If no one has pointed it out to you yet, until your motor is up to a certain temperature it will only operate in open loop and when in open loop the engine basically runs of the preset values in your PCM's VE table.
If the values are set on the high side it will generally run rich, if they are on the low side it will run lean.
That's one of the things we change when we tune, to try get the values in the VE tables close to what the engine likes best so to speak.
In theory once you get the VE table set up correctly you could effectively drive around in open loop all the time but I certainly wouldn't recommend that.
During W.O.T. the PCM defaults to open loop and ignores the O2 sensor, thats why it's important to get the values in the VE table correct.
When the engine eventually goes into closed loop the narrow band O2 sensor takes over and tries to trim the fuel to near stoichiometric.
So if your truck seems to be rich when in open loop but better when warmed up and in closed loop, that's just the O2 sensor doing it's job.
Tuner Pro can actually be set to display the point that it changes from open loop to closed loop.
Any time you change your fuel pressure you will need to potentially redo your VE tables.
More fuel pressure = richer across the board
Less fuel pressure = leaner across the board
You need to make sure you have enough fuel pressure with your stock injectors so that are not maxing out their duty cycle at the top end. Then you tune from there.
A bit of a crude description but hopefully it gives you a bit of an idea.
Some of the more knowledgeable guys here might ask you to upload a TunerPro log from your truck and they can then comment on what they see is going on in your log.
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