I noticed alot of people had this problem. I have a 2005 volvo s80 2.5L turbo. I attempted to crank the engine to start car. It wouldnt turn over, it would sometimes turn over and stall in seconds. I disconnected the fuel filter and cranked engine over to see how much fuel would come out. NONE. so i replaced fuel pump. shortly after I started getting a p0089 code. fuel pressure regulator. My model volvo has a built in regulator in the fuel pump. it senses pressure on the fuel rail by the fuel pressure sensor and then works in tune with what is called the PEM. performance enhancement module to deliver fuel to the fuel rail. I then started driving and at high demand for fuel the car would sputter during acceleration. if the fuel tank got below 1/4 tank it would really act up. if I filled the car with gas it was fine. I would have to stop the car while driving and turn the key a few times to get the pump to build pressure

My next step of course was to find out if I actually did have a pressure problem. I disconnected the strator valve on the fuel rail and connected a gauge. (the gauge has to read at least 90 psi) I was getting 45 psi at start up. under high demands I would get 25 psi. and then when i shut the car off i would get 0 psi. the car should hold pressure for at least 20 minutes after running.

Finally after alot of research and finally getting ahold of the schematics I noticed that the one problem was the PEM that was completely corroded. I can not think of a worse place for a piece of electronic equipment. It is located right behind fuel filter. I took it off cleaned the connections with isopropyl alcohol. plugged it back in and "bam" pressure is now holding at 45psi under high loads and holds pressure when shut off.

I could not find a good solution on any forum and want to save you all the frustration of this up and coming common problem

 

PEM is Pump Electronic Module. And you should not have 95 PSI at any time. But yes, corrosion on the PEM is common. They make a relocation kit for the XC90, not sure about the S80.