Vauxhall Vivaro fuel starvation ?

Unless they've altered it the pump is in a plastic module combined with a level gauge.
There is a small mesh filter on the bottom of the fuel pick up but this sits in a chamber in the base of the plastic filter/pump module and it sits in around a 1'2 pint of fuel that's always there if there's fuel in the tank.
And the tank has to be removed to get at/out the pump module.
Not that I'm trying to put you off removing the tank to get the pump out but a simple test is to squeeze the primer bulb up near the filter when it does this.
If It takes a lot of pumps to pressurize and go hard them is a fuel supply or vacuum loss.
If the bulb is solid then the fuel system is charged and the problem isn't fuel supply.
But these vehicles are much more prone to electrical shorts.
And when you go round a corner the engine/gearbox does move and can stress the harness.
The main engine harness runs from the ECU,
vertical next to the fuse/relays.
harness then runs down through a nasty turn left across to the gearbox then splits down to the crank angle sensor on the box front,
The up split feeds everything else and anywhere it turns/touches metal is suspect,
Up to the injector connectors shorting onto the head and the CAM sensor doing the same.
I had one in a few years back that had yours and many other symptoms,
I gave up counting after finding over 8 wiring short circuits in the harness.
So test the primer bulb first,
soft is empty,
firm is full,
If it's full suspect the wiring!!!

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