What's wrong with this spark plug? (burn marks on center electrode)

[redfishsc]

This came out of a 99 Suburban (5.7 Vortec, unmodified) with around 211k miles on the engine, and MAYBE 4K miles on the plug. It's hardly 7 months old.

This is a copper AC Delco R44LTS6, which is the copper Delco recommended by Rock Auto. I wanted to just try cheap coppers since the plugs in this truck aren't terribly hard to reach.


I found it today trying to find the source of a rough idle. It didn't lope or surge, it was just 'shaky' and the problem usually went away as soon as I'd start rolling.

Check out the center electrode insulation and those black burn marks. There are two main burn marks with some burn in between them.

Ignore the cobwebs, that's pocket lint

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I popped in a replacement plug (same style) and now it idles as smooth as ever. I'm not 100% sure I've solved it since i can't really say that without driving it for a few days.

Next question, what is your favorite spark plug for this engine? I do not get any pinging or anything, and I don't tend to foul spark plugs with crap. The truck uses maybe 1-2 quarts of oil over a 6000 mile OCI (penz plat 5W30) depending on how hard I am on the throttle.

 

[skylark]

I run NGK. Just the $2 v power plugs. I've been very happy with them in my gm products.

 

[Tom P]

What skylark said. NGK makes the best plugs in my opinion. Although delco plugs have never let me down either. Never seen an electrode dome up like that before. Check your spark plug wires, coil and cap and rotor... Don't bother with the platinum or iridium plugs. IF they do produce any benefit, it will never be enough to notice during daily driving. Running the V powers in my 7.4L as we speak.

 

[redfishsc]

What skylark said. NGK makes the best plugs in my opinion. Although delco plugs have never let me down either. Never seen an electrode dome up like that before. Check your spark plug wires, coil and cap and rotor... Don't bother with the platinum or iridium plugs. IF they do produce any benefit, it will never be enough to notice during daily driving. Running the V powers in my 7.4L as we speak.

The only benefit to platinum/iridiums that I can tell is their longetivity. My former 03 Deville had a Northstar, and the rear bank of plugs was so freaking hard to reach that there's not a chance in hell they were going to get replaced but once every 100K.

On the Vortec, they're pretty straightfoward.

On the plug you see in the pic, that dome isn't all buildup, that's part of the plug. It's just these strange carbon streaks. It's almost like the spark was going from the center of the plug down to the bottom (valley), and that would potentially also explain the nasty idle.

It still idles like a kitten from a cold start this morning, I'm about to go for a drive, so we'll see. I think it's fixed but cant' say that for sure. I've learned that on these old Chevys, you really never know they were actually fixed until they break down again.

 

[SAATR]

I'd say either an overrich mixture causing excessive carbon buildup or a plug whose heat range is slightly too cold. You may well try the NGK's and find that their heat range spec is a bit closer to what you need to keep the plugs clean. It can vary a bit from manufacturer to manufacturer, hence why some plugs don't run as well as others in an otherwise sound engine. I've been running the 41-993 Iridiums for quite a few miles now and have no complaints. One of the benefits of an iridium finewire electrode is that the tip runs hotter than a standard copper/nickel electrode due to its smaller thermal mass, which makes it slightly more resistant to carbon fouling. That and longer service life (both advertised and observed by me) make them an easy choice for my DD.

 

[Hipster]

oil fouled

 

[redfishsc]

Which NGK would you guys recommend?

 

[SAATR]

TR55 if you want to stay with a standard plug.

Sent from my XT1585 using Tapatalk

 

[Tom P]

TR55 if you want to stay with a standard plug.

X2

 

[redfishsc]

That's what I'll order then.

So far the idle is still WAY better than it was, but there's still some lingering shake that was definitely not there before I put in the Delco coppers. It's possible my coil is getting weaker and it's not the plugs' fault, but finding that one messed up plug in a cylinder that has not been fouling anything up make me suspicious.