Just replaced hpfp and cleaned tank and all lines and fuel rail with new injectors and intermediate pump and I have a p0088 . I have not had the new injectors programed in yet but I have checked connectors on rail and they look good what could be ppossible cause. I don't have a scan tool for it
Try cleaning your Rail control solenoid again. It has the tiniest Filter on the very tip that is easily clogged and will not allow fuel to pass, maintaining proper pressure, in your case, lower!!
The valve is normally Open and easily flushed and blown backwards (backwash) from the 4 radial holes.
This is the filter - less than 1/8" diameter:
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Once flushed, verify open / close with a weak 9 volt battery. Briefly cycle (1-2 seconds) the leads on and off. You will hear an obvious click.
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Now with a rubber hose, blow in the end to hear it open and close.
Blow air in, while cycling the leads. It will be obvious when it's open and closed.
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Now find a friend with VCDS to adapt (code) your new injectors!!!
Hope this helps!!
Andrew