Spark Plug Color Chart - Reading Your Plugs

This page is nothing new. Just needed some text that was readable. The pictures and text are from the Autolite technical manual and just made a bit easier to read in the grand scheme of EZ-Read charts here. You can find Images and PDF's from Autolite, NGK, and other on how to read the color and condition of your spark plugs. This is a simple re-hash of that with mostly the same text as the original from Autolite. The pictures are about as good as I could find on the web, and guessing these are 30 years if a day.

The plugs are numbers as in the Original Autolite book page as is the text, again not taking any credit for these Autolite did the work. Wish I had some better pictures, but what can you do. My commentary (If I have any will be in
BLUE). When in doubt change the plugs if you can.

You ask what type of plugs I use? I use a combination of Autolite Racing and NGK Racing plugs. Fan of either brand, but I'm not so picky that I wouldn't use Champion or other brands. I just prefer Autolite and NGK.

Here is a Tool Review on Spark Plug Magnifiers, will help read the plug as our eyes start getting worse!

Normal Plug

Correct heat range of spark plug is being used. Replace with the equivalent Autolite spark plug at the
proper interval.

Appearance - Grayish-tan to white color
Normal Spark Plug
Normal Red Spark Plug

Normal With Red Coating

Coloration is from the use of additives in unleaded fuel.

Appearance - Pinkish-red color on the ceramic insulator tip, the center electrode, and the ground electrode.
Fuel Fouled Spark Plug

Fuel Fouled

Indicates the cylinder from which the spark plug came is not using all the fuel supplied to it. Check for faulty or sticking choke, overly rich fuel mixture, ignition problems, leaking fuel injectors, or spark plug heat range is too cold.

Appearance - Firing tip is damp with gasoline, usually the odor of fuel is present on the spark plug. The insulator is often tinted the color of charcoal.

These usually look WET and smell of gasoline. In race cars make sure you check the plug after a quick shut down. I find that I get a bit on the fouled side (not as bad as this) when stuck at idle due to unruly camshafts and such.
Spark Plug with Bad Detonation


Caused by low octane fuel or over advanced timing. Can be noticed as engine knock. Check for faulty EGR system, detonation sensor, and correct spark plug heat range.

Appearance - Insulator is usually cracked, chipped, or broken. Ground electrode can also exhibit damage.
Worn Out Spark Plug

Worn Out

Spark plug used beyond its intended life.
Replace with a new set of Autolite spark plugs.

Appearance - Center and ground electrodes are eroded, have rounded edges, and are excessively worn away. Difficulty starting engine and misfiring during acceleration may occur.

Yes, do get a new set of Autolite plugs. As mentioned I run Autolite Racing Plugs with cut back electrodes as well as NGK plugs. Cheep enough to change them often!
Glazing or Over Heated Spark Plug

Glazing (Over Heating)

Spark plug is operating too hot at high speeds. Replace with a colder heat range of Autolite spark plug.

Appearance - Ceramic insulator tip appears to have a melted, glazed coating.

You sometimes see a very bright white electrode with black dots on it also as a heat range issue in some cases cracked electrodes. See the Hall of Shame for one
Carbon Fouled Spark Plug

Carbon Fouled

Spark plug heat range is too cold and/or caused by extensive low-speed, short distance driving. Replace with the correct heat range of Autolite spark plug. Also caused by weak ignition system and/or rich fuel mixture. Fuel injection engines would produce carbon fouling from clogged fuel injectors, vacuum leaks, and/or problem with carbon canister/purge valve operation. Carburetor equipped engines cause carbon fouling from improperly adjusted or malfunctioning choke.

Appearance - Black, sooty coating on firing end.

I generally think of this as a less severe case of PLUG 3, Fuel Fouled, both are bad as the carbon really causes the plug to not work results are bad misfires.
Sustained Preigniton Spark Plug Damage

Sustained Preigniton

Check for correct application of spark plug (heat range too hot, wrong spark plug for engine), cross firing of ignition cables, over advanced timing, lean fuel mixture, defective EGR valve, accumulation of combustion chamber deposits, hot spots in the combustion chamber due to poor heat dissipation, improper installation torque applied to spark plug, and/or head gasket protrusion into the combustion chamber.

Appearance - Melted center and ground electrodes and damaged ceramic insulator tip. Initial and sustained  preignition are two extremes of the same engine problem.
Spark Plug with Ash Deposits

Ash Deposits

Caused by the use of leaded fuel, fuel additives, and/or oil additives. Check for worn piston rings and/or valve guides. Misfiring may occur due to the deposits on the electrodes.

Appearance - Center electrode, ground electrode, and/or
ceramic insulator tip are coated with tan colored deposits.
Oil Fouled Spark Plug

Oil Fouled

Caused by presence of oil in the combustion chamber. Check for worn rings, worn valve guides, and/or worn valve seals.

Appearance - Center electrode, ground electrode, and/or
ceramic insulator tip are coated with a black, oily substance.

Usually when a plug is oil fouled smoke (gray) is seen. This can sometimes have a wet look, but don't confuse it for Fuel Fouled Plug 3, which will smell a lot like gasoline.
Mechanically Damaged Spark Plug

Mechanical Damage

Locate and remove foreign object from inside of cylinder. Check catalog for proper spark plug application. Improper spark plug thread reach can protrude into cylinder and sustain damage.

Appearance - Center electrode and ground electrode are
bent out of position, down or to one side of the spark plug. Ceramic tip is broken and missing from the firing tip.

Likely if you have a plug that looks like this a new plug is not going to fix the problem unless you just used the wrong plug length. Expect something bad inside the cylinder.
Lead Fouled Spark Plug

Lead Fouled

Occurrence is from use of leaded fuel or fuel additives containing lead which become conductive over the firing tip. Install new spark plugs.

Appearance - Ceramic insulator tip is coated with a
brownish-yellow glazed coating.

Hard to tell this one from a good plug from the picture. Fortunately you only find leaded gas in racing these days, so if you suspect something like this just change the plugs.

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