I would agree... Reading plugs is pretty much an obsolete art ... once the lead is gone.  You can still get a vague idea ..a nice tannish finish on the centre electrode is good.. But is doesn’t take much ..a bit of idling in traffic etc before the evidence is messed up.

Cheers, Ian

Back in the days before unleaded fuel or alcohol, the best mixture produced a "light tan" coating on the porcelain. An un-serviced plug would gradually build up a stone-like deposit over the ground electrode, making the combustion less efficient. A too-lean mixture would result in erosion of the porcelain and a burning away of the ground electrode. The porcelain might also crack and chip away.

Another sign of good combustion was a light sandy looking coating on the inside of the tailpipe. With the advent of unleaded fuels, (circa 1975), tailpipes went black. Then came catalytic converters...

There are lots of plug charts, of course. I’ve seen a few that would call the “lean” plug in my example “on the lean side but still OK.” My O2 gauge indicates a darkish tan or a light chocolate electrode is optimum, although I understand the Mini produces max power when it’s running a bit on the rich side. 

Hmm, interesting.  I would have thought the optimal plug in your example would be too lean.  Thanks 

I'm almost embarrassed to admit that I check my plugs every 200 - 300 miles. I have a mass O2 gauge and so I'm able to keep my mixture pretty much spot on. My plugs look like the "good" example in the old school" plug charts. Occasionally, when I let my car idle in the driveway for a couple of minutes before shutting it off, my plugs are a bit sooty looking.

I have noticed that when I've used an octane booster, the ground electrode has a reddish hue due to the manganese compound contained in most octane booster products. It seemed to me that the manganese was masking the true color of the plug so I stopped using it. I'd be curious to know if ethanol effects plug color as well. My local fuel contains 10-15% ethanol.



 

What part of a plug would give the best indication of engine health?  I've heard that the color of the deposits on the nose of the insulator will tell, others have said to check the color on the ground electrode.  Or does the use of unleaded fuel contaminated with ethanol (editorial comment) invalidate the old methods of reading a spark plug?