Rick's Free Auto Repair Advice

Spark plug condition — How to read a spark plug

Spark plug condition

Every day on auto forums I see people post a picture of their spark plug and ask, “How’s my spark plug? Should I put it back in?” If drives me crazy for two reasons; first, their pictures are so bad you can’t even see the center and side electrode condition and that’s what determines whether you can reinstall it, and second, spark plug color only tells you how the engine has been running up to this point. It tells you nothing about how it will run in the future.

spark plug condition images
spark plug tip comparison
So what determines spark plug condition?

Let’s start with the question; “Is this spark plug any good and can I put it back in my car?” The answer to that lies in the condition of the center and side electrodes. Let’s dig deeper into that

There’s a direct relationship between electrode sharpness and spark plug performance

Why? Because the spark wants to jump from a sharp edge on the center electrode to a sharp edge on the side electrode. Sharp edges produce the hottest sparks because they start earlier and the spark lasts longer than a spark from a round electrode.

So, when you ask if your spark plugs are still good, at least give us a close up of the center and side electrodes. Without those images there’s no way we can give you advice.

There’s also a direct relationship between spark plug condition and gap size

The center and side electrodes wear with use and that creates a wider gap. A wider gap requires a higher firing voltage to ionize the air/fuel mixture and that results in a cooler spark with short duration. In addition to reducing engine performance a wider gap stresses your ignition coils, causing them to overheat and fail early.

Is this spark plug good?

Is the center electrode still sharp or is the edge rounded over? Rounded: replace
Is the side electrode still sharp or is the edge rounded over or worn? Rounded: replace

Posted on by Rick Muscoplat

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