What does this spark plug mean?

[Big6ft6]

So I've been burnin' about 1 qt of oil every 300 miles in my 91 k2500 TBI 350....I finally got around to pullin' all the plugs, and all of them looked really good except this one.

What do these two things mean?.... the way the plug looks, but also that only one plug/cylinder seems to be the problem? Do either of these pieces of information point to a specific idea of what is cuasing my oil consumption? 1qt in 300 miles through one cylinder seems pretty crazy. I have no visible oil leaks whatsoever, so it is all being burned.

Here is how my other plugs looked.

 

[yellowK20]

Bad rings in that hole, or bad valve guide in that hole.

what your seeing on that plug is ash from the burning oil

 

[Big6ft6]

Is either one of those more likely? At first it seems if the engine was run dirty, wouldn't all the oil rings in each cylinder be gummed-up/stuck....but then I thought about it it seems equally weird that only one valve guide was bad.

Is it possible to burn a full quart of oil in 300 miles though bad rings?

Is there an easy way to test for valve guide wear? I assume I'd have to remove the valve springs...I suppose that is stil easier than checking the oil rings!

 

[Big6ft6]

Another thought...what about an intake leak in the runner that goes to that cylinder? Would it "suck up" that much oil from the lifter galley?

probably not huh? have to be a pretty darn big leak.

 

[K85 Octane]

check the compression and leak down of that cylinder

 

[Big6ft6]

Compression was good, all 8 cylinders were very close to 150. I don't have a leak down tester unfortunately.

I bought the truck about 4k miles ago. I noticed the ecessive oil consumption only a few hundred miles after purchasing. The first thing I did after noticing oil consumption was check the compression The previous owner had changed the plugs/wire/cap cuz he said it had a stumble at idle... (now I know why) so the plugs were all clean at that time since they were new. I had sampled a couple plugs over the months since then, but only yesterday did I check them all and found the bad one.

 

[yellowK20]

sounds like a valve guide then, does it smoke at startup?

 

[Big6ft6]

No, that is the wierd thing, it actually never smokes. When I first start I get the steam like every car I've owned, that goes away by the time I'm to the end of my street.

the only tailpipe evidence of oil is that my tail pipe is coated on the inside with black soot, and if I hold my hand right by the tailpipe whiel she's idling I get little black flecks on my hand. But never the classic blue smoke from burning oil.

Since it doesn't burn oil at start up, people keep telling me it must be my rings, but now that I know it is one cylinder...that just seems like a lot of oil to be squeaking past oil rings doesn't it? Or not?

 

[yellowK20]

Could also be a manifold leak maybe and its sucking oil into tue intake and burning it there