Hi,

I found this article by putting in “what is the vapor pressure of gasoline?” I got more than numbers.

https://www.punchlistzero.com/gasoline-boiling-point/

I did this because I know what happens to things that get pumped.
If you study a refrigeration cycle you get tuned in on things like this.
We use a variety of gases “blended” to achieve a certain characteristics of heat removal of the fluid to gaseous state of change. We have low, medium and high temperature rated compressors being used as there are volumetric differences to be dealt with.
Just like the article states, it’s about blends and equipment.

You can trust me on this as Volvo engineering would not put in two pumps if it wasn’t necessary.
Bean counters are all over the place.

Major things that do change are the environments surrounding tanks. They change in levels or amounts contained.
These are not a closed system like refrigeration is but its still about temperature and pressure of working fluids.
Imparting energy into a fluid will affect both of these items.
The pumping action does exactly that.
The main pump actually circulates more fuel than is needed and it goes back to the tank.
You don’t have to guess where those gasoline lines just came from, up front, to know that things are in dynamic phase of changes going on in the system.

So what the tank gets back, is different all the time. Just as much as the environment changes.
The in-tank pump was put there to compensate for this, as the main pump does not like cavitations of any kind.
The injectors issue out what ever they get or don’t get.
The fuel management system can only compensate so much.
It claims to be self tuning but that’s under laboratory built conditions, with all of the components working, to its creators design.

That little pump is a back up for the variables.

Hope this helps all of us to give credit to the engineers and ourselves, for maintaining these cars over the years, despite quality changes that you are noticing.

Phil