 Spark Plug Reading
As stated on the opening page, a lot can be learned about your engine condition and what is happening in the combustion process by properly reading and "understanding" your spark plugs. This page will show you pictures as well as explanations on causes and affects of what your spark plugs are experiencing.
Each picture is a thumbnail, meaning when you click the small image you will be presented with a larger image in a NEW browser window.
Normal Condition:
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The Spark plug to the left is what a normal plug should look like.
Grayish-tan to white in color indicates the plug is operating at the proper heat range as well as correct jetting and the cylinder is running healthy. Note that on modern unleaded fuel burning engines that it may be difficult to see the coloration that is so obvious on the plug above. Using a magnifying glass or a lighted spark plug viewer offers more accurate inspection.
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 RACER TIP: That vertical color band on the ceramic shows you where the plug is indexed. Meaning that band is aimed slightly toward the exhaust valve. Optimum location places the band in the location shown by the image at right. If the band is anywhere but here, it means that the plug is not pointed at it's optimum location. There is still discussion as to whether indexing a plug is worthwhile, but on may applications looking for that last horsepower, it doesn't hurt. If you consider the opposite effect, with the stripe hidden behind the ground strap, the spark kernel is being quenched and not allowed to expand at an optimum rate. |
Worn Out Condition:
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Excessive electrode wear, misfire during acceleration and hard starting.
Simply put as in the title description, it's worn out ... it looks pretty good regarding the color, so just replace it with same plug or at least with a plug of compatible heat range. You've all heard the term " If it works, don't fix it". Don't look for flaws with this plug ... just blame yourself for not changing it sooner.
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Mechanical Damage:
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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.
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.
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Detonation:
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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.
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.
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Overheated:
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On this symptom you will notice a chalky appearance, white insulator, rapid electrode wear as well as an absence of deposits. The actual shell may also be discolored.
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.
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Ash Deposits:
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These are light-brownish deposits that are encrusted to the ground and/or center electrode(s). This situation is caused by oil and/or fuel additives. This condition can cause misfires.
The cure for this is to 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.
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Oil Fouled:
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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.
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.
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Initial Pre-ignition:
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This will usually look as a melted center electrode and/or ground electrode.
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).
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Sustained Pre-Ignition:
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This will be pretty obvious ... melted and/or missing center and/or ground electrodes as well as a destroyed insulator.
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).
If you happen to see a spark plug like this in your engine, you'd better look for possible internal engine damage as well. (pistons, cylinder walls, valves, rings, etc.)
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Splashed Deposits:
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These look as if they are small islands of contaminants on the insulator. This is usually a dirty carburetor bores or air intake as well as the possibility of a dirty or faulty injector.
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.
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Carbon Fouled:
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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).
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").
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