So I’ve posted here a few times about my p38 4.6 v8 petrol, running rough, low and rough idle, and missing at a steady rpm, had a full list of parts replaced, out of curiosity my local Land Rover guy clamped off the fuel return from the rain and it started to run absolutely mint, ( I didn’t drive it like this and didn’t want to cause further problems) it’s had the in tank fuel pump, filter and pressure regulator, any idea why blocking the return would enable it you run that much smoother?

It will raise the fuel pressure although you've already checked it and it was correct? It might be worth seeing what the fuel pressure is with the return clamped.

Gilbertd wrote:

It will raise the fuel pressure although you've already checked it and it was correct? It might be worth seeing what the fuel pressure is with the return clamped.

Fuel pressure with ignition on, 34-37 psi, running it was around 30-31psi when revved it would drop to around 28psi, will check psi with return clamped, could that drop be due to injectors as everything else has been replaced?

34-37 is correct, when revved the intake manifold wouldn't have any vacuum in it so it should stay at that, it will only drop on the overrun when there is vacuum in the intake. It should also stay at the specified pressure at a constant open throttle.