What does this plug tell you?

[BlackBart]

Sorry off-topic, not a Volvo. It's my son's '86 BMW E30 straight six.

This is a bit different from things I've seen. One cylinder only, #6. All others had normal color and minor wear. My son says it wasn't gooey like it looks, it was hard, he scraped it off.

I'd guess "deposits" on a basic plug chart -
https://ngksparkplugs.com/en/resources/read-spark-plug

I wonder if if's oil, coolant? Looks like there's a bit of oil on the threads (?). It was running fine, no missing.

[amblerman]

It's funny, whenever I try and use those spark plug charts, I come away thinking my engine is suffering from "all of the above".


The problem with the "deposits" type of example is they usually show you one example. And I'm sure the deposits would look different based on what chemical/additive was responsible for the deposits (ie fuel additives , fuel cleaners, etc).

The example they show looks grey and fluffy.

I actually think the color/texture of your son's curved electrode looks normal. It's just the wart on the center electrode that looks funny. That sort of matches the color/texture of the "melting" example on that spark plug chart.

But I have no idea how likely melting is, how it occurs, etc.

[BlackBart]

Agreed. "Ohhh noooo!" My experience is only brown and normal, black sooty rich, or dripping wet flooded engine.

I'm wondering if that's burned oil. Could be rings, valve guides.... Is it coolant? A BMW guy just told me the #6 cylinder is the most likely spot for a head gasket failure.

[BlackBart]

Compression test last weekend :

Tested each one at least twice. Kept climbing through 20-25 pulses.

1 - 180 / 180

2 - 163 / 165 added teaspoon of oil: 190+

3 - 162 / 165 added teaspoon of oil: 187

4 - 180 / 180

5 - 175 / 176

6 - 180 / 180


So those are just at the edge of 10% apart. Looks like worn or sticky rings. Interesting normal numbers at #6 with the plug. Maybe an intake valve guide?