Hi, I’m hope to get some insight on what to look for next. I am leaning towards pulling and tearing down but before I get that deep wanted to get some thoughts. engine history Complete rebuild in 1989 (bore, pistons, rings, bearings, etc… and 7k mi since rebuild which included head work. New ss valves, guides, seals, etc… Out of the 7k, ~2k in last 3-4 years what’s happening. - Couple weeks ago noticed miss at low speed cruise below 50. - Car idles fine. - Car is not smoking at idle or while cruising - Recently at cold start passenger exhaust black soot condensation at initial start up instead of clean condensation like drivers side - Pulled plugs #2 & #4 were sooty. (See pictures plugs 1-8 L to R). All others looked good. and slight oil on spark plug threads on #4. Pictures of pistons with scope. - Replaced plugs, checked wires (resistance o.k.), replaced cap and rotor and no difference with the Miss at low speed cruising below 50. . - Vacuum is at 19 and timing at 8 initial and 34 at ~2,500 rpm - no excessive vacuum if i block breather hose to air cleaner and put hand over oil fill. - recent oil change just before miss started and have used a 1/2 quart of oil in last 200 miles - recently noticed some oil puddling by the intake bolt just above #4 next to choke coil and it not coming from valve cover. - did a compression test all cylinders between 150-160 lbs warm except for #3 at 115lbs . Did another test on #3 wet and only went to 125lbs. - Started a leak down test before my tester took a crap and I have no confidence in the readings since it’s not a quality tool and the gauge kept sticking. I ordered a new tester. What I did find was it sounds like #3 exhaust valve is leaking since I can hear the venting out of the tailpipe. All others venting into crankcase but again I can’t trust or provide readings until I retest. I believe #3 could be what’s causing the miss but does not solve for sooty plugs on 2&4 or the oil puddling and consumption. I will do another leak down once I get my new tester but want to see what opinions are out there on next steps. I am confident I have an exhaust valve problem w #3 but not sure what’s going on with #2&4 until I do the leak down. Thanks,
Wow, your pistons look fantastic. No carbon build-up, very clean. Don't have a clue on the plugs. Would like to see what the leak down test shows. How about your PCV, check to see if it's still operational?
Describe what kind of mis you have. If in fact your valve/s aren’t sealing, your engine will run, especially at idle, with a distinct da da da. IF it’s intermittent like ignition, it will idle smoothly then a da da thrown in. I know my description sounds funny, but the way an engine runs (idles) tells a lot.
I think you may have two separate issues taking place here. 1) oil usage . 2) low compression. I have had cases where oil usage / burning oil from failed valve seals and or rings did keep a valve unseated when it should have been seated . I think you need to pull that head with the exh valve leak and see why it is leaking since you have confirmed that already. You will then also get to see if there are a any scratches or gouges in the bore that would be signs of a busted ring or cracked ring and on the piston.
Intake gasket sucking oil into 2,4? Stock rockers? If adjustable #3 exhaust too tight? Are you sure both valves were closed on test?
I was going to say the same as Dan about the intake gasket........I can't see any other reason it would puddle there. Even the composite gaskets tend to wick oil through then, especially without a splash pan.......maybe just an intake retorque will help Pull the rocker shafts and retest leak down.........you can tap on top the valve and sometime and few sharp cycle helps to knock off some carbon build up........with the leak down tester installed you get instant feed back. As stated any intake valve sealing issue can be seen by a bouncing rapidly vacuum gauge
I wouldn't be surprised if the valve seals were starting to fail given the age of the rebuild. A quick peak under the valve cover might show rock hard or even broken/missing seals. I would also spray the intake gasket with some carb cleaner while running and see if the engine responds. You could also do a smoke test on it and see if smoke starts coming out the breather hole.
Vacuum is steady at ~18.5 and doesn’t bounce. Performed the leak-down test. #3 had the low compression and the exhaust valve is definitely Leaking as I can really hear the air coming out of tailpipe. Leak down shows definite problem. cold- 1-17%, 2-15%, 3-85%, 4-22%, 5-20%, 6-15%, 7-13%, 8-21%. I pulled the rocker shaft and there’s something floating at the top of the spring on #3 exhaust valve which I presume is the valve guide seal. However, I didn’t think I would have seals on the exhaust valve (see video) https://youtube.com/shorts/9cBTM_wPxB8?feature=share I think the next step is to remove the spring?
It kinda looks like the inner dampner spring is broke.Seals fit pretty tight on the guide and have a retaining band on them with a fair amount of tension on them to keep them from coming off. The spring needs to be removed to repair whatever is going on in there. Chris
85% is big........I dont like seeing much over 15-20%, but alot of motors tend to run just fine at those numbers. A cracked or broken spring could cause you issue........at least forbthis you have it very much narrowed down to where your problem lays.
Just watched your video. Hard to tell what that loose thing is until you remove the spring. Looks like the dampener is still in one piece tho.
Just thought of something, IF someone put a seal on the exhaust guides WITHOUT machining the guide to the proper size the seal will not stay on the guide. Buick DID NOT use seals on the exhaust valves, ONLY on the intake valves.
Mark, that’s what I thought too, regarding seals on exhaust valve. I need to pull spring. As far as a removal. Is the T/A tool my only option with the head on?
Many tools out there. I'm sure you know this but make sure the cyl. is pressurized or the piston @ the top.
You can use a single spring tool like this https://www.autozone.com/test-scan-...O35h--Yt-DqoS_GJBbBoCrHoQAvD_BwE&gclsrc=aw.ds
The air works good but I prefer to fill the cylinder with nylon rope and then rotate until the piston squishes the rope up and holds the valves up. Its a little safer if something goes wrong. You can also lower the piston some to check the valve guides
It it was a valve guide seal on exhaust. Springs look fine. Short video of what the valve looks like. I wish I could see the seat too.
