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VIDEO: Analyzing Spark Plug Colors

If the spark plugs have a matte black or grey appearance, it could be carbon fouling. This video is sponsored by Autolite.


A Four-Cylinder Engine With ignition coils over the spark plugs, or coil on plug, has misfire code for cylinder three or P0303, the plug has a dark grey matte coating on the spark plug insulator what is the least likely cause of the spark:


a) Spark Plug
b) Leaking fuel injector in cylinder 3
c) Carbon deposits on the intake valves
d) MAF Sensor defective

The answer is b. Why? The dark grey matte finish is indicative of a cylinder that has too much fuel during the combustion process.

If the spark plugs have a matte black or grey appearance, it could be carbon fouling —something typically caused by a fuel mixture that is too rich. During normal combustion, most of the fuel oxidizes and changes into carbon dioxide and carbon monoxide gas. When there is more fuel than oxygen, the carbon in the unburned fuel polymerizes into carbon deposits. These molecules like to stick to the hot spots in the combustion chamber and this includes the spark plug’s tip and insulator.


Curing the problem will typically point to the fuel system and how the engine is breathing. If a fuel injector is clogged or sticking open, extra fuel can cause carbon problems. If the mass air flow sensor or oxygen sensors are not accurately reporting the air coming into the engine or the oxygen content in the exhaust stream, it could cause a rich-running condition that can cause carbon to foul the spark plugs. Another factor is how the air flows past the valves. If the air is restricted or has to flow past carbon deposits on the intake valves, the air will be turbulent and disturb the flame front and fuel droplet size in the combustion chamber. This means that the fuel injected into the intake port or combustion chamber will not entirely burn. If a spark plug has a glossy black appearance, well, that usually indicates excess engine oil in the combustion chamber. This video is sponsored by Autolite.

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