i would think the problem is still the fuel pump..these must have no less the 60psi .even though you have 70psi when you shut off the return..i think it is not delivering enough volume
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this still isn't right..the pressure needs to be 60psi..nothing else can cause the low pressure other then the pump or regulator
You may have a vacuum leak at the intake gaskets. The vacuum leak is hard to find because the leak is under the intake, not on the top. GM has had problems with the intake gaskets on these engines, so much that they redesigned the gasket in 2005. If the intake gaskets have never been replaced before on this engine its a good place to start. The old gaskets are orange and the new ones are green. The only way to tell if you have the new gaskets is to remove the intake, which is not a hard job. The orange gaskets go flat and cause the vacuum leak. This will cause a rough idle and a few codes to set such as the p0300 and p0171/p0174.
The fuel pressure seams to be OK, if your pressure gauge is off by 5%, then you would have a normal pressure reading.