Cylinder #7 (left, rearmost) is oil-fouling the sparkplug and burning oil / smoke from that side exhaust.

Heads received valve-job from a reputable machine-shop and New (IIRC) Viton Oil Seals,Valve-Springs, Retainers, 8-months earlier.

Figured it must be worn rings. Yesterday, compression check. The "Bad" #7 cylinder produced 143 PSI which is average to a bit higher (Entire Engine, Lowest = 137 PSI, Highest = 148 PSI.)

Squirting some oil into #7 cylinder and retesting compression, no change or increase.

So, could the oil-rings go bad, yet compression still be Ok? What is the likely problem? It Smokes and fouls that one plug only. Seems to run Ok.

Engine is an AMC V8, Factory Original engine, in a Jeep CJ5. Basically, a daily-driver with a moderate aftermarket cam, intake, ignition, headers, dual-exhaust, Holley 4BBL.

Is the valve-guide and / or valve seal more likely the problem?

If this a AMC try to see if you have room for a double seal on the guide I have used the intake and exhaust and solved a similar problem years ago on a 304. The intake gasket can also be the problem but I would not be tearing into that engine with the comp you read

Thanks Paul.

Yes, it "IS" a 304 V8. When you say "double" valve seal, is there a particular Brand to look for, or do I install Two standard Seals on each valve?

I would look into the heads first.

The 350 I used to have in my truck drank a bottle of oil a week and had two plugs that would get sooted up real bad requiring me to rotate them around. Interestingly the truck never blew any smoke out the exhaust unless you floored it, and even then it wasn't very much.

I did a cranking compression test and it pulled very close to 150 on every cylinder so that eliminated the rings.

I pretty much assumed my issue was related to the heads, either the guides or the seals, but I never really dug into it. What was weird was the engine never really used oil and then it suddenly started using a lot almost overnight.

Is it using oil or just a little burning? Wiped cam lobe (intake)would foul the plug but still give good compression and let some oil out the tailpipe. Exact same thing happened to me in a 440 10 yrs ago.

I had a small block chevy that would smoke like a pig if I hit the brakes hard but would not smoke on start up acceleration or deceleation. I did a compression test and was perfect all the way across hot and cold I did a leakdown test and that was perfect also yet I still had gray smoke when I hit the brakes hard. I checked it with the brake booster disconnected thinking it was bad and the engine was sucking in brake fluid making the smoke but I still had smoke when I hit the brakes hard. I finally pulled the heads off and found the #1 cyl wrist pin was coming out of the piston and had a 1" wide 1/8 " deep trench dug in the side of the cyl. The wrist pin must have been sealing in the trench giving me a good compression test but when I hit the brakes hard the oil would slosh to the front of the motor getting in the trench and making the smoke

Yes, the compression ring may be good enough to give a good pressure reading but may have a bad/broken oil ring.

Appreciate the replies and experiences.

It is both burning and using oil, but the oil use isn't horrible. It smokes particularly bad once it is warmed up.

It even smokes heavily at idle, waiting at stop-signs and stop lights, but also when accelerating. Not as much when I first start the engine (cold) but heavily when I re-start it after it has been driven a while.

I bought some new valve-cover gaskets and an on-engine valve-spring compressor. Already have the air-line to hold the valves shut while pulling the valve-keepers/retainers/springs.

Valve-Stem side-play in the valve-guides is supposed to be 0.001 to 0.003 according to the Jeep FSM.

I'll inspect the guides and seals as best able and may try to squeeze two valve seals onto each valve-stem of the affected cylinder.

3 things that come to mind:
Intake gasket allowing oil into intake port
Worn guides, worn or no valve stem seals
Scored cylinder wall

A good list.

I should give the intake manifold oil leak more consideration.

Ditto on the intake gasket. I had the same problem a while back... The intake was loose and a small part of the gasket was blown out. The car was smoking on one side and using more oil than usual. Replaced the gasket, torqued the intake manifold and the problem was gone

Remove the 2 valve springs on #7 hole and replace the seals. While the springs are off, check for valve guide ware. Worn or not, put it back together and see if the problem has gone away. If it's still using oil, Pull the intake and check to see that the intake gasket is sealing properly around that hole ( valley area inparticular ). If all looks ok, pull off the left side head. At this point in disassembly, you should be able to find the problem.

Yes, that's approximately what I plan to do.

Thanks Again.

Need to clarify
Make sure the seals are placed firmly against the seat there was time back in the mid 1980s that the 360 engines valve seals were lifting up off the seat and actually helping push oil down past the lousy valve guides. I would use a intake valve seal on both then cap it with a 1/2 cap exhaust valve seal. Please keep in mind that if the guides are worn badly nothing will stop it with out knurlling {sp}or have new guides put in the heads. By the way older AMC engines really preferred the straight weight oils over the 5-20-30s etc. Jeep trucks with 401s I had would go threw light weight oil but not the 30 weight?? Why ?