I just completed my 4.3 to 5.3 conversion (2002 silverado) with complete wiring harness, accessories, etc. I used hptuners to flash the 4.3 ecm to 5.3 parameters. Everything took on the tune and I made sure that it had all of the correct info in there for engine size, etc. The problem is that the truck will just crank over and not start. No security light coming on or flashing but no spark. Its gets fuel and pulses a noid light on the injectors. I have the 5.3 computer that came with the engine and wiring harness. I can delete vats out of it and the truck will run fine but the damn security light stays on all the time, even with the key off. I have done the security relearn on it per gm quite a few times to see if that was the issue but no luck, the light still stays on. My question is, why will the truck run with the 5.3 computer, but not the 4.3 computer flashed to 5.3 specs, do I need to disable VATS on the 4.3 computer even though it was the one that was on the truck to begin with? I dont want to run the 5.3 computer because of the security light among other things. Any suggestions are appreciated. Thanks!

 

When you flashed the tune to the V6 PCM did you write calibration only or write entire? To me that sounds like it isn't firing the coils, and it's still a V6 setup running on V8 parameters. This is all on the assumption you didn't write entire.

 

I dont remember now that you mention it. I will go back and write entire and see what happens. Thanks!

 

I went back and did the write entire with no luck.

 

I'm out of ideas then, still new to the tuning thing myself. Sub'd for when you get it figured out though.