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that is really kinda low, dont think i would go under 55
you need the 80 psi for loading purposes...must be a hd, dont need to run 80 all teh time
ground the sending unit wire and see if the gauge responds, might be a bad ground at tank or sending wire shorted
do you mean the mass air flow sensor? 91 code 31
cam sensor circuit
EGR circuit (88-90)
MAP sensor (shorted)