDITION
This is how your plugs should look after they've been used for a few kilometers. Light grey or tan deposits and slight electrode erosion indicates the plug is operating at the proper heat range as well as correct jetting and the cylinder is running healthy.

 

MECHANICAL DAMAGE
Bent electrode and a broken insulator, dents often present on electrode.

Symptoms: Misfiring.

Causes: Plug nose is too long for engine head.This is caused by foreign objects in the combustion chamber or an improper plug reach where it contacts the piston. Even a piece of carbon can do this.

Cure: To solve this, make sure you have the correct length tip spark plug as well as removing any foreign materials in the combustion chamber. In some cases you may have excessive carbon buildup on the backs of the intake valves that will have to be addressed.

 

BROKEN INSULATOR - DETONATION
Insulator is cracked or split.

Symptoms: Misfiring.

Causes: Incorrect tool/torque applied during installation or removal. In cases of severe detonation, insulators may become cracked or chipped. Improper spark plug gap settings will also cause the insulator tip to crack or chip. Careless gap setting.

Cure: Detonation is tricky ... make sure that you are using the correct octane fuel first and then verify correct ignition timing. Next check for an inoperative EGR system (if equipped) as well as proper function of the Knock Sensor (if equipped). Also, you will want to make sure you are using the correct heat range plug.

 

OVERHEATING
You will notice a chalky appearance, an extremely white insulator with small black deposits and premature electrode erosion. The actual shell may also be discolored.

Symptoms: Loss of power at high-speeds or during heavy load.

Causes: Plug insufficiently tightened. Engine insufficiently cooled. Ignition timing too advanced. Plug heat range too hot.

Cure: To cure this you must first verify that the plug is the correct heat range, the ignition timing settings are correct, the air/fuel mixture is not too lean, there are no vacuum leaks and that the EGR valve (if equipped) is functioning properly.

 

OIL FOULING
Wet, oily black deposits on the insulator and electrodes.

Symptoms: Poor starting, misfiring.

Causes: Wrong piston rings, cylinders and valve guides. New or recently overhauled engines. Fuel mixture oil content too high. Oily coating caused by poor oil control. Oil is leaking past worn valve guides, piston rings, or on some race engines a possible intake gasket leak and then entering the combustion chamber.

Cure: Check for worn valve guides (Never knurl valve guides), intake gasket sealing alignment, as well as worn cylinder walls and piston rings. A leak down test is a good place to start for what is causing this.

 

PRE-IGNITION
A melted or burned centre and/or ground electrode, blistered insulator and aluminium or other metallic deposits on the insulator.

Symptoms: Loss of power causing engine damage, pre-ignition occurs when combustion begins before the timed spark occurs.

Causes: Plug insufficiently tightened. Engine insufficiently cooled. Ignition timing too advanced. Plug heat range too hot.

Cure: Check for incorrect heat range plug, over-advanced timing, lean fuel mixtures, inoperative EGR valve or Knock Sensor (if equipped) and also look for hot spots or deposit accumulation inside the combustion chamber.
If you or your engine builder took the time, all areas of combustion chamber should have been de-burred to eliminate this problem. This includes the sharp edges on the chamber, piston top, and cylinder wall valve reliefs (if applicable).

 

SUSTAINED PRE-IGNITION
This will be pretty obvious ... melted and/or missing center and/or ground electrodes as well as a destroyed insulator.

Symptoms:

Causes:

Cure: Check for incorrect heat range plug, over-advanced timing, lean fuel mixtures, inoperative EGR valve or Knock Sensor (if equipped) and also look for hot spots or deposit accumulation inside the combustion chamber.
If you or your engine builder took the time, all areas of combustion chamber should have been de-burred to eliminate this problem. This includes the sharp edges on the chamber, piston top, and cylinder wall valve reliefs (if applicable).
After you see this, you'd better look for possible internal engine damage as well. (pistons, cylinder walls, valves, rings, etc.)

 

SPLASHED DEPOSITS
These look as if they are small islands of contaminants on the insulator.

Symptoms:

Causes: This is usually a dirty carburetor bores or air intake as well as the possibility of a dirty or faulty injector.

Cure: You must use aggressive carb and choke cleaner or other solvent cleaner (a pressurized fuel injection service on fuel injected vehicles or injector removal and cleaning) before installing new spark plugs.

 

CARBON FOULING
This is very common visual condition on our race engines. Soft, black, sooty, dry-looking carbon. This indicates a rich mixture, weak ignition or wrong heat range plug (too cold).

Symptoms: Poor starting, misfiring, faulty acceleration.

Causes: Faulty choke – over rich air/fuel mixture. Delayed ignition timing, Bad ignition leads, Plug heat range too cold.

Cure: You will first need to verify plug heat range. On carbureted engines, check choke as well as choke pull-off (if equipped) for proper function and adjustment. On fuel injected engines,  check for clogged injectors and the cold-start valve and circuit. You also need to check for correct fuel pressure settings.
As a general rule on all computer-controlled engines, you need to also make sure that all input signals to the computer are working and accurate. This includes, but is not limited to, all temperature and pressure sensors as well as the EFI system components.
Lastly on all engines, check for vacuum leaks and weak spark or low voltage output. (Good reason for a better coil and aftermarket ignition unit "amplifier").

 

WORN OUT CONDITION
Excessive electrode wear,

Symptoms: Misfire during acceleration and hard starting.

Cause: Simply as in it's description, it's worn out ...

Cure: Replace it with same plug or at least compatible heat range.

 

 ASH DEPOSITS
These are light-brownish deposits that are encrusted to the ground and/or center electrode(s).

Symptoms: This condition can cause misfires.

Cause: This situation is caused by oil and/or fuel additives.

Cure: Verify worn valve guides or valve seals, not using fuel additives, or you might even try changing fuel brands. By the way, a hotter plug is what most people try to fix this problem. You need to first understand that the plug is NOT typically the problem.