It kinda sounds like your cam bearings are not installed right, to me.

 

Yup this is what is happening.

 

This is kinda fuzzy and I could be wrong, but I seem to remember a small block plug (like a miniture freeze plug) in the right oil gallery at the back of the block. It's at about the same height on the block as the passage for the barbell, but on the other side of the block. Maybe that plug was left out.

Can anyone confirm this - or correct me if I am wrong?

 

would the 5.3 be the same? I can check that out on the old motor to see if it has that.

 

Yes, the 5.3 has the same oiling system.

 

I'll check to see if there is a plug.

 

The 1999 5.3 only has the barbell back there and no other hole or plug.

 

 

 

I had the exact same problem.. I started my truck without the valve covers on, and there was little or no pressure from the ls6 pump I installed. There would be loud valvetrain noise that would come and go.

I changed the o-ring on the oil pump and it went away.

Here are side by side pics.