I would suggest you run a thread chaser in that hole and any others that appear to have issues with the threads. Then install new plugs, clear the memory and take an extended test drive with the scanner reading pertinent data lines. A properly installed plug backing out raises a lot of troubling questions.
No, spark plugs were not cross-threaded: positive about this
I checked all the plugs, and they were all more or less loose, but it is only one which almost fell off
spec is 18 ft-lbsā¦ this is way too low in my view: it did not even squash any of the compression o-rings on the plugs
this time I made them to go 1/4 turn over spec torque, all 6 of them
assembled everything, andā¦ still the same 0.30V
so far I have 2 theories:
1 -leak in exhaust before the catalyst
2- bad injector
Iām made a leak-test using a shop-vac blowing into an exhaust pipe and soapy water sprayed
so far I found one faint leak (slooooowly making bubbles) in the welded seam between the cat and its top flange, then biger (but still not that big) leak down from the cat
from the looks of it it is a laughable to throw A/F that much as I see: it would account for the offset when at idle, but not under 2000-3000 RPMS, where gas pressure is substantial
so, theory #1 is highly unlikely
as to the theory #2ā¦ first of all, Iāve never replaced injectors before, but it is likely manageableā¦ how do I identify this to be really a problem, not just shoot in the dark??
An exhaust leak after the upstream O2 is unlikely to be the culprit here. And while it is possible that an injector is the problem I would imagine that a bad injector would have caused a cylinder missfire code at some point in all your testing. Have you checked manifold vacuum with a mechanical gauge? Is there any indication of poor performance from a single cylinder at any rpm/vehicle speed? Does the car have a history of extended stop and go traffic operation? A coked up valve stem may cause that problem but I have never seen a single valve stem coked on an engine. Of course the problem will always show up somewhere first.
Engine actually runs very smooth
MPG is on the low side (~16) once it started, it was ~18.5 before
checking here: https://www.2carpros.com/articles/how-to-test-a-fuel-injector
āA common misconception about cylinder misfires is they cause the engine to run rich, in the absence of combustion the cylinder contains 14 parts air to 1 part fuel (14:1) which causes a lean condition.ā
Iām actually considering it a prime suspect now
and I have to confess: I used āinjector cleanerā fuel additive right before it threw this code, not sure if I clogged one of injectors with dirt
I think what happens is:
- ECU ādoes not knowā if any one cylinder is NOT burning or burning extrelemy lean: it has only one sensor to monitor 3 cylinders
- cylinder pumps as much air as it is supposed to, it is not getting any (or low) fuel
- A/F ratio is lean for obvious arithmetic here, and obviously it is a reason why it is always 0.30V: because it pumps one cylinder of air with no or low fuel
- any leaks on exhaust would show āasymmetricā response to RPM change, but I do not see it, so I have a āleakā, which is proportional to RPMs
- to detect misfire: it has to be either detonation present (no such thing for no fuel) or should be no spark (which is not the case either by my testing)
Answering on other your Qs:
I do not know how prior owner ran this car, but he made 150,000 is 10 years and I do have a good highway stretch on it regularly, so ānoā on stop&go
Car and engine are in great shape: very smooth operation and almost no engine gasses blowback, which is an indicator that low compression is not on top suspects list
Did not check manifold vacuum with a gauge, but with a finger seems to be nice&strong
Update after a lot of new work.
I removed and cleaned injectors: all were in good shape, but Iāve cleaned them under pressure with carburetor cleaner, then ultrasonic with water and degreaser, then carb cleaner again.
At this point, they all have very consistent spray pattern and can be declared to be in top shape.
All the o-rings were replaced, so vacuum would not leak there for sure.
To make things even more certain about injectors, Iāve swapped banks 1 and 2 to check if problem follows.
Result: CLOSE TO NOTHING
Now I have 0.31V reading on Bank 1, instead of 0.30V before.
I looked down to the intake ports and valves looked OK, I would probably even classify as CLEAN.
At this point I go for compression test on cylinders at bank 1 and then Iām out of idead 
One more test I did: I used a rug to block exhaust pipe, not create an excessive pressure there (few seconds at a time).
Zero correlation with sensor reading on bank 1, not even 1/100 of volt
Do you have access to a lab scope?
I donāt do not even know what it is.
Update on compression: cylinders 1/3/5 are 13.1 / 12.7 / 13.0 - not bad in my view for 150K+ mileage
I tried spraying curb-cleaner to the gasket connecting intake manifold (aluminum pie-shaped one, not plastic plenum) to where it meets cylinder head looking to get any revs or at least spikes on sensor: nothing.
Still, Iām entertaining an idea about failing gasket between bank 1 heads and this pie-shaped intake manifold.
The trouble about it is that bank 1 is where plastic plenum/airbox obstructs all the view. Iāve pretty much pissed that carb cleaner into the dark, hoping for miracle, but I did not probably cover even 30% of potential contact/failure zone
I was explained by a friend what ālab scopeā is, and YES, I can get it
I can hear some hissing from the top, not from the wheel well
gonna look for smog machine or just use propane?
If you hear a hissing you have likley found the problem.
I hope, sir! 
Unfortunately I could not confirm it with a carb-cleaner spray, just to be sure.
Gasket set is not very expensive, and by this point I can get all the top removed and reinstalled with closed eyes.
I wonder WHY it would leak?
Connected worry is if anything was cracked.
Sure, I was applying some brute force as I was working to loosen nuts on the catalyst, on this side sure enough, but could not imagine it might somehow pop up on the heads-to-intake connection??
Intake manifolds are prone to mate unevenly. There are various techniques to get the manifold to nestle into position evenly and only with close attention or a great deal of luck can the mating be successful. Even the small block Chevy intakes took some care to get settled squarely in place so as not to leak.
not very reassuring
still, how happened it failed after 150K milesā¦
anyway, will post update once Iām there
I thought you had removed and replaced the upper plenum. The seal between the upper and lower sections would be the difficult joint.
I used ācrazy Russianā method to make a smoke machine on a dime:
- plastic bottle
- hot-glued piece of hose to the cap
- hot-glue compressor trigger-handle to the bottom
- cigarette into the capās hole
- lit it up and blow
I had some smoke through the tail-pipe, but importantly, I have smoke from under the lower intake parts, so diagnosis is official at this point - somehow Ive got a hole there
anybody knows if Nissanās VQ40DE (4.0L V6) has coolant circulating through the lower intake?
I managed to avoid draining cooling system so far, as I could take the plenum tilted and shuffled aside doing all other work, so ould we a great surprise to get coolant all around removing aluminum intake header
Who was it that said āthe harder I work the luckier I get?ā