Low fuel pressure 1997 k1500

[Bubba the lemon]

I've got a 97 k1500 5.7, the truck recently started having a bad stumble around 3500 to 4500rpm and I have an afr gauge so I could see the truck going dead lean. Checking fuel pressure I got 52 while running. After replacing fuel pump I still got 52, then I checked the resistance and voltage at the pump and everything seemed good, I replaced the regulator fuel pressure down to 48 running after doing that. Replaced the pump again on mechanics advice last week and got 48psi again. Thinking either bad fpr or injector unit ? Any help appreciated

 

[east302]

To my knowledge, GM only published a spec for pressure with key on, engine off (60-66 psi). How does your pressure compare? Is the fuel filter new?

From memory, my 98 runs in the 50-55 psi range at idle.

Do you have any misfire counts?

 

[Schurkey]

Yup. Key On, Engine Off has an actual fuel pressure spec. They say you may need multiple "prime" cycles to achieve full pressure. Key on, pump runs 2 seconds. Key off for 10 or 15 seconds, turn ignition on, pump runs for 2 seconds, etc.

Engine running will be "lower" because the fuel pressure regulator is vacuum-referenced, when the intake manifold vacuum is high, the fuel pressure is reduced.

As said--fuel filter is a possibility. Connect a scan tool, look for individual cylinder misfire counts, fuel trims, and verify ALL the sensors and computer outputs--EGR, IAC, spark advance, etc.

If a cylinder misfires for any reason, the O2 sensors are likely to send false "lean" signal to the computer.