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You may want to do some fuel pressure tests before you change the pump (s). That system is quite cumbersome really and by the sound of things, you do need to start renewing some stuff.
Firstly, check that both pumps are running when the car is on, this is easier if you remove the rear seat and with a cheap mechanics stethascope listen for a low whirring noise. If that pump doesn't lift fuel out of the tank and give the high pressure pump a good feed pressure, then the high pressure pump will struggle.
Check that the high pressure pump is runing too, jack up the drivers side and support it, the feel/listen for that one running too!
Its a good idea to buy a fuel pressure test kit, its just a gauge, some hoses and fittings that allow you to measure the pressure the pump is delivering at the fuel rail. You should have a pump capable of giving you 4-5 barg (and more) and the fuel pressure regulator then controls the pressure in the rail by spilling fuel back to the tank as needed. If you get a good constant pressure, try pinching off the return line momentarily and see the pressure climb rapidly, it if moves up to 10-12 barg all is well with the pump.
We run an endurance race car and had lots of fuel issues. To simpliy things, we took the small pulse dampener out all together, its in the line after the filter. I suggest taking that out, it doesn't do much and can fail. Change the filter too and check the metal fuel lines under the car are not dented or pinched, especially the return line and breathers.
With these cars getting older, 2 new pumps and a filter isn't a bad idea.
Good luck
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