5.3 rebuild with cam, flattop pistons and 243 heads in 99 reg cab swb. Got my ecu tuned by Michigan motorsports. Hooked everything up where I could at least get it to start and build oil pressure before I finished putting front clip back on. Turns over and spits and sputters. Only cylinder 5 and 6 firing. I checked to see if the coils and injectors were getting power and they seem to be. What else could it be ? Is it a harness problem ? Harness came from a 01 Silverado 4x4.

 

Coil bracket(s) have sharp edges, and sometimes "wear through" the coil wire harness especially when 20+ years old.

 

clogged injectors
bad fuel pressure
bad crank sensor
i mean really could be anything.

will it run correclty if you spray carb cleaner down its throat?

 

Are you saying you changed the harness from the last time this engine ran correctly?
If yes, then mixed up injector or coil wiring is a possibility. Maybe got swapped right to left?

The carb cleaner suggestion might help decide if its an injector or coil problem.