MAF reading / measurements. Nissan Altima 2000

2.3 g/s at idle is a little low, Nissan MAF's tend to read a bit more than the engine displacement in g/s at idle. For a 2.4L I'd expect to see a reading more like 2.8~3.0 g/s. That would jive with the usual cause for a P0171, which is a vacuum leak, not the MAF. Stream your STFT (Short Term Fuel Trim) with your scanner. If it goes very high at idle but drops to near +0% at 2K RPM or more, you have a vacuum leak.

It's an intake leak, exhaust can cause codes but not a P0171. Check for cracks in your intake boot, leaky gaskets at the Throttle Body/MAF, and the intake manifold gaskets particularly around the #4 runner. Those are all common issues on KA24's, but it's an old engine, a lot of your rubber stuff could potentially be dried out and cracked. Spray around the all the induction parts south of the MAF and all the vacuum hoses with something combustible like Gumout or Brakleen. Listen for idle surges, the combustibles will get sucked into any vacuum leak and temporarily richen the mixture, causing the RPM's to rise.

Zurab said:

So far, I was unable to find any leak (used whole can of carb cleaner, to spray at different spots).
Can open stuck PCV valve be the culprit?

Click to expand...

Yes. So can a pinhole in the brake booster diaphragm, or a diaphragm failure any other device with a vacuum pot. The best way to diagnose those is by systematically pinching off vacuum lines while watching the STFT.