2004 Sierra PO171,PO172

[chuck's]

These two codes were stored in the computer but did not activate the engine light. The vehicle went to a transmission shop to diagnose a suspected transmission problem. These two codes were pulled and since both codes deal with a lean burn situation, it was determined that there is performance issues with the engine. The tests done to the tranny all passed. I am thinking that the fuel pump is beginning to fail. I did a pressure test and at idle the pump is running at 60 psi. Shutting the engine down ,the pressure drops and holds steady at 57psi. I then restarted the engine and monitored the pressure when I quickly depressed the throttle to the floor and then released. The pressure reading drops down to 40 psi at wide open throttle and then to 60 psi back at idle. Is this a normal for the fuel pressure to drop this low and then bounce back? I found that the transmission acted up when trying to climb a hill and demanding more fuel to the injectors. Is there a test to check the maximum pressure the pump delivers? What other test would be recommended? Is this the first signs of fuel pump failure? Assistance would be greatly appreciated.
chuck's

[maxwedge]

Not normal, actually the pressure should up a full throttle under load. Remove the hose from the press. reg. see what the press. goes up to.

[chuck's]

I only saw one fuel line up on top of the engine. I would say that is the pressure line. Would these newer pumps have the regulator in the tank to dump unneeded fuel directly in the tank?
chuck's

[kenman1717]

possibly a vacuum leak at the intake if you have lean codes. I have an '03 Silverado I have been getting those lean codes off and on, and it runs like crap in the winter time if its not plugged in, had it into a dealer they reprogrammed for idle surge, did no good, I did other things, didn't help, then I found a bulletin online about lean conditions on 4.8, 5.3 and 6.0l engines. there are new updated intake gaskets.

and you mentioned it was acting up climbing hills, mine did that too, and it was due to one cat plugging up and one was coming apart, and it started misfiring like crazy right after that.

not saying this is in fact your problem, just sharing a possibility. if you only have one fuel line to the intake, its a returnless system, and if I remember correctly the regulator is part of the pump.