see here: http://volvospeed.com/vs_forum/topic/168329-location-of-evap-purge-valve-on-a-2004-xc90-t6/ (use the embedded link within for actual parts diagram) I would start with the easiest - gas cap. Make sure it is tight and the sealing ring is in good condition. From there I would tell you to try ...

As above, my personal experience- https://forums.swedespeed.com/showthrea ... mall-leak)

May not be your issue but easy enough to validate and omit.

If you drop the spare and look in the cavity towards the front of the car, think you'll see it. Dredged this link up for reference-https://forums.swedespeed.com/showthread.php?213487-PO442-Check-engine-light Someone else had a problem with the hose coupler that connects the front and rear long evap ...

Below is S60 video, but it is virtually identical for XC90 2.5T:

Probably have a 'burned' exhaust valve that is missing a small chunk. Had that happen on a 960 w/ 6 cylinder that i bought w/ a misfire. Drove it on 5 cylinders for a year before I pulled the head and fixed.