Hello Volvo forums, I come to you in a time of great frustration.
TL;DR my car has had constant nonsense going on which seems related to the PCV valve and my patience is wearing very thin.

Wife bought the car a couple years ago, and the CEL cam on shortly after for lean burn. After some research (including these forums) I concluded it was the PCV valve. I bought an entire cover+valve replacement piece and swapped it, the light went away for a while, then it would come back occasionally and stay for a few weeks. No problem.
Since last summer, the light has been on constantly, first only for lean burn, then for lean burn +rich burn (as if that were possible at the same time). This time, I changed the valve again, but not the whole cover. This did not change anything, and the CEL remained, as did the values of the short term and long term fuel trim, which basically stayed the same.
Today, I smelled oil when I was driving. Arriving home, I opened the hood to see that the cap had come off completely and the valve was sitting beside it. Fortunately no pieces were missing, so I slapped it back on and started the car.
Now, I have rich burn codes, catalyst efficiency codes, and a short term and long term fuel trim that are both above +40%.
If anyone has some good or bad or silly ideas as to what I should swear at next, I would very much appreciate them.
My own initial thoughts:
-valve isn't seated correctly, because I slammed the cover back on in great anger
-MAF sensor is stoned
-Air filter is ruined
-Huge vacuum leak somewhere that I am not seeing

Thanks for your patience.

 

my first question is whether you used an OEM part or aftermarket. Aftermarket parts frequently have fit issues so your next step may be to redo with Genuine Volvo parts. As to your hunch about a vacuum leak, that's probably where I'd start. Inspect hoses, air boots etc. spray carb cleaner around the intake tubing, vacuum tree, lines etc. If nothing seems amiss, you may need to do a smoke test. Regarding the MAF, if you sucked oil into the intake from a wonky oil separator (aka flame trap), try cleaning up the intake and then clean the MAFs grid with a MAF cleaner product like CRC MAF cleaner (don't use carb cleaner as it can be too harsh ).

There should be some good vids on Youtube for installing a new PCV oil separator, how to check the gasket for a good seal etc.

 

the part I changed out seemed to be OEM, the only missing thing was the FoMoCo logo. I tried cleaning the MAF, and it seems to have normal readings, so I reckon it's not the problem. Next step I think I'm gonna hire someone to do a smoke test, as I sprayed carb cleaner on all the spots I can access by standing in front of the car, and nothing was amiss.