How to Distinguish Between Good and Bad Spark Plugs?
2 Answers
Methods to distinguish between good and bad spark plugs are: 1. If the spark plug is blackened as if smoked, it indicates that the cold/hot type of the spark plug was incorrectly selected, the air-fuel mixture is too rich, or engine oil is seeping up. If the spark plug is reddish-brown or rust-colored, it indicates the spark plug is normal; 2. If the spark plug is severely eroded, with scarring, black streaks, cracks, or melted electrodes at the top, it indicates the spark plug is damaged and must be replaced immediately; 3. Let the engine idle and listen carefully for changes in engine speed and sound. Obvious changes in speed and sound indicate the spark plug is normal, while insignificant changes indicate the spark plug is in an abnormal state; 4. Unscrew the spark plug and place it on the engine block, then test for spark with a high-voltage wire. If there is no spark or the spark is weak, it indicates the spark plug is leaking electricity or not working.
As a seasoned driver with over a decade of experience, I find it quite simple to distinguish good spark plugs from bad ones—mainly by examining their appearance. If the electrode tip is noticeably worn and rounded or the gap exceeds one millimeter, it’s basically time for a replacement. Excessive carbon buildup is also troublesome; a thick black coating on the surface, like the grime accumulated from cigarette butts, indicates incomplete combustion. I remember last time my car had trouble starting and was shaking—turned out the spark plugs were covered in rust. After replacing them, the power improved significantly, and I even saved a few bucks on fuel. Neglecting regular checks can accelerate engine aging, leading to repair costs of hundreds of dollars—definitely not worth it. I recommend inspecting them every 50,000 kilometers or periodically with a magnifying glass to prevent major issues. It’s both safe and economical.
