At first i've noticed a lack of perfomance, while the milage dropped. No codes, no messages.
Checked the vehicle with ELM327 and, later, with IDS and found that LTFT and STFT were a mess on a both banks.
The LTFT jumped from +10 to -10 while STFT were shifting from -15 to +15.
Needless to say that O2S readings were a mess to.
First i've checked for a vacuum leaks. Actually all of the tubes were leaking.
Ok, i've removed a throttle and started a job
1. New pass. side PCV hose. Now it's made from rubber.
2. Checked a intake tube for a cracks. None were found.
3. Removed a T-fitting below a throttle body, which runs for a drivers side PCV and purge solenoid.
4. Pulled a drivers side PCV hose. It broke Replaced with a rubber.
5. Replaced a purge solenoid hose for a rubber hose
6. Replaced a small vac.hose of a purge solenoid
7. Replaced a fuel pressure regulator vac. hose
8. Found that brake servo vac. line was leaking badly. Pulled it off from a intake manifold.
9. Fabricated a rubber plug and plugged an intake manifold provision for a brake servo hose.
10. Installed a T-fitting between a throttle and purge solenoid and connected a brake servo hose to it.
11. New TB gasket.

After this massive amount of job there was still a mess with a LTFTs. Both were -10.
Checked a MAF sensor - it was fine.
No leaks.
O2S are fine.
After some digging a, by an accident, disconnected a RH PCV tube while an engine was running. STFT's jumped up to +15. Ok. I've plugged a RH PCV and STFT settled to a +10; after some time LTFT were 0 and STFT were 0 too.
But it's not good to drive a car with a plugged PCV.
Then i've found one simple thing. I guess that the 2.5mm PCV restictor in a LH valve cover is broken and do not restrict a flow to the PCV.
LH PCV sucks too much air from a crankcase. The only way for an air to get in a crankcase is a RH PCV hose. And a MAF sensor meters that big amount of air too. ECM calculates the fuel for an amount of air, metered by a MAF sensor, but the mixture is too rich, so it decreases a fueling by LTFTs
I've installed a 2.5mm tube in a LH PCV hose, reconnected everything back again and... Now i have almost ideal LTFT, STFT and O2S readings.
The perfomance is quite good again and a milage is even better than before.

 

Well done on the discovery and fix Moscow!

 

Leaper:
I am not understanding your explanation. If the LH PCV is sucking "too much" air, and that air is coming through the RH tube, then it is measured air and should not affect the mixture. I suspect the LH tube is drawing the excess air from atmosphere, likely the oiul dipstick gasket or other crankcase leak.

 

This excessive amount of measured air is not always taking part in a mixture formation, especially under a low load conditions.
No crankcase leaks - all seals and gaskets are new, not leaking air (i've pressurised a crankcase with a smokemachine). Oil dispstick and cap seals are new too.

 

Thank you for this post, it gives us a very useful insight.
Glad to hear you got it worked out.