1991 Chevy Silverado Throttle Body Idle issues

Replacing sensors without properly testing for spec. voltage and functionality is a big no no. Just because those sensors are new does not mean that they are working properly. The computer should be reprogramed (NOT JUST REFLASHED) or replaced. Increasing the cubic inches (from 350 to about 420) is a lot for a computer to compensate for.
A 19 year old wiring harness should be replaced. The moving and/or removal of the wiring harness. as required with any engine change, could have caused damage to some of the wires..

I recommend fuel delivery from the injectors is insufficient. After reading your message and understanding your vehicle has been under the supervision of ASE Certified hands and they have not reported any faulty issues with the replacement components I would consider 454 injectors and an adjustable fuel pressure regulator then consider reprogramming the ECM. Keep us posted.[quote:356807c914="Silverado Blues"]Engine Performance problem
1991 Chevy Silverado V8 Two Wheel Drive Automatic 89,900 miles

I just completed an installation of a 400 SBC into my 1991 Silverado C3500. Transmission is a 4L80 automatic. The engine is .030 over with a EFI cam and 9.5:1 compression with aluminum 63cc heads. Originally equipped with a 5.7 it ran great 3 days earlier but upon installation and proper cam break in (according to the engine builder) I found the truck will not hold an idle nor a steady cruise. Being an expert in hit and miss diagnostics I decided to install a new Air Idle Control Valve, Throttle Position Sensor, Knock Sensor, Temp Sensor, MAP sensor, O2 sensor, Ignition Module, Fuel Pump, Filter & Pressure Regulator, New cap Rotor & Ignition Module and missed every time, I decided to limp it over to a local ASE Certified Shop After a week and two hundred bucks in incidentals later, they were baffled and suggested that I take it to the engine builder to consider an engine build issue.
After a week at the reputable builders hands and several hundred more dollars they said the engine was solid and sound and maybe a new computer or new distributor or even a new TBI on top. I suspected a faulty ground strap so I confirmed a quality ground from battery to engine block to chassis to cab and even grounded the exhaust pipe (single wire o2). With no issues of any vacuum leaks, just don't know why the truck can't find a steady idle, it seems to jump idle up and down and stalls frequently. What do you suggest? Do I need to have the computer flashed? 454 injectors? ECM? Distributor? P.S. I have deleted the EGR valve.[/quote:356807c914]