After a few hundred more miles my oil consumption issue seems to have stopped. I've added a People's Liberation Army oil catch can and can't get it to smoke anymore either.

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Maybe a stuck ring loosened up and started sealing again.

I think it was this:

On ‎10‎/‎13‎/‎2019 at 10:00 PM, Reptoid Overlords said:

Could it be that when I filled the bone dry engine with 4.7 quarts of oil and ran it, I had to add a quart to make up for areas that normally do not drain oil under normal circumstances?

 

I went and got a harbor freight leak down tester. Not sure how accurate they are, but the readings are a consistent 12% approx loss on all 6 cyl.

I can hear a faint bit of hissing through the oil filler hole, but can't feel any air through there or the dip stick.

Vacuum at hot idle is 22hg. It shoots up to about 25 if you blip the throttle. No strange movements of the needle. Nice and steady.

The plug condition has leaned out a bit too.

I'll still be keeping an eye on things.

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It’s a bit contradictory that you have that high of vacuum numbers but 12% loss on leak down? My engine runs 10 at idle but only 3-5% leak down per cylinder . Plugs don’t look terrible, maybe still burning of residual oil?

2 hours ago, madkaw said:

It’s a bit contradictory that you have that high of vacuum numbers but 12% loss on leak down? My engine runs 10 at idle but only 3-5% leak down per cylinder . Plugs don’t look terrible, maybe still burning of residual oil?

Camshafts could contribute to the discrepancy

3 hours ago, madkaw said:

It’s a bit contradictory that you have that high of vacuum numbers but 12% loss on leak down? My engine runs 10 at idle but only 3-5% leak down per cylinder . Plugs don’t look terrible, maybe still burning of residual oil?

Assuming that the cam is stock, what would be an acceptable percent loss on leak down given those vacuum numbers?  I'm not arguing your comment, I just really don't know.  Most information regarding leak down test results state anything over 20% is bad news, and some say 20% is okay.  

52 minutes ago, Patcon said:

Camshafts could contribute to the discrepancy

I have no real history of this motor.  It came out of an abandoned car I got with no title.  Some PO in it's lifetime, for better or worse, certainly had it apart.  It's not entirely out of the realm of possibilities that the cam is not stock, or at least not anymore..  It's got all the OEM markings on it, but I guess it could have been ground.   That might explain why when i drove the donor car it seemed so fast.  And it might explain why is seems substantially faster than my old stock motor, and the lope at idle.  Moreover, it might explain why I'm having issues running rich.  

Today I installed an oil separator in line with the crankcase PCV system. I found excessive oil in the rubber line from the crankcase tubing to the PCV valve in the manifold.  If I'm unknowingly running a cam with longer than stock duration could the valve timing be late, causing high vacuum?

 

I have no baseline from which to judge how a new or newly rebuilt L28 "ought" to run.  Anyway, I hope I don't seem argumentative or unappreciative, I always appreciate all the fine help I get here.   

I realized after rereading the thread that this is an untouched used bottom end - so your numbers are healthy for sure . I would be happy with the vacuum readings for sure . I have a 54 block with 110k miles that I need to put a head on . I don’t plan on touching it either since the bores look perfect .

David L been following this. Interesting stuff. 
Stephen’s has been just as interesting too

i have lost track of how many project you have now?
Don P

I would expect higher vacuum on a cam with less over lap and possibly less duration

I thought the leak down numbers were really good all things considered

I'm not entirely out of the weeds. Whatever oil consumption issue I had, wether it be throughout the combustion process, or the PCV system is totally gone.

However, today I observed the oil level too low after another few hundred miles. According to the dipstick, after letting the hot engine sit about 30 min, about a half quart is gone.

I looked around underneath and discovered a bit of oil seeping down the bottom of the transmission that I hadn't seen until now.

So, being the reasonable person without a problem that I am, I got whatever inconvenient equipment that was in the way out, and discovered that the rear main leaks pretty good.

It's a Fel-Pro. Looks like I'll be stopping in by the Nissan dealer on the way home from work tomorrow.

One thing I didn't do, and it's no one's fault but my own, is install the trans to block brackets that came with the motor. Not sure if they are for structural support, but I'll be installing them this time.

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They're for "NVH" reduction, I believe.  I think that stands for Noise, Vibration, Harmonics (or  Harshness),  They stop the transmission from wobbling around on the back of the block.  Not going to help your oil leak but might make for a quieter ride.