background:
97 gsr with 150k miles
last valve adjustment + spark plug change at 100k
CAR RUNS FINE - no stumbles or loss of power, no visible smoking

So I was driving on an uphill freeway, minding my own business, when the check engine light came on. When I pulled the codes, it was cyl 3 & 4 misfire. I reset the codes and they haven't come back yet. I looked at the spark plugs, and #1 seemed the best, with only a light tan coating. (I read somewhere that under normal driving conditions, the car runs slightly lean.) The worst was #4. It had oil on the threads of the plug. The electrodes were kind of black (not wet with oil, but maaaaaaybe slightly moist, not sure) and had symptoms of "ash deposits", which is light brown and black crusties that I can easily scrape off. #3 was slightly better than #4, but still had a little oil on the threads plus blackness, and #2 was only slightly worse than #1. I don't know if this pattern is meaningful of anything.

I should also mention that the car burns about 1 qt of oil every 1000-1500 miles (I think it's been doing that ever since 70k when I was over-enthusiastic about dumping seafoam and water down the intake in order to try to clean out deposits). But no noticeable smoke out the exhaust.

The last valve adjust was at 100k, but looking at the receipt, I only see labor cost and no mention of any parts, such as valve cover gasket or spark plug orings.

Basically, I would like to know if this can be due to spark plug seals (easy to replace at next valve adjust), or if it is something more expensive like piston rings or valve seals.

 

Star with the cheap stuff, like replacing the spark tube seals and go from there. If it's burning oil then there is probably a burnt gasket/seal somewhere. You can test for compression to see if it's the rings or valve seals/guides.

BTW: NEVER pour water into the intake. Water only goes in the coolant system.

 

do a vacuum test and leakdown compression test. chances are a couple cylinders will read great and that one with the heavy ash will be your problem

what's causing it? could be rings, guides or seals, often a combo of all 3

if the vacuum test has any flutter in the needle you have a cylinder low on compression. since these literally take a minute to do, start there, then move to the compression test

 

Here's a picture of my plugs currently (about 2 weeks old). You can see that two look awesome and two look black.



It is a dry black soot. How can I tell if it is oil burning or excess fuel? Another weird thing is that in the mornings, the piston tops for these two cylinders look wet. When I dab this moisture and spread it on aluminum foil, it doesn't stink like gas, yet it also dries up and evaporates after a few hours. Almost like water...

Compression tested 210-230 across all cylinders, and there is no smoke at startup or after engine braking, so I assume for now that oil consumption is related to the oil rings.

 

rings lose tension over the years. that's part of how you can have good compression and burn oil. by the way, 210 to 230 is a big difference. healthy engines typically ready within 5psi of each hole.

hard valve seals and slop in the guides is the other way oil tends to burn. the exhaust valves get hot, so do the seals, they harden and oil can seep down