I'm going to be helping him with his tune, hopefully this week. I'll log it with EFI and get it tuned in.

I have a feeling its the 5.3L tune thats the cause. Sure a 5.3 tune will run a 6L fine, but the the VE will be different and the stock 5.3L tune won't be setup for high timing at the increased airflow ranges. So I'm sure his fueling is off and timing is low.

 

I take it that this is a transplaned 6.0 in a half-ton. If you're running the factory stock cam and the engine is a 2001 or later, I would cut and paste the VE values from a factory stock tune into the Main VE table and see how things go by monitoring AFR with a wideband. The factory Main VE table is pretty close for that cam. If it's a 1999 or 2000 LQ4, I think that the factory VE table is pretty terrible in that it builds up in value to about 4,800 RPM as I recall and never falls off. It'l like the tuner setting up the table did a quick and dirty job and never finished it. The high and low octane spark tables are not very optomized either. Another thing, on my 2000 K2500 LQ4 PCM, power enrichment was effectively entirely disabled. The PE mode had a time delay of 60 seconds and the PE versus RPM values were all set to an equivalence ratio of 1.0.

All my best,

Steve