Hi Geoff

To test your theory you could interupt the crank signal and see if the ecu tries to drive the FPR or not.

I would think the rail pressure might still be regulated to start up pressures, with loss of crank signal but the ecu wouldn't fire the injectors(I may be wrong)

When the fuel pressure in the rail is not great enough for the vehicle to start can you see the ecu trying to drive the FPR to create more pressure?

What is the rail pressure when it wont start?

You should have a look at the supply pressure from the pump(s), including either side of the filter at different load conditions.

Also looking at supply pump current can give you answers somtimes.

hth

James