Intermittent loss of compression

[grannyknot]

So one or more of the valves were sticking before the engine warmed up completely, if they were sticking closed that could indicate a failed or lazy lifter, too tight valve guide or misaligned guide, or a burr. If sticking open that points to too tight vale guide or misaligned guide or a burr. That slap you heard could have been the piston hitting the valve and closing it.

If the mechanic over reamed the guides the valves could be binding because of too much slop or misalignment, if he knurled the guides they could just be too tight because he didn't ream enough and as the engine warms up they expand and allow easier movement, it's called morning sickness.

Now it's drinking oil, anyway I look at it you still have to pull the head, examine and measure those guides.

[zeeboost]

So you're thinking that the sticking is more from the guide, but not the seal itself? At this point I figured at a minimum I would be removing the changing out the stem seals, since I'm suspicious they're the culprit for the oil burning. With the seals off I can do a rough visual on the movement of the valve inside the guide to see if there's any obvious excess slop. If I have to pull the head off then it's going back to the machine shop, and I don't know how long that will take again...frankly I'm really just hoping the stem seals will do the trick. Though it was kind of odd that literally all cylinders had no compression on startup towards the end of this issue. 

Thank you all for the replies, sounds like it's pretty narrowed to either valve stem seal or guide. 

[madkaw]

There’s also the option that the machinist used bronze inserts vs knurling the guides and they were/are too tight. Maybe material from wearing on the inserts ruined the seals ? Several possibilities 

[zeeboost]

Thank you all for the replies, I'll follow up with this thread once I get time to tear into the engine. 

[zeeboost]

Brief follow up:

Replaced all of the valve stem seals, the old new ones were brown (didn't find part numbers) and my replacements are black (Ishino Stone). When I held the brown one side by side to the black one, the inner diameter of the top opening was definitely smaller on the black, but it could just be because it hadn't been worn in yet. As I popped the brown ones off, they still snugly rode up the valve and caught the lip of the valve stem. I did notice a under a few of the valve stem seals, small pieces of what appeared to be old carbon build up or sludge were on the valve or guide. I forgot to bring my micrometer to measure the side-to-side play, but it seemed minimal, to the point that I couldn't see it preventing a new stem seal from doing it's job. I understand that concrete numbers and measurements trump how I feel about it, but I can go back in and measure it some other time if I need to. 

So the new seals are in, still smokes, though it does seem to be a little better at idle...but probably just my imagination. Friday evening I pulled all the plugs and have had ATF soaking on every piston.  Today I rotated the crankshaft a couple times, and I'll mess with it after christmas break. I remember when I first opened up the engine, it had a bit more sludge and oil buildup than I care to see. So there's a chance the oil rings have too much buildup to do their jobs, but again, the oil consumption was a recent development so I didn't think rings would be an issue. 

I also "rebuilt" the lifters as per cygnus's write up. This should be a mandatory service with anyone that has a p90a head; lots of nasty gunk built up inside of there.  I had been reading that these lifters can have a tendency to over pump, so to speak, and it made me wonder if that was the reason for the initial intermittent loss of compression while cranking. 

 

I'll keep this thread updated, until then, Merry Christmas to you guys!