fuel pressure regulator vs. fuel pressure sensor

First of all I have the 2000 XC70 AWD Turbo.

The car is hard to start so I hooked up a fuel pressure gauge and discovered that as soon as the key is turned on pressure hits 30 psi. I start car and it runs around 30-35. When I turn off, the pressure immediately bleeds off to zero in a minute or so. This isn't right. Pressure should be maintained for quite a while after shut down. So, I'm pretty sure the fuel pressure regulator is on its way out.

Now the insanity begins. Lots of searching on all the Volov forums and VADIS for FPR location. VADIS says it's on top of the fuel tank for this year and model. Some people said it was on front axle (nope not there, I checked). So dropping the tank is a huge job to get to it.

But now more mystery is thrown into the equation. When I go to check on prices and part availability at the dealer, they tell me the FPR is actually built inline in the fuel hose that runs from the fuel filter to the engine ($410) yikes!!!

The part on top of the tank that I thought was the fuel pressure REGULATOR is actually called the fuel pressure SENSOR.

So my questions are 1) what is the function of the fuel pressure sensor on top of the tank? 2) would failure of this fuel pressure sensor cause the fuel pressure to bleed off after shut down or this is symptom a function of the fuel pressure regulator solely?

Anybody really understand this set up?

Would love to know the definitive answer since I'm in the same camp as you with hard starting issue.

I too have a 2000 XC70. The fuel pressure regulator is in the middle of the fuel rail. I don't know where the pressure sensor is. The pressure on my car should be and is 53 PSI.

They didn't use a fuel pressure sensor in 2000. They used a tank vapor pressure sensor but it has no bearing on retaining fuel pressure in the fuel lines.
The fuel pump has an integral check valve to stop backflow and the fuel pressure regulator in the fuel line at firewall on 98 and in back on 99-2000 (I'm talking XC70's here but it is the same on FWD models) is the item stopping fuel drain via the return line.
There is not an easy way to diagnose between which of the two is your problem.
On older cars there was rubber hose at FPR you could crimp closed with Vise-grips while checking fuel pressure key off and if it held with the vise-grips in place it was the FPR. If the pressure dropped still it was fuel pump. You can't crimp the hard plastic line without great risk of breaking it. I believe the FPR has a rubber small hose on it that vents back to fuel tank and if that hose has fuel in it then FPR is the fault but it can leak down return line or the vent hose and vent hose fuel check only finds the ruptured diaphragm fault.

Ask JRL. Send him a PM. He will know the answer and tell you if it is the check valve...

I ended up ordering and replacing the fuel pump assembly with a brand new Volvo pump. That fully cured my hard starting issue and it now start like new. If your fuel pump is still the original, it might be wiser to replace it.

Expensive but worthwhile for me since the old pump (not so old, date stamped in 2008 but it's a Scan-Tech) was leaking for a long time, even from when it's installed by the previous owner given the attempts to seal it (even with a worm clamp around the lock ring).