Keep in mind how these car's "Controllers" drop the voltage to F/pump considerably @ IDLE / Off idle . Mine goes full B+ battery voltage (less the power transistor junction drop) ABOVE 2k rpm's ('93 J30) , Q's will do this ,but I don't know if they have a "gradual' or "stepped" increases in power to F/pump - unlike my Big jump to FULL voltage after the 2k rpm!
So , a weak pump may not make it while "controlled on the down-Lo" , all these things I mention WILL show in
a pressure gauge tapped @ the rail .