lift pump
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When you bump the starter pump should run for 25 sec. If pressure is below 10psi replace the fuel filter and try again. Stock pumps will put out max of 12psi but are not good enough to keep psi up at wot. If psi is below 10 at wot its time for a new lift pump or filter. Have you bled the injectors?
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With out seeing it it's tough, I;d put a gauge on first thing,see what it does in this 25 second "run",check fuelfilter itself,check the input voltage to liftpump again durring the "run"to make sure it is staying powered up,yes this is a bit to do but you will know what you have going on here,could be something elsebut ...