Three years ago a master tech showed me how to read spark plug color and it changed my diag game completely

I was working at a shop in Portland back then and this old guy named Chuck came over while I was scratching my head over a misfire on a 2005 Civic. He just pointed at the plug I pulled and said "that's lean, check your injectors". I thought I knew how to read plugs (brown is good, black is rich, white is lean) but he walked me through the whole thing. The carbon fouling patterns, the little blisters on the insulator, the way the ground strap tells you about timing. It was like someone turned on a light bulb. I spent that whole week pulling plugs on every car that came in just to practice. Has anyone else had a moment where some simple old school trick totally changed how you work?

3 comments

3 Comments

fisher.charlie26d ago

Read somewhere that modern plugs can be way harder to read because of all the additives in fuel now.

the_elizabeth25d ago

Honestly, @fisher.charlie, I gotta push back a little on that "harder to read" part. The additives in modern fuel are mostly ethanol and detergents, and they actually burn cleaner than old leaded gas did. Those old plugs used to get crudded up with lead deposits that made them look dark and fuzzy no matter what. Now a plug can look almost too clean because the fuel burns more completely, but that doesn't mean it's harder to read. You just gotta look for different things like the color of the porcelain and the wear on the electrode instead. The basic idea of reading a plug hasn't really changed, it's just that the clues are a little different than what your grandpa might've learned back in the day.

uma_baker9925d ago

That lean reading on the insulator is what trips me up the most Ellie, did you ever get a plug that looked textbook perfect but the engine was still running lean from a vacuum leak? Hey @fisher.charlie I wonder if those additives make the porcelain stay extra white even when the air fuel ratio is off, like masking the real story.