96 GMC K1500 5.7l running rough

[Longhunter44]

Hi all, I picked up a 1996 Z71 K1500 with 230K miles. Rebuilt the transmission and have replaced the rotor, cap, wires, plugs, coil and ignition control module and have run a can of Seafoam through the tank and crankcase. The truck starts fine and runs fine until it warms up, then it will misfire and run rough with low power unless I get it over 3000 rpm, then it runs extremely strong. It's hard to hold speed on the highway, it tends to slowly lose speed, especially up hill until I give it more gas and then it down shifts, kicks the rpms up and runs very strong. I have also replaced two of the O2 sensors that were throwing codes. No codes right now, but the SES light will blink when it's running rough and then go off when I get it above 3000 rpm.

 

I'm at a loss on how to correct the situation and in need of some help and direction. Any help would be greatly appreciated.

 

Thanks,

John

[383S-10]

It might be the fuel pump getting ready to fail on you.....that was a symptom of my 98 Sierra...Im on my fourth fuel pump but at 278,xxx plus miles.........the pump usually starts to get louder when they are about to go out....my trick for changing out one is unbolt the bed, place cardboard between bumper and bed then raise it like dump truck support with 2 32" 2x4 on frame rails to under bed.....swap takes about 40 mins to complete.

[1997SierraSLT]

As stated above, it could be the fuel pump wearing out or it could be catalytic converters starting to get plugged up.