Ooof, that mid range STFT IS 27%, that’s a huge adjust. Something is leaking or fuel pressure is kaput, have you checked that?

If that is reproducibly rpm depenendent in the mid range, I would look at the intake manifold area for a leak. That’s induction dependent vs a leaky hose

Smoke the intake, you have a huge hole.

Ok, so where can I get a smoke machine? I checked the Autozone loaner tool program and doesn't look like they have one....I'd rather not buy one, unless I can get one for cheap... Build one? Or ask in shops around your area if they can diagnose the leak for you. I have this one: https://www.amazon....

I have replaced the whole pipe. It is known to develop cracks over years, plus I needed a new PTC valve: the original was filed wider to compensate for blow-by gases and extra flow resistance of external oil catcher can. After the engine has been rebuilt, the can was thrown out, yet the fuel trim wa...

Check valve in the brake line, too

Recall letters:

https://static.nhtsa.gov/odi/rcl/2021/R ... 0-4366.PDF

I usually put silicone around there to make a tight seal. That thing always seems to leak a bit when smoke testing.

Pretty good start here ….

https://www.matthewsvolvosite.com/which-xc90-to-buy/

I had an XC90 for a short while. Just a fix-n-flip thing so I don’t have that much everyday driving experience. You sit higher up than in a sedan which is nice. You also have to be long to reach stuff under the hood since the front sits so high up if you don’t stand on the spare or something. Not ve...

the belts don't break, the pulleys wear out and throw the belt.
the water pump is due too.