So you've pulled the spark plugs from your car, motorcycle, or nitromethane-powered phone charger, and they seem dark. Sure, spark plugs, which ignite the fuel-air mixture in engines and make them go, are supposed to have a little color to them, but these are really, really dark — almost black. Is that okay? Is something wrong? Never fear, dear reader, for spark plugs are easy to read once you know a couple little things.
The color of a spark plug can tell you a few different things about your engine's health. In extreme cases, a blackened spark plug could be fouled by oil in the combustion chamber, but more often a spark plug's color will tell you about your engine's air-fuel ratio. A dark plug means too much fuel, while a light one means too much air. Both are bad, though for different reasons — an engine running rich will use too much fuel and lose power, while an engine running lean can get too hot and cause some real damage to its internals.
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Ideally, your spark plug will have a bit of color on it, but it won't be entirely black. If it's truly dark, though, first give it a sniff — you're looking for the smell of oil. If you get a whiff of that (and it'll be a strong smell), you're looking at a plug fouled by excess oil in the combustion chamber. That points toward a blown piston ring, a gasket issue, or a problem with your PCV system.
In the absence of an oil scent though, a very dark plug means a blocked air intake, a dirty air filter, or an issue in your fuel supply (whether carbureted or fuel injected) that's giving your engine too much fuel. This will hurt your fuel economy and power, but it can actually be a bit safer than a spark plug that's too light — running too lean risks heating your engine's combustion chambers too much, which can cause issues from pre-ignition all the way to overheated pistons expanding too much and welding themselves to the cylinder.
Reading spark plugs is becoming a bit of a lost art, but there are still good guides out there with visual references to help you figure out exactly what color they should be. Sure, your modern car will likely report an issue to you from its computer well before you ever see its spark plugs, but your lawn mower or generator won't — knowing the proper color of a spark plug is still important in small engines. Plus, it makes you feel more like Sean from "Tokyo Drift," and who doesn't want that?
