I have an 88 k1500 with a 5.7. Everything that I can think of is new under the hood. the computer is giving me a Code 44 which is a Lean Exhaust code. I have a new O2 sensor put in by the mechanic. Also this problem carried over from the old engine to the new one. When it is cold out the truck runs rough for the first 5-10 min. once the engine warms up it smooths out but still doesn't have the power it should. The only thing that isn't new is the TBI. Any ideas?

 

Check with vacuum gauge and read THIS. Check for vacuum leaks. Check fuel pressure.

If all the NEW parts are working properly (New parts can fail) I would guess, TBI.

Are the fuel pump and fuel filter new?