5.3L Swap Hard Start and Sputtering

[Green67Stepper]

I have a '99 5.3l LS swap in my 67 C10 with about 250-300 miles on the swap so far. It runs and drives fine overall, but i do have a few issues.

1: It never wants to start right up. It typically needs a slight touch of the throttle to start and usually wants to idle down and die if you dont gradually ease off. Once its idling, its fine, usually around 550 rpms. At Key On, Fuel Pressure surges to 58-60 psi and then stops and holds at 49-50 psi. It also idles around 49-50 psi.

2: Under normal driving, it runs great. Anytime its at 50% throttle or more though it wants to hesitate and sputter.


I have pretty much cleaned all the sensors with MAF cleaner (MAF, IAC, MAP) i have also removed the throttle body and Fuel Pressure Reg and cleaned them. I have tested the Fuel Injectors and they all work. I also removed them all and cleaned them with some Carb Cleaner. I unplugged all the plug wires at the coils and each one was sparking. I unplugged the MAF and it really runs bad unplugged, so i assume that means the MAF is working. The entire fuel system is new, lines, tank, pump, filter. I have also run Seafoam through the fuel system and vacuum line to clean injectors, etc.

Where should I go from here?